Remote Sensing of Shelf Sea Hydrodynamics: Proceedings of the 15th International Liege Colloquium on Ocean Hydrodynamics 0444423141, 9780444423146, 9780080870762

Table of contents :
Content:
Edited by
Page iii

Copyright page
Page iv

Foreword
Page v
Jacques C.J. Nihoul

List of Participants
Pages vii-ix

Water Colour Imaging from Space Original Research Article
Pages 1-24
J.F.R. Gower

Contribution of Remote Sensing to Modelling Original Research Article
Pages 25-36
Jacques C.J. Nihoul

Optimal Remote Sensing of Marine Environment Original Research Article
Pages 37-49
I.V. Muralikrishna

Satellite and Field Observations of Currents on the Eastern Sicilian Shelf Original Research Article
Pages 51-68
E. Bohm, E. Salusti

Kinetic Study of Self-Propelled Marine Vortices Based on Remotely Sensed Data Original Research Article
Pages 69-105
T. Nishimura, Y. Hatakeyama, S. Tanaka, T. Maroyasu

Study of Vortex Structure in Water Surface Jets by Means of Remote Sensing Original Research Article
Pages 107-132
Sotoaki Onishi

Surface Temperature and Current Vectors in the Sea of Japan from Noaa- 7/AVHRR Data Original Research Article
Pages 133-147
T. Sugimura, S. Tanaka, Y. Hatakeyama

Study of Mesoscale Processes in the Shelf Zone of the Black Sea Using Remote. Techniques Original Research Article
Pages 149-157
R.V. Ozmidov, V.I. Zatz

Surface-Wave Expression of Bathymetry Over a Sand Ridge Original Research Article
Pages 159-185
C. Gordon, D. Greenwalt, J. Witting

Wave-Current Interactions: A Powerful Mechanism for an Alteration of the Waves on the Sea Surface by Subsurface Bathymetry Original Research Article
Pages 187-203
James M. Witting

Remote Sensing of Oil Slick Behaviour Original Research Article
Pages 205-215
P.P.G. Dyke

An Intercomparison of Geos-3 Altimeter and Ground Truth Data off the Norwegian Coast Original Research Article
Pages 217-234
Asle Lygre

Satellite Imagery of Boundary Currents Original Research Article
Pages 235-256
T. Carstens, T.A. McClimans, J.H. Nilsen

Turbulence Distribution off Ushant Island Measured by the Osurem Hf Radar Original Research Article
Pages 257-275
P. Piau, C. Blanchet

A Quasi Geostrophic Model of the Circulation of the Mediterranean Sea Original Research Article
Pages 277-285
Laurent Loth, Michel Crepon

Some Applications of Remote Sensing to Studies in the Bay of Biscay, Celtic Sea and English Channel Original Research Article
Pages 287-315
R.D. Pingree

Remote Sensing of Chlorophyll in the Red Spectral Region Original Research Article
Pages 317-336
S. Lin, G.A. Borstad, J.F.R. Gower

Satellite Representation of Features of Ocean Circulation Indicated by CZCS Colorimetry Original Research Article
Pages 337-354
C.S. Yentsch

Citation preview

REMOTE SENSING OF SHELF SEA HYDRODYNAMICS

FURTHER TITLES IN THISSERIES 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 20. 21. 22. 24. 25. 26. 27. 28. 30. 31. 32. 33. 34.

35. 36. 37.

V. VACQUIER, Geomagnetism.in Marine Geology W.J. WALLACE, The Development of the Chlorinity/Salinity Concept i n Oceanography E. LISITZIN, Sea-Level Changes R.H. PARKER, The Study of Benthic Communities J.C.J. NIHOUL (Editor), Modelling of Marine Systems 0.1, MAMAYEV, Temperature-Salinity Analysis of World Ocean Waters E.J. FERGUSON WOOD and R.E. JOHANNES (Editors), Tropical Marine Pollution E. STEEMANN NIELSEN, Marine Photosynthesis N.G. JERLOV, Marine Optics G.P. GLASBY (Editor), Marine Manganese Deposits V.M. KAMENKOVICH, Fundamentals of Ocean Dynamics R.A. GEYER (Editor), Submersibles and Their Use in Oceanography and Ocean Engineering J.W. CARUTHERS, Fundamentals of Marine Acoustics P.H. LeBLOND and L.A. MYSAK, Waves i n the Ocean C.C. VON DER BORCH (Editor), Synthesis of Deep-sea Drilling Results in the Indian Ocean P. DEHLINGER, Marine Gravity F.T. BANNER, M.B. COLLINS and K.S. MASSIE (Editors), North-West European Shelf Seas: The Sea-Bed and the Sea i n Motion J.C.J. NIHOUL (Editor), Marine Forecasting H.-G. RAMMING and Z. KOWALIK, Numerical Modelling of Marine Hydrodynamics R.A. GEYER (Editor), Marine Environmental Pollution J.C.J. NIHOUL (Editor), Marine Turbulence A. VOlPlO (Editor), The Baltic Sea E.K. DUURSMA and R. DAWSON (Editors), Marine Organic Chemistry J.C.J. NIHOUL (Editor), Ecohydrodynamics R. HEKINIAN, Petrology of the Ocean Floor J.C.J. NIHOUL (Editor), Hydrodynamics of Semi-Enclosed Seas B. JOHNS (Editor), Physical Oceanography of Coastal and Shelf Seas J.C.J. NIHOUL (Editor), Hydrodynamics o f the Equatorial Ocean W. LANGERAAR, Surveying and Charting of the Seas

Elsevier Oceanography Series, 38

REMOTE SENSING OF SHELF SEA HYDR0DYNA M ICS PROCEEDINGS OF THE 15th INTERNATIONAL LIEGE COLLOQUIUM ON OCEAN HYDRODYNAMICS

Edited by JACQUES C.J. NIHOUL Professor of Ocean Hydrodynamics, University of L i2ge Lisge, Belgium

ELSEVl ER Amsterdam - Oxford

-

New York - Tokyo 1984

ELSEVIER SCIENCE PUBLISHERS B.V. 1, Molenwerf, P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands Distribution for the United Stares and Canada:

ELSEVIER SCIENCE PUBLISHING COMPANY INC. 52, Vanderbilt Avenue New York, N.Y. 10017, U S A .

Library of Congress Cataloging in Publication Data

International Lisge Colloquium on Ocean Hydrodynamics (15th : 1983) Remote sensing of shelf sea hydrodynamics. (Elsevier oceanography series ; 38) Bibliography: p. 1. Ocean circulation--Remote sensing--Congresses. 2. Ocean currents--Remote sensing--Congresses. 3. Continental shelf--Remote sensing--Congresses. I. Nihoul, Jacqaes :. J . 11. Title. 111. Series. GCZ8.5.156 1933 551.47 84-1b72 ISBN 0-444-42314-1 (U.S. )

ISBN-0-44442314-1 (Vol. 38) ISBN 0-44441623-4 (Series) 0 Elsevier Science Publishers B.V., 1984 All rights reserved. No part of this publication may be reproduced, stored i n a retrieval system or transmitted i n any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V., P.O. Box 330, 1000 AH Amsterdam, The Netherlands

Printed in The Netherlands

V

FOREWORD

The International Liege Colloquia cn Ocean Hydrodynamics are organized annually. Their topics differ from one year to another and try to address, as much as possible, recent problems and incentive new subjects in physical oceanography. Assembling a group of active and eminent scientists from different countries and often different disciplines, they provide a forum for discussion and foster a mutually beneficial exchange of information opening on to a survey of major recent discoveries, essential mechanisms, impelling question-marks and valuable recommendations for future research. The Scientific Organizing Committee and all the participants wish to express their gratitude to the Belgian Minister of Education, the National Science Foundation of Belgium, the University of Liege, the Intergovernmental Oceanographic Commission and the Division of Marine Sciences (UNESCO) and the Office of Naval Research for their most valuable support. The editor is indebted to Dr. Jamart for his help in editing the proceedings. Jacques C.J. NIHOUL

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VII

LIST OF PARTICIPANTS BALLESTER, A., Prof. Dr., Instituto Investigaciones Pesqueras, Barcelona, Spain BOHM, E., Dr., Dipartamento di Fisica, Universita Roma, Italy. BOUKARY, S., Mr., University of Niamey, Niger. CARSTENS, T., Prof. Dr., Norwegian Hydrodynamic Laboratories, River and Harbour Laboratory, Trondheim, Norway. CHABERT D'HIERES, G., Eng,, Universite Scientifique et Medicale de Grenoble, Institut de Mgcanique, Grenoble, France. CLEMENT, F., Mr., MCcanique des Fluides Geophysiques, Universitg de LiPge, Belgium. CREPON, M., Dr., Laboratoire d'Oc6anographie Physique, Museum d'Histoire Naturelle, Paris, France. DANIELS, J . W . , Mr., Department of Oceanography, University of Southampton, U.K. DISTECHE, A., Prof. Dr., Laboratoire d'Oceanologie, Universitg de LiSge, Belgium. DJENIDI, S., Eng., Mgcanique des Fluides Ggophysiques, Universite de LiPge, Belgium. DUPOUY, C., Miss, Laboratoire d'Optique AtmosphCrique, Universit6 des Sciences et Techniques de Lille, France. DYKE, P.P.G., Department of Mathematics and Computer Studies, Sunderland Polytechnic, U.K. GASPAR, Ph., Mr., Institut d'Astronomie et de GCophysique, Universit6 Catholique de Louvain, Belgium. GIDHAGEN, L., Mr., Swedish Meteorological and Hydrological Institute, NorrkGping, Sweden. GILLOT, R.H., Dr., Joint Research Centre, Commission of the European Communities, Ispra, Italy. GOFFART, A . , Miss, Laboratoire de Biologie Marine, Universit6 de Ligge, Belgium. GORDON, C.M., Mr., Naval Research Laboratory, Washington, U.S.A. GOWER, J.F.R., Dr., Institute of Ocean Sciences, Sidney, Canadp. GRILLI, S . , Hydraulique GBn6rale et Mecanique des Fluides, Universit6 de Liege, Belgium. GROSJEAM, P., Mr., MBcanique des Fluides GBophysiques, UniversitB de L S g e , Belgium. HECQ, J.H., Dr., Laboratoire de Biologie Marine, Universit6 de Liege, Belgium.

VIII

JACOBS, W., Mr., Institut fcr Geophysik und Meteorologie der Universitat K61n, Germany. JAMART, B., Dr., Unit6 de Gestion, Modele Mathgmatique Mer du Nord et Estuaire de l'Escaut, Cellule de Liege, Belgium. LEBON, G., Prof., Dr., Thermodynamique des Phenomenes Irrgversibles Universite de Liege, Belgium. LE CANN, B., Mr., Laboratoire d'Oc6anographie Physique, Universitg de Bretagne Occidentale, Brest, France. LIN, S., Mr., The Second Institute of Oceanography, Hangchow, Zhejiang, People's Republic of China. LOFFET, A., Eng., Belfotop Eurosense, Wemmel, Belgium. LYGRE, A., Mr., Continental Shelf Institute, Trondheim, Norway. MARUYASU, T., Prof., Dr., The Science University of Tokyo, Noda, Chiba, Japan. MASSIN, J.M., Dr., Ministsre de l'Environnement, Direction de la Prevention des Pollutions, Neuilly, France. MONREAL, M.A., Mrs., Consejo Nacional de Ciencia y Tecnologia (Conacyt), Mexico. MORCOS, S., Dr., Division des Sciences de la Mer, UNESCO, Paris, France. MURALIKRISHNA, I.V., Dr., National Remote Sensing Agency, Balanagar, India. NEVES, R., Mr., Instituto Superior Tecnico, Lisboa, Portugal. NIHOUL, J.C.J., Prof., Dr., Mecanique des Fluides Geophysiques, Universite de Liege, Belgium. NISHIMURA, T., Dr., The Science University of Tokyo, Noda, Chiba, Japan. ONISHI, S., Prof., Dr., The Science University of Tokyo, Noda, Chiba, Japan. PIAU, P., Eng. Institut Franqais du Petrole, Rueil-Malmaison, France PINGREE, R.D., Dr., Marine Biological Association, Plymouth, U . K . POULAIN, P.M., Mr., Mecanique des Fluides Geophysiques, Universite de Liege, Belgium. RONDAY, F.C., Dr., M6canique des Fluides Geophysiques, Universite de Liege, Belgium. SALAS DE LEON, D., Mr., Consejo Nacional de Ciencia y Tecnologia (Conacyt), Mexico.

IX SALUSTI, S.E., Dr., Istituto di Fisica, Universita Roma, Italy. SENCER, Y., Eng., Mithatpasa cad, Ankara, Turkey. SMITZ, J., Eng., Mecanique des Fluides Ggophysiques, Universit6 de Ligge, Belgium. TANAKA, S., Dr., Remote Sensing Technology Center, Tokyo, Japan. VENN, J.F., Mr., Mathematics Department, City of London Polytechnic, U.K. VAN DER RIJST, H., D r . ,

Elsevier Publ. Company, Amsterdam,

Holland. WITTING, J.M., Dr., Naval Research Laboratory, Computational Physics, Washington, U.S.A. YENTSCH, C.S., Prof., Dr., Bigelow Laboratory for Ocean Sciences, Maine, U.S.A.

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XI CONTENTS J.F.R. GOWER

:

Water Colour imaging from space

. . . . . . .

J.C.J. NIHOUL : Contribution of remote sensing to modelling

........................

I.V. MURALIKRISHNA : Optimal remote sensing of marine environment . . . . . . . . . . . . . . . . . . . . .

1

25

..

37

E. BOHM and E. SALUSTI : Satellite and field observations of currents on the Eastern Sicilian Shelf

51

T. NISHIMURA, Y. HATAKEYAMA, S . TANAKA and T. MARUYASU: Kinetic study of self-propelled marine vortices based on remotely sensed data

69

. . . . . . . .

. . . . . . . . . . . . . . . . .

S. ONISHl : Study of vortex structure in water surface jets by means of remote sensing

. . . . . . . . . . . . .

T. SUGIMURA, S. TANAKA and Y. HATAKEYAMA

:

107

Surface

temperature and current vectors in the Sea of Japan from NOAA-7/AVHRR data . . . . . . . . . . . . . .

. . .

133

R.V. OZMIDOV and V.I. ZATZ : Study of mesoscale processes in the shelf zone of the Black Sea using remote techniques

149

C. GORDON, D. GREENWALT and J. WITTING : Surface-wave expression of bathymetry over a sand ridge . . . .

159

.......................

. . .

J.M. WITTING : Wave-Current interactions. A powerful mechanism for an alteration of the waves on the sea surface by subsurface bathymetry

187

P.P.G. DYKE : Remote Sensing of oil

............ slick behaviour . . . .

205

A. LYGRE : An intercomparison of GEOS-3 altimeter and ground truth data off the Norveqian coast

217

T. CARSTENS, T.A. McCLIMANS and J.H. NILSEN imagery of boundary currents

235

. . . . . . . . . . . :

Satellite

..............

P. PIAU and C. BLANCHET : Turbulence distribution off USHANT ISLAND measured by the OSUREM HF Rad?r

......

257

XI1

L. LOTH and M. CREPON

A quasi geostrophic model of the circulation of the Mediterranean Sea :

. . . . . . . . . .

R.D. PINGREE : Some applications of remote sensing to studies in the Bay of Biscay, Celtic Sea and English Channel

. . . . . . . . . . . . . . . . . . . .

277

287

S. LIN, G.A. BORSTAD and J.F.R. GOWER : Remote Sensing of

Chlorophyll in the red spectral region

. . . . . . . . .

C.S. YENTSCH : Satellite representation of features of ocean circulation indicated by CZCS colorimetry

. .. .

317

337

1

WATER COLOUR IMAGING FROM SPACE J.F.R. GOWER Institute of Ocean Sciences, P . O .

Box 6000, Sidney, B.C.,

Canada

V 0 L 4B2

ABSTR~CT

Water colour images from the Coastal Zone Color Scanner on the NIMBUS 7 satellite can now show physical and biological processes in the ocean with greater clarity than has ever been possible before. Examples are presented here of turbulent flow patterns in the Gulf Stream affected by the New England seamounts, coastal upwelling off South Africa, the surface pattern formed by the Alaskan Stream, and regions of high phytoplankton concentration on the continental shelf of Argentina. The processing steps now being used to obtain these results are described, with references to more detailed treatments. Possibilities for future improvements in this type of remote sensing measurement are discussed, with particular reference to the possibility of mapping naturally stimulated phytoplankton pigment fluorescence from space.

INTRODUCTION The colour of the sea has been used by sailors for centuries as a check on navigation and as an a d to locat ng productive waters for fishing. Several seas round the world are named after their colours, the most commonly cited example being the Red Sea, named

after

its

sporadic

Trichodesmium ( = Oscillatoria).

blooms

of

the

phytoplankton

Currents bring together water

masses with more subtle colour differences.

The Kuroshio ( "dark

water") is named for this difference, and the colour change at the edge of the Gulf Stream can also be distinguished by eye from a ship. Near the coasts the colour changes can be due to resuspension of bottom sediments silt-laden water.

in shallow water or to river discharge of

Water from the Yangtse Kiang river in China,

for example, gives the Yellow Sea

its name.

Colour changes

further from shore must be due to the growth of phytoplankton where conditions of nutrients and sunlight are favorable.

Such

2

growth can cause colour changes from blue through blue-green to green and in extreme cases to yellow, brown or red. Patches and streaks of

strongly discoloured water were widely

early travelers.

reported by

Darwin (1845) for example, cites references and

describes passing through large areas off South America where "the colour of the water as seen at some distance was like that of a river which has flowed through a red clay district; but under the shade of the vessel's side it was quite as dark as chocolate.

The

line where the red and blue water joined was distinctly defined. The weather for some days previously had been calm and the ocean abounded to an unusual degree with living creatures." Darwin here points out two important connections--with calm weather, allowing near surface stratification, stability and high growth rates, and with the other

"living creatures" in the ocean who

depend on

phytoplankton as the first link in their food chain. Early observers were greatly intrigued by the fronts and narrow bands exhibited by the patches of visibly discoloured water.

In

fact only minute elements of the f u l l patterns can be seen from a ship.

The satellite images presented below show more of the full

complexity of s&ucture due to current streams, and mesoscale eddy fields influenced by larger scale water movements. The water colour images are from the Coastal Zone Color Scanner on

NASA's

techniques

7

NIMBUS used

in

satellite. deriving

Processing these

images

and

correction

are

discussed.

Opportunities for future developments are suggested, including the possibility of mapping naturally stimulated phytoplankton pigment fluorescence from satellites. SATELLITE WATER COLOUR IMAGERY Early weather satellites provided visible and thermal images of clouds, to show the locations of weather systems by day or night. The thermal imagery could also faintly distinguish the sea surface temperature structure associated with major boundary currents such as the Gulf Stream. (Scanning

Improvement of these sensors, through the SR

Radiometer)

to

the

VHRR

(Very

High

Resolution

Radiometer ) to the present AVHRR (Advanced Very High Resolution Radiometer) now gives clear and sharp images, such as Fig. 1, which can resolve temperature and spatial differences as small as 0.2OC

and

1 km

respectively.

This

image

shows

the

thermal

patterns of the surface skin of the ocean, associated with the northeastward flow of the Gulf Stream off the coast of N o r t h

3

Fig. 1. Thermal infrared image (TIROS N AVHRR) showing the Gulf Stream at 19.22 GMT on May 7, 1979. America on May 7, 1979. clouds appear white.

Warmer water is dark, and cold, high

Eddies shed by the stream can be seen to

the north and south of the main current. The degree of detail and the geometrical fidelity of these images have made them a major tool of physical oceanography.

By

contrast, the associated visible images (Fig. 2 ) have been useful only indirectly, providing additional information on the presence of low cloud in daytime passes, though in some cases ocean information can also be deduced from sunglint patterns (La Violette et al, 1980). the water

In Fig. 2 the only contrast visible over

are the white patches due to cloud, with a

faint

brightening at the lower left due to sunglint. The amount of light upwelling from beneath the sea surface gives only about 1% of the signal from sunlit clouds, so that variations in this quantity should indeed be hard to detect on an image designed for cloud mapping. Thermal contrast due to the Gulf Stream, on the other hand, can easily amount to 5 % of the full scale signal, making this an easier target for satellite remote sensing.

4

Fig. 2.Visible image recorded at the same time as Fig. 1. A specialized ocean colour sensor can, however, do much better

than Figure 2 would suggest.

Sensitivity can be increased and

the signal allowed to saturate over cloud.

A mirror can be used

to tilt the field of views away from areas where sunglint is expected, and narrow, optimally placed

can be is dramatically illustrated in Fig. 3 which shows processed data from the Coastal selected.

The

result

of

this

Zone Color Scanner on NIMBUS 7,

spectral bands

improvement

for the same area of ocean at

nearly the same time (3.5 hours earlier). Shades

of

grey

in

the

image

represent

phytoplankton

chlorophyll 5 and phaeophytin pigment concentrations (a standard measure of phytoplankton concentration) with the darkest shades corresponding 10 mg.m-3.

to

.05

mg.m-3

and

the

lightest

to

over

Comparing this with Fig. 1, the water colour image

is able to show more structure in the water, though Fig. 1 could possibly be further enhanced to bring out more structure in the colder water. very

similar,

anticorrelation concentration.

The features that appear on the two figures are illustrating between

the

commonly

temperature

and

observed

high

phytoplankton

The Gulf Stream is again darker in Fig. 3, but

5

F i g . 3. C Z C S p r o c e s s e d ( l e v e l 2 ) p i g m e n t image s h o w i n g t h e same a r e a a s F i g . 1 a t 1554 GMT o n May 7 , 1 9 7 9 ( 3 . 5 h o u r s e a r l i e r ) 1000 m d e p t h c o n t o u r ( d o t t e d ) a n d N e w E n g l a n d s e a m o u n t c h a i n ( t r i a n g l e s ) have been superposed. Grey t o n e s t e p wedge, below g i v e s pigment value.

here

because

compared

of

to more

its

low

pigment

t h a n 0.3

concentration

mg.m-3in

t h e more

( < 0.1

mg.m-3),

productive w a t e r s

further north. T h e complex mesoscale

eddy

field

Gulf Stream i s h e r e w e l l i l l u s t r a t e d . be q u a n t i z e d i n t e r m s of

(Gower

et

al.

1981) and

t h o s e s u g g e s t e d by t h e

in

the w a t e r

north of

the

Such s p a t i a l p a t t e r n s c a n

t h e i r t w o dimensional s p a t i a l spectrum variations

in

increase i n high

this

spectrum,

such as

frequency s t r u c t u r e on

t h e r i g h t s i d e o f t h e image, would be e x p e c t e d t o c o r r e l a t e w i t h t h e changing dynamics o f d i f f e r e n t ocean a r e a s .

case,

t h e c h a i n o f New England seamounts

crosses t h e

image w h e r e

this

change

in

In the present

( p l o t t e d as t r i a n g l e s ) structure

is observed.

T h e s e a m o u n t s e x t e n d u p from t h e b o t t o m a t 5000 m e t e r s t o d e p t h s

6

o f b e t w e e n 1000 a n d 2000 m e t e r s , a n d t h e r e f o r e i n t e r c e p t t h e G u l f Stream,

which

Richardson

flows

in

the

(1981) h a s r e p o r t e d

top

2500

meters

the e f f e c t o f

t h e s u r f a c e f l o w a s t r a c e d b y buoy t r a c k s .

of

ocean.

the

these seamounts on

H e o b s e r v e d meanders

a n d s m a l l ( 2 0 km s c a l e ) e d d i e s , n e a r a n d e x t e n d i n g e a s t w a r d s f r o m i n d i v i d u a l seamounts.

Fig.

3 i l l u s t r a t e s t h i s e f f e c t more f u l l y ,

w i t h s e v e r a l i n s t a n c e s of s m a l l e r e d d i e s n e a r t h e s e a m o u n t s , a n d a g e n e r a l l y more d i s o r d e r e d f l o w t o t h e e a s t .

The 1000 m e t e r

depth

contour

i s s u p e r p o s e d o n t h e image t o

show t h e p o s i t i o n o f t h e e d g e o f t h e c o n t i n e n t a l s h e l f .

The area

of h i g h p r o d u c t i v i t y c a u s e d b y t i d a l m i x i n g o v e r G e o r g e s Bank c a n be s e e n a s a l i g h t e r t o n e d a r e a e a s t o f Cape Cod.

Fine structure

i n t h i s a r e a f o l l o w s t h e form o f t h e s h a l l o w ( a b o u t 10 m ) s h o a l s on t h e b a n k . F i g . 4 shows a n . a r e a o f f t h e w e s t c o a s t o f S o u t h A f r i c a . Town i s a t t h e l o w e r r i g h t c o r n e r o f t h e image.

Cape

The l i g h t a r e a s

i n d i c a t e h i g h p r o d u c t i v i t y due t o coastal upwelling w h e r e pigment concentrations

can

reach

30

mg.nr3.

CZCS

images

of

these

Fig.4. CZCS p i g m e n t image s h o w i n g e f f e c t s o f u p w e l l i n g ( r i g h t ) and p o s s i b l y t h e Benguela c u r r e n t (bottom r i g h t ) , off S o u t h A f r i c a on November 3 , 1979. 1000 m d e p t h c o n t o u r ( d o t t e d ) superposed.

c o a s t a l a r e a s h a v e b e e n d i s c u s s e d b y Shannon e t a 1 ( 1 9 8 3 ) .

The

c h a n g e i n s p a t i a l s t r u c t u r e f u r t h e r o f f s h o r e , a t t h e lower r i g h t of

the

image,

again

a

suggests

dynamic

input,

here

from

the

Benguela c u r r e n t . Fig.

5 c o v e r s a s m a l l s t r i p o f t h e n o r t h e a s t P a c i f i c Ocean

along the s o u t h e r n edge o f the A l e u t i a n I s l a n d c h a i n ,

and s h o w s

t h e s u r f a c e s t r u c t u r e a s s o c i a t e d w i t h the Alaskan Stream on J u l y 1 0 , 1 9 7 9 i n terms o f p i g m e n t l e v e l v a r i a t i o n s i n t h e r a n g e 0 . 4 t o 1 m9.m-3.

stream

This

l e a v e s t h e Gulf

is

of Alaska.

major

the

current

by

which

water

f l o w s westward as a narrow

It

jet

a l o n g t h e c o n t i n e n t a l s l o p e , whose l a n d w a r d e d g e i s i n d i c a t e d i n Fig. the

5 by t h e 1 0 0 0 m c o n t o u r

image

confirms the

(dotted).

narrow

The s t r u c t u r e v i s i b l e i n

( 6 0 km)

width

deduced by

Royer

a n d shows t h e s t a r t o f a

(1981) f r o m c u r r e n t m e t e r observations,

r e c i r c u l a t i n g e d d y n e a r t h e bottom c e n t r e o f t h e image,

south of

Dutch Harbor on U n a l a s k a

i n which

Gulf

of

Alaska

to

observed Thomson

water

occur

(1972).

at

Island.

mixes

a

Such r e c i r c u l a t i o n ,

into

range

The position

the

of of

Pacific

longitudes

this

eddy

Ocean,

as

has

been

discussed

by

is i n the s t a r t o f

t h i s range and n e a r t h e p o s i t i o n f o r the s t a r t o f

recirculation

r e p o r t e d by W r i g h t ( 1 9 8 1 ) f o r March 1980.

F i g . 5. C Z C S p i g m e n t image showing t h e A l a s k a n stream o n J u l y 10, 1979 w i t h t h e 1000 m d e p t h c o n t o u r ( d o t t e d ) s u p e r p o s e d . Fig.

6

shows high concentrations of

phytoplankton

along the

e d g e o f t h e c o n t i n e n t a l s h e l f o f f t h e A r g e n t i n a c o a s t , b e t w e e n 40 and 45' S o u t h o n December 1 0 , 1 9 7 8 .

P r o d u c t i v i t y here i s r e l a t e d

t o mixing by s t r o n g t i d a l c u r r e n t s over the shelf.

A r e a s of high

0

Fig. 6. CZCS pigment image showing areas of high phytoplankton concentration on the edge of the continental shelf off Argentina on December 10, 1978. 1000 m depth contour (dotted) superposed. phytoplankton concentration have been elongated by current shear parallel to the coast, and further strips of pigmented water are visible

further

offshore.

Similar

reports by Darwin and others of

strips must

have

led

to

"great bands" of discoloured

water. Location of depth contours and seamounts on figures presented above makes use of latitude and longitude marks provided round Inaccuracies of about 30 k m in

the edges of processed images.

3

5

and

partially corrected by reference to visible coast features.

No

positions

of

these

marks

were

noted

in

Figs.

and

such correction is possible in Fig. 6 and the depth contour may therefore be mis-located by a similar distance. In all these figures the data is processed so that grey shades of the image will correspond to definite levels of phytoplankton chlorophyll by

a

and phaeophytin pigment concentration as indicated

the grey wedge shown under Fig.

observed

as

being

above

a

given

3.

near

All

land and cloud,

infrared

brightness

9

threshold are masked to black so as to suppress grey tones for which this correspondence will certainly not apply. The form of the colour change being detected in these images is shown in Fig. 7 (NASA, 1 9 8 2 ) . Low concentrations of pigment will absorb blue light at wavelengths shorter than 500 nm, leading to a change from a blue to a bluelgreen colour for the water. At higher pigment concentrations backscatter from the associated cellular material in the water increases the radiance observed at longer wavelengths as indicated, leading to a yellow or brown colouration.

^E

10

S

I

I

I

I

I

I

I

I

I

I

I

I

I

I

i d

1

ti

\

3 2)

0. 1

(D

S .A

2

Qz m

0.01

0

4

5

0.001

LD

4 0

t 0.0001

-c c (

400 450 500 550 600 650 700 Wavelength (nm)

Fig. 7. Sea-water leaving radiance spectra chlorophyll 5 pigment concentrations. (NASA, 1982)

for

several

The algorithms used in the processing are accurate only in so called case 1 water where phytoplankton and their covarying detrital material play the dominant role in determining the This is true in optical properties (Morel and Gordon, 1980). open ocean and many coastal areas. In other areas (case 2 water) suspended material from a shallow bottom, or dissolved or suspended material from land will be important. In clear shallow water light reflected from the bottom will also form part of the optical signal.

10 8 shows a p a r t

Fig.

of

the Gulf

of

Mexico a n d Grand Bahama

Bank a r e a w h e r e m o s t o f t h e g r e y s h a d e s a r e d u e t o a d d e d r a d i a n c e where

pigment

error,

but

encountered

are

levels it

by

typically

near

instrument's

the

I n m o s t cases t h i s w i l l n o t be a s e v e r e s o u r c e

detection l i m i t . of

clear waters

from the ocean b o t t o m t h r o u g h t h e v e r y

reflected

indicates

an

ocean

the

colour

variety

of

optical

scanner.

In

problems case

this

an

algorithm t h a t i n t e r p r e t e d observed o p t i c a l radiances i n t e r m s of

water

depth

a n d bottom

reflectance

(Lyzenga 19811,

might

well

produce u s e f u l r e s u l t s .

Fig.

8.

CZCS p i g m e n t

tge

image showing shallow w a t e r a r e a s i n

G u l f o f Mexico a n d o n Grand B a h a m a Bank on D e c e m b e r 2 , 1 9 7 8 .

PROCESSING OF CZCS WATER COLOUR IMAGES Water c o l o u r d a t a i s c o l l e c t e d b y t h e CZCS i n 4 b a n d s 2 0 nm w i d e c e n t r e d a t 443 6 7 0 nm ( r e d ) .

(blue),

(blue/green),

520

550

(green)

and

A f u r t h e r t w o b a n d s a t 750 nm a n d 11 u m a r e u s e d

f o r m a s k i n g c l o u d or l a n d a n d f o r p r o v i d i n g s i m u l t a n e o u s t h e r m a l images

respectively.

l o w e r q u a l i t y t h a n t h e AVHRR, data

with

a

sufficiently

band

thermal

The

i n t e r m i t t e n t l y and ceased working

i n 1981.

which c a n ,

small

time

operated

i n principle,

difference

t h e r m a l c h a n n e l o n t h e CZCS of l i m i t e d u s e .

only

Its output w a s

to

of

provide

make

the

11 Pigment concentrations and attenuation coefficients are computed using algorithms based on observed correlations of these quantitites with upwelling radiances from the ocean in the blue and green spectral regions (Clark, 1981). To deduce these radiances from CZCS data, the outputs of the first three bands need to be corrected for atmospheric and surface effects. The fourth band at 670 nm is used in making this correction as described below. The processing of CZCS images as carried out by NASA provides two levels of output (Hovis et a1 1980, Hovis 1981). Level 1 gives a set of "quicklook" images of the data in each band recorded by the satellite, and level 2 gives images of: computed sub-surface radiances, corrected

for

atmospheric and

surface

effects: the aerosol signal at 670 nm: the phytoplankton pigment concentration: the diffuse attenuation coefficient and the thermal radiance where this band was operating. Grey levels on the level 2 images relate to quantitative values of all these variables.

Figs. 3-6 and 8 above are examples of level 2 pigment

images. A number of papers have been published describing improvements that have been made in arriving at the present process, the most recent being Gordon et a1 (19831. The first step is €0 convert the measured signals into radiance units. This step has been complicated by a degradation of the reflection of the Sensor tilt mirror while in orbit. This is not monitored by the on-board sensor calibrations, but can be accurately followed by its effects on the resulting data, and time dependent calibrations have now been derived. Modifying the calibration for each band in this way will also compensate for errors in the assumed solar

.

spectrum The major

computation in the processing is to remove the

signal due to Rayleigh scattering of sunlight in the atmosphere over the slightly reflecting ocean, with allowance for ozone absorption in the upper atmosphere. Gordon et a1 (1983) have found that a single scattering approximation works well, but the computation must be carried out for the rather complex geometry of the sensor scan, about an axis tilted to avoid sunglint, over a curved earth. Since the signal depends on the total of gases in the atmosphere, it can be predicted fairly accurately, giving a well defined problem easily handled by computer software. The signal varies smoothly across the scene and can be interpolated after relatively few computations.

12

The Rayleigh signal and upper atmosphere ozone concentration have a slight seasonal and latitudinal dependence that is allowed for in five possible steps. Variations in atmospheric pressure, and in the surface water reflection with wind and waves, including foam cover, cannot be compensated without more data. Some

correction

is

provided

in

the

next

stage

of

aerosol

correction. Aerosol scattering in the atmosphere adds a signal which is much more variable in intensity, but which has a smooth spectrum which can be reasonably well approximated by a power law. At 6 7 0 nm the water radiance becomes very small, and the remaining signal after Rayleigh correction can be used as a measure of the varying aerosol signal in the scene at this wavelength. Extrapolation to the wavelengths of other bands, however, requires a knowledge of the exponent of the power law spectrum. In retrospect, at least two bands, at 6 7 0 nm and at a longer wavelength, would have been useful for measuring this exponent at each pixel. The 7 5 0 nm band included in the CZCS is of low sensitivity and is not suitable for this purpose. However Gordon (1981) showed that this exponent was often constant over large areas and Gordon and Clark (1981) proposed the currently used method of determining it from one "clear water" point in the scene, and applying the resulting aerosol spectrum to the whole image. The method makes use of the fact that the upwelling radiance from case 1 water containing phytoplankton at a pigment concentration of less than 0 . 2 5 mg.m-3 and no other significant scattering material, will be close to fixed values at 5 2 0 and 5 5 0 nm and will be very low at 6 7 0 nm (see Fig. 7 ) . The mean aerosol spectrum power law deduced from these three wavelengths

can then be extrapolated to 443 nm. The resulting "aerosol" correction can contain contributions from improperly corrected Rayleigh radiance, surface foam and residual calibration errors, and

will

tend

to

reduce these

effects where conditions are the same as at the "clear water" point. The correction will be less perfect in other areas of the scene especially if the aerosol properties change. Errors will also exist wherever any suspended material raises the water leaving radiance at 6 7 0 nm, since the signal in this band is used to map the varying aerosol contribution whatever its spectrum. An iterative process in which deduced pigment concentrations are

used to estimate the 6 7 0 nm radiance due to higher concentrations

13

of phytoplankton was proposed by Smith and Wilson (19811, but this has not been implemented in the standard NASA process. The final stage of the atmospheric correction consists of computing the subsurface radiances that will give observed, corrected satellite radiances. This must allow for the facts that surface refraction reduces the signal from beneath the water by about half, and that Rayleigh scattering and ozone absorption attenuate the signal passing out through the atmosphere. These subsurface radiances are then used as inputs to the pigment and attenuation coefficient algorithms. Since these algorithms are based on observations in case 1 waters where optical properhies are determined by phytoplankton concentration only, the two outputs are in fact highly correlated. The algorithms are in the form of mean power law relations with ratios of subsurface radiances in bands 1 and 3, and 2 and 3 as given by

Gordon

et

a1

(1983) and

SASC

(1983).

These

two

documents give details of most of the above processes, with the exception of the time dependent calibration, which is still being refined, and the method used to automatically select clear water areas. The above processing system seems to work well in that images are produced in which atmospheric aerosol patterns are largely suppressed. A limited evaluation given by Gordon et a1 (1983) shows that pigment concentration estimates can be accurate to 530%. However this is for scenes showing large clear areas containing good “clear water” reference areas, and refers only to Shannon et a1 the pigment concentration range 0 to 1.5 mg.m-3. (1983) studying CZCS images of the relatively cloud free Southern Benguela current region (Fig. 4) find differences between ship and satellite chlorophyll 5 pigment estimates over the range 0.1 to 20 mg.m-3, of about a factor of 2 . In many areas the observations will need to be made in smaller clear areas among cloud. Here the existence of good clear water reference areas becomes particularly critical.

Gordon et a1 (1983) show that the

effect of only 0.27 ~ng.m-~ of pigment in the “clear water’’ area can lead to a factor of 2 error in deduced‘pigment for other areas. A drawback of the present processing system is that the position of the assumed clear water pixel is not recorded on the final data, so that users cannot easily assess possible errors. It must be emphasized that the present problem in making aerosol corrections is largely due to the present design of the

14

CZCS.

Morel and Gordon

(1980) proposed

an

improved set of

spectral bands, since refined in the MAREX report (NASA, 1982), which would greatly reduce this problem. The examples shown above demonstrate the value of the data. The MAREX report (NASA, 1982) suggests how an improved, follow-on sensor could be used in a large scale program of primary productivity mapping with applications in fisheries, climate studies and physical oceanography.

TECHNICAL IMPROVEMENTS POSSIBLE FOR SATELLITE WATER COLOUR MEASUREMENTS Improvements which can increase accuracy and coverage of satellite water colour data have been mentioned above and by Morel and Gordon (1980), NASA (1982) and SCOR (1983). Table 1 summarizes these proposals, several of which are being implemented on the next Ocean Color Imager due to be launched by the U . S . by about 1986 on one of the NOAA weather satellites. A further major problem found with the CZCS was in the complexity of the required data processing, and the resulting long delays before data became available. The problems now seem to have been overcome and the data backlog, in some cases extending back five years, is now being reduced. Technical developments in the field of integrating optics with solid state electronics have resulted in sensor arrays that can be used for remote sensing, either in a pushbroom mode (where a one-dimensional line of sensors looks at contiguous points along a line of view which is moved at right angles to the line by motion of the satellite) or in an imaging spectrometer mode (where a two-dimensional array of sensors operates as many pushbroom scanners, each at a different wavelength). Such sensor arrays offer high sensitivity and the possibility of observing in more, or more precisely chosen, spectral bands. A typical sensor array might have 300 by 300 elements, which would allow pushbroom imaging of a 15" field of view with an angular resolution comparable to the CZCS, and a spectral resolution of 1.5 nm in the wavelength range 400 to 850 nm. Several arrays would be required to cover the wider CZCS field of view. If the outputs from all elements were read and digitized at the rate required for satellite imaging (about 10 times per second) then the volume of data would be enormous (about 50 times

15

TABLE 1 Suggested improvements in satellite ocean colour imagers (OCI) ~

Technical requirement

Current action

Improve aerosol correction

add infrared bands

include in next OCI

Improve pigment characterization

add visible bands

include in next OCI

Improvement

I

Map smaller pigment changes

I

Improve area coverage

I

increase sensitivity

include in next OCI

add onboard processing to reduce data volume

include in next OCI

add colour sensors to geosynchronous satellites

proposed but not yet implemented

add band near 400 nm

proposed but not imp1ement ed

Map natural fluorescence

increase sensitivity and add special bands

flexible airborne sensor being constructed

that

CZCS).

-

Distinguish yellow substance from phytoplankton pigments

r

from

digitally

the

present

combined

into

The

outputs

predetermined

can

spectral

however bands

be

thus

reducing the data band width to that required by mechanical scanners, and giving much greater flexibility and precision in selection of the bands. The selection can be changed under software control, allowing a variety of specialized band combinations to be formed for mapping different target signatures. This type of sensor is particularly suitable for attempting the mapping of naturally stimulated phytoplankton pigment fluorescence as discussed in the next section.

16

MAPPING OF NATURALLY STIMULATED PHYTOPLANKTON, CHLOROPHYLL A FLUORESCENCE IN SEA WATER The broad band colour changes that are mapped by the CZCS are caused by a combination of absorption and backscattering of incident light by phytoplankton. The resulting colour changes are illustrated in Fig. 7 and are often adequately characterized by green to blue ratios deduced from measurements in CZCS bands. Another familiar feature of phytoplankton chlorophyll a pigments is their fluorescence, which for the most commonly occurring process leads to emission at 685 nm. A slight increase in the radiance at this wavelength, due to natural stimulation of this fluorescence by sunlight, can be seen for all four spectra plotted in Fig. 7, where the amount of this increase, above a smooth baseline, is roughly proportional to the chlorophyll concentrations listed. Use of this signal for airborne remote sensing surveys was first suggested by Neville and Gower (1977) and Gower (19801, and for satellite observations by Gower and Borstad (1981). The fluorescence signal has been found to be proportional to the chlorophyll concentration, though the value of the proportionality constant has been found to vary in the case 2 waters where most tests have been made (Fig. 9). Observations of naturally stimulated fluorescence have been used successfully in airborne surveys of the British Columbia coast (Borstad et al, 1980) and in the eastern Canadian Arctic (Borstad and Gower, 1983). Gower and Lin (1983) report a characteristic vector analysis of reflectance spectra for coastal waters for which fluorescence appears to provide superior estimates of pigment concentrations compared to the estimates derived from green to blue ratios. This analysis has been extended to examine variations in the fluorescence emission for different phytoplankton (Lin, et al, this volume). An 8 band ocean colour scanner with a band centred at 685 nm, 23 nm wide, was also flown on the Space Shuttle in 1981 (Kim et al, 1982). Other similar bands at 655 and 787 can be used to interpolate a baseline from which the radiance difference at 685 nm may be related to chlorophyll 5 fluorescence. Unfortunately, apart from the low sensitivity of the sensor and the non-optimal widths and positions of the bands, there were problems with weather and timing of this shuttle flight. The best scene of the limited resulting data set is shown in Fig. 10

17

20.0

Fi *

5

/

(3

3

15.0

.2

/

t

,/ .3

a -I -I

I

a 0

a

10.0

0 -I

...-

I

0

w

0

5.0

a

3 u)

0.0

FLUORESCENCE LINE HEIGHT

Fig. 9. Relations between naturally stimulated fluorescence (expressed as apparent reflectance increase at 685 nm x lo5) and phytoplankton pigment concentrations observed in surveys on the British Columbia coast in 1979 ( 1 and 2), 1981 (3 and 4) and 1976 (5). with uncorrected radiance at 685 nm

(top) and the calculated

radiance difference at 685 nm from the linear The scene shows parts of the Yellow Sea and off the mouth of the Yangtze River so that colour changes will be related to suspended

baseline (bottom). the East China Sea most of the water sediment. Some of

the brightening in the lower image, for example near the coast of Korea (top right), may be due to pigment fluorescence. The sensitivity is such that fluorescence due to a few mg.m-3 of pigment should be detectable. The lower scene is much less affected by the aerosol change near Cheja Island (centre) and by the strong limb brightening both visible on the top image. This data has not yet been processed using the techniques described above. Apart from its use as an estimator of chlorophyll 5 concentration, the

fluorescence

signal will

provide

another

tracer of water flow, or mixing patterns. Fig. 11 shows a variation in the observed fluorescence signal between spectra (A

18

Fig. 10. Images from the OCS experiment on the OSTA-1 Space Shuttle flight on November 13, 1981, showing the mouth of the Yangtze River (left) and southern Korea (top right). Uncorrected 685 nm band (top), partly processed fluorescence image (bottom). and

B)

taken

a

few

minutes

apart

in

Kiel

harbour

(Gower,

unpublished). Curve C is the difference plotted with 20 times more sensitivity. The proportional change at 550 nm is much smaller than that in the fluorescence signal. For airborne and satellite remote sensing the fluorescence signal has the advantages of a narrow band width, which distinguishes it from the variable, broad band signals due to aerosols or water surface effects, and a position at the red end of the optical spectrum where the Rayleigh scattered radiance is low.

Absorption of light by the atmosphere occurs at wavelengths

close to that of the fluorescence signal, particularly on the longer wavelength side where water vapour absorbs with varying strength from 690 to 745 nm and oxygen from 687 to 694 and from 760 to 770 (Fig. 12).

19

400

500

600

700

800

WAVELENGTH (nm)

Fig. 11. Water radiance spectra (A and B ) observed at two points in Kiel Harbour on April 26, 1982 from the deck of a ship. The difference ( C ) is plotted at 20 times the vertical scale. The right hand peak in curve C, interpreted as caused by a change in chlorophyll 5 fluorescence, can be fitted by a Gaussian centred at 682 nm with a half height width of 24 nm (residual shown dotted). Spectrometer resolution is 12 nm. Observing bands will need to be fitted between these features with the relatively high precision of a few nanometers. Measurement of the fluorescence signal will be by analysing the radiance spectrum shape in the range 660 to 690 nm supplemented by measurements in the window at 745 to 760 nm, or in the almost transparent window at 708 to 714 (Fig. 12) to remove the smoother shape of the background radiance. Although such observations could be made with a

specially

configured mechanical scanner, an array sensor such as described above provides greater sensitivity and flexibility. Such a sensor, the Fluorescence Line Imager (FLI), is now being built as part of the remote sensing program of the Canadian Department of Fisheries and Oceans. This is an airborne prototype imaging

20

Fig. 12. Atmospheric optical depths between 500 and 8 5 0 nm due to absorption by oxygen and water vapour. Note the expanded vertical scale which shows faint features especially at wavelengths shorter than 680 nm.

spectrometer whose properties are listed briefly in Table 2 . Figure 13 shows the sensor head with four of its five cameras, which will together cover a 7 0 " field of view. Fig. 14 shows the layout of one of the cameras in which light is dispersed by a transmission grating and focussed onto the CCD array on the left side. Some of the readout electronics is also visible. Computer control will allow spectral band specification and will perform the processing needed to form these bands by signal summation. A real-time output is available for display of a mathematical combination of different bands. Flight programs are being planned to test use of this sensor over a variety of targets. Although the instrument was designed specifically for water colour observations, its parameters make it ideal for other remote sensing studies, for example in the fields of agriculture, forestry, geology and atmospheric sciences and for simulating the spectral responses of other optical imagers. Scientists interested in joint observing programs should contact the author.

21

Fig. 13. The sensor head of the Fluorescence Line Imager (FLI), being built for the Canadian Department of Fisheries and Oceans, with four of the five CCD cameras in position.

Fig. 14. One of the F L I cameras with covers removed, showing the layout of the optics and some of the digitizing electronics.

22

TABLE 2 Properties of Fluorescence Line Imager (FLI) Size of arrays used Number of arrays Total field of view Total number of pixels Total number of spectral elements Spectral coverage Spectral resolution Number of bands Location and width of bands Digitization Signal to noise Scan rate

385 x 288 5 70" 1925 288 410 to 850 nm 2 nm 8 under software control to 1.5 nm 12 bits 2000:l for a 30 nm band 10 per second

CONCLUSIONS

Processed

CZCS

imagery demonstrates the potential of ocean

colour imaging from space for physical as well as biological oceanography. Improved sensors should lead to more results, covering wider areas with greater regularity.

precise Imaging

of natural fluorescence also appears possible and should lead to further improvements.

23

REFERENCES Borstad, G.A., Brown, R.M., and Gower, J.F.R., 1980. Airborne remote sensing of sea surface chlorophyll and temperature along the outer British Columbia coast. Proceedings 6th Canadian Symposium on Remote Sensing, Halifax, N.S., May, pp. 541-549. Borstad, G.A. and Gower, J.F.R., 1983. Ship and aircraft measurements of phytoplankton chlorophyll distribution in the eastern Canadian Arctic. Arctic, in press. Clark, D.K., 1981. Phytoplankton pigment algorithms for the NIMBUS-7 CZCS. In: J.F.R. Gower (Editor), Oceanography from Space. Plenum Press, Marine Science, 13: 227-238. New York. Darwin, C.R., 1845. The voyage of the Beagle, 2nd Ed., Everyman Library Paperback, Dent, London. Gordon, H.R., 1981. A preliminary assessment of the NIMBUS-7 CZCS atmospheric correction algorithm in a horizontally inhomogeneous atmosphere. In: J.F.R. Gower (Editor), Oceanography from Space, Marine Science 13: 257-265. Plenum Press, New York. Gordon, H.R. and Clark, D.K., 1981. Clear water radiances for atmospheric correction of coastal zone color scanner imagery. Applied Optics, 20: 4175-4180. Gordon, H.R., Clark, D.K., Brown, J.W., Brown, O.B., Evans, R.H. and Broenkow, 1983. Phytoplankton pigment concentrations in the Middle Atlantic Bight: Comparison of ship determinations and CZCS estimates. Applied Optics, 22: 20-37. Gower, J.F.R., 1980. Observations of in situ fluorescence of chlorophyll 5 in Saanich Inlet. Boundary Layer Meteorology, 18: 235-245. Gower, J.F.R., Denman, K.L. and Holyer, R.J., 1980. Phytoplankton patchiness indicates the fluctuation spectrum of mesoscale turbulence. Nature, 288: 157-159. Gower, J.F.R. and Borstad, G.A., 1981. Use of the in vivo fluorescence line at 685 nm for remote sensing surveys of surface chlorophyll a. In: J.F.R. Gower (Editor), Oceanography from Space, Marine Science, 13: 329-338. Plenum Press, New York. Gower, J.F.R. and Lin, S., 1983. The information content of different optical spectral ranges for remote chlorophyll estimation in coastal waters, International -Journal of Remote Sensing. In press. Hovis, W.A., Clark, D.K., Anderson, F., Austin, R.W., Wilson, W.H., Baker, E.J., Ball, D., Gordon, H.R., Mueller, J.L., El-Sayed, S.Z., Sturm, B., Wrigley, R.C., and Yentsch, C.S., 1980. NIMBUS 7 Coastal Zone Color Scanner: System description and initial imagery. Science, 210: 60-63. Hovis, W.A., 1981. The NIMBUS 7 Coastal Zone Color Scanner (CZCS) program. In: J.F.R. Gower (Editor). Oceanography from Space, Marine Science, 30: 213-225. Plenum Press, New York. Kim, H.H., Hart, W.D. and van der Piepen, H., 1982. Initial analysis of OSTA-1 Ocean Color Experiment Imagery. Science, 218: 1027-1031. and Gower, J.F.R., 1980. LaViolette, P.E., Peteherych, S . Boundary Layer Meteorology, 18: 159-175. Lyzenka, D.R., 1981. Remote sensing of bottom reflectance and water attenuation parameters in shallow water using aircraft and Landsat data, International Journal of Remote Sensing, 2: 71-82.

24

Morel, A.Y. and Gordon, H.R., 1980. Report of the Working Group on Ocean Color. Boundary Layer Meteorology, 18: 343-355. NASA, 1982. The Marine Resources Experiment Program (MAREX) Report of the Ocean Color Science Working Group. Goddard Flight Center, R. Kirk (Coordinator). Neville, R.A. and Gower, J.F.R., 1977. Passive remote sensing of phytoplankton via chloropyll 5 fluorescence. Journal of Geophysical Research, 82: 3487-3493. Richardson, P.L., 1981. Gulf Stream trajectories measured with free-drifting buoys. Journal of Physical Oceanography, 11: 999-1010. Royer, T.C., 1981. Baroclinic Transport in the Gulf of Alaska Part I. Seasonal Variations of the Alaska Current. Journal of Marine Research, 39: 239-250. SASC, 1983. NIMBUS 7 CZCS derived products scientific algorithm description. Report no. EAC-7-8085-0027. Systems and Sciences Corporation, Hyattsville, MD., USA. SCOR, 1983. Remote Measurement of the Oceans from Satellites. Scientific Committee on Oceanic Research,.Workinq Group 70 report, in preparation. Shannon, L.V., Mostert, S.A., Walters, N.M. and Anderson, F.P., 1983. Chlorophyll concentrations in the Southern Benguela current region as determined by satellite (Nimbus 7 Coastal Zone Color Scanner). Journal of Plankton Research, 5: 565-583. Smith, R.C. and Wilson, W.H., 1981. Ship and satellite bio-optical research in the California Bight. In: J.F.R. Gower (Editor), Oceanography from Space, Marine Science, 13: 281-294. Plenum Press, New York. Thomson, R.E., 1972. On the Alaskan Stream. Journal of Physical Oceanography, 2: 363-371. Wright, C., 1981. Observations in the Alaskan Stream during 1980. NOAA Technical Memorandum ERL. PMEL-23.

26

CONTRIBUTION OF REMOTE SENSING TO MODELLING Jacques C.J. NIHOUL GHER, University of LiBge, Belgium

1. Application of remote sensing to the identification of processes

and structures and to the formulation of mathematical models of the marine system. One of the most decisive contribution of remote sensing has been the supplying, for the first time, of synoptic views of large sea areas and the identification of mesoscale and macroscale horizontal structures which had been overlooked in field studies and ignored in mathematical models. Digital image analysis of Landsat data has revealed, for instance, the penetration in the Harima Sea (Japan) of a pair of large scale vortices formed by amalgation of two series of coherent vortices, produced in the free boundary layers in the wakes of the Naruto Straits'Capes. The vortex pair, apparently carried along by the tidal currents in a first stage, was found to continue penetrating into the Harima Sea, after tide reversal, under self-induced driving forces (Maruyasu et al., 1 9 8 1 ) . This mechanism which plays a cogent role in local mixing could not have been identified without synoptic remote sensing views of the Set0 Inland Sea. NOAA 6 images of the Western Mediterranean have shown complicated seasonal circulation patterns, - including eddies, planetary solitons, upwellings, fronts, water intrusions, coastal currents which could not have been apprehended by restricted experimental

-

investigations (e.g. Philippe and Harang, 1 9 8 2 , Preller and Hulburt, 1 9 8 2 ) (fig. 1). The meandering of large scale currents like the Gulf Stream and the subsequent shedding of synoptic eddies has never been properly perceived and understood until remote sensing images of the area were available (e.g. Behie and Cornillon, 1 9 8 1 ) .

26

F i g . 1. c h a r t of s p r i n g s u r f a c e t e m p e r a t u r e f r o n t s i n t h e A d r i a t i c Sea (23-29 A p r i l , 1982) communicated by Lannion C e n t e r . General legend f o r f i g u r e s 1 t o 4 . Mean p o s i t i o n of a t h e r m a l f r o n t (AT 2 1 ° C ) p e r s i s t i n g t h e whole week; w a r m water on t h e d a s h e d s i d e .

7'7 ///

O c c a s i o n a l t h e r m a l f r o n t (AT 2 1 " C ) w i t h t h e d a t e o f observa t i o n . Permanent t h e r m a l b o r d e r w i t h o u t marked f r o n t a l f e a t u r e s .

ri"rOd actcea soi fo noabl stehrevramt iaol n bi on rddi ec ra t ewdi.t h o u t marked Up

,--.Upwelling

$1TC :: W a r m - _ _
-.-.-< a

80

55 50

- 80

Radar draction = 302' dlstonce = 26 km

\

\

Time after high water at Brest F i g . 13. Turbulence measurement. Dependence on t h e range o f t i d e .

The t u r b u l e n c e i s seen t o be connected t o t h e c u r r e n t v e l o c i t y , i . e . t o i t s v a r i a t i o n i n t h e s e m i d i u r n a l t i d e p e r i o d , and t o t h e t i d e range. When somebody measures t h e c u r r e n t

versus t i m e a t a p o i n t i n t h e sea, he

c o n s i d e r s as normal v a r i a t i o n s o f t h i s c u r r e n t o f 10%. Here, t h i s v a l u e l e a d s t o 13 cm/s f o r t h e maximum c u r r e n t w i t h t h e maximum t i d e range ( a p p r o x i m a t e l y t h e c o e f f i c i e n t 1 1 0 ) . The r e s u l t s o b t a i n e d here a r e more t h a n two t i m e s h i g h e r . The d i s c u s s i o n o f why t h e t u r b u l e n c e i s so h i g h i n t h a t area i s n o t t h e purpose o f t h i s paper. We may suppose i t i s a c h a r a c t e r i s t i c o f t h a t area, b u t a l s o t h a t s p a t i a l measurement o f t h e t u r b u l e n c e i s n o t u s u a l . Everybody knows t h a t ocean i c d i f f u s i o n i s o f t e n h i g h e r t h a n d i f f u s i o n c a l c u l a t e d w i t h s i m p l e models. So turbulent dispersion i s usually calculated w i t h empirical coefficients. 6 . DISCUSSION

T h i s s e c t i o n argues about t h e r e s u l t s t o show t h a t no a r t e f a c t was found w h i c h would have been a b l e t o g i v e such w i d e Bragg l i n e s , e x c e p t f o r t u r b u l e n c e . Oceanographic remote s e n s i n g i s a new s c i e n c e . The sensors used i n t r o d u c e

273 new concepts, and i t i s n o t easy t o connect t h e i r measurements w i t h more convent i o n a l ones. I t i s always necessary t o s t u d y t h e i r r e s u l t s c a r e f u l l y i n o r d e r t o e l i m i n a t e e v e n t u a l a r t e f a c t s which may damage t h e r e s u l t s . I n t h i s s e c t i o n we w i l l r e v i e w t h e p o s s i b l e phenomena w h i c h may produce t h e broadening o f t h e Bragg l i n e s .

6.1.

The s i d e l o b e s o f t h e r e c e p t i o n antenna

The r e c e p t i o n antenna i s composed o f s i x t e e n s i m p l e o m n i d i r e c t i o n a l antennas. The f i r s t s i d e l o b e i s

-

16 dB below t h e main l o b e . There i s a n a t u r a l p r o t e c -

t i o n a g a i n s t l o b e s a t more t h a n 90" f r o m t h e r a d a r beam by more t h a n 10 km o f land. An a t t e m p t was made t o s t u d y t h e importance of s i d e l o b e s . We chose a p e r i o d when t h e wind was up t o t h e r a d a r . I n t h i s case t h e e f f e c t o f s i d e l o b e s i s below t h e l i m i t o f

-

I0 dB which i s used. There i s no d i f f e r e n c e between these

r e s u l t s and t h o s e o b t a i n e d i n o t h e r w i n d d i r e c t i o n s . Another i m p o r t a n t i t e m i s t h a t t h e mean c u r r e n t i s e f f e c t i v e l y compared t o t h e t i d e stream, and t h a t even i n upwind c o n d i t i o n s , t h e mean c u r r e n t i s t h e same f o r t h e n e g a t i v e and p o s i t i v e Bragg l i n e s . The shape o f t h e Bragg l i n e s i s o f t e n f l a t l i k e i n F i g u r e 4. S i d e l o b e s would have produced o t h e r shapes, w i t h o u t any f l a t c e n t r a l p a r t . F i n a l l y , t h e r e i s no d i f f e r e n c e between t h e two f r e q u e n c i e s 7 and 14

MHz.

Perhaps t h e l o b e s a r e t o t a l l y d i f f e r e n t . So we may say t h a t t h e r e s u l t s a r e n o t due t o t h e r a d a r c h a r a c t e r i s t i c s and

t h a t a l l t h e s i g n a l s came from t h e s e l e c t e d r e s o l u t i o n c e l l .

6.2.

The e f f e c t o f t h e wind

We t r i e d t o f i n d a c o r r e l a t i o n between t h e w i d t h o f t h e Bragg l i n e s and t h e wind speed i n t h e area. The w i n d was o f t e n v e r y l o w . O b v i o u s l y no c o r r e l a t i o n was found i n t h i s case. B u t even w i t h s t r o n g winds, we o b t a i n e d t h e same r e s u l t . We suppose t h a t t h e w i n d i s r e s p o n s i b l e f o r some d i s p e r s i o n i n t h e r e s u l t s f o r t h e mean c u r r e n t . B u t s i m p l e c o r r e c t i o n s which were s u c c e s s f u l f o r t h e Marsen experiment 161, were n o t i n t h i s one. Stokes d r i f t i s u s u a l l y low, much l o w e r t h a n t h e r e s u l t s o b t a i n e d f o r u

1 9 1. W i t t e [ l o 1 found t h a t d i s p e r s i o n was a l o t h i g h e r t h a n t h a t e x p l a i n e d by t h e Stokes d r i f t . S i n c e t h e r e i s a s t r o n g w a v e - c u r r e n t i n t e r a c t i o n i n some p a r t s o f t h e a r e a c l o s e r t o t h e c o a s t [111, we s t u d i e d i t s e f f e c t on t h e Doppler spectrum. I n t h e case o f c r o s s w i n d c o n d i t i o n s , t h e Bragg l i n e s would be symmetrical about a frequency n e a r z e r o . B u t t h e shapes observed a r e t h e e f f e c t o f a 2 fB t r a n s l a t i o n o f one Bragg l i n e on t h e o t h e r , i n s t e a d o f a symmetry.

274

Other f e a t u r e s The u value d o e s n ' t depend on the time s c a l e . Measurements were made f o r 3 and 20 minuts. The r e s u l t s a r e already the same. So we a r e not measuring basic a l l y time-varying process. Since the r e s u l t s a r e independent of the radar frequency, they d o n ' t depend on the current v a r i a t i o n with depth i n t h e f i r s t meters. This i s not s u r p r i s i n g f o r a turbulence on t h e s c a l e of kilometers. 6.3

Finally, t h e r e a r e a few v a r i a t i o n s of 0 with the distance from the c o a s t . of t h e c e l l . So t h e r e i s no B u t the f a r t h e r we go the g r e a t e r i s t h e width d e f i n i t e conclusion on t h a t point. CONCLUSION The HF narrow-beam radar may measure a turbulence c h a r a c t e r i s t i c , i n addit i o n t o wind d i r e c t i o n , long-wave directional s p e c t r a , t o t a l wave height and current speed. This turbulence value, which i s the s p a t i a l standard deviation of t h e current speed, i s not a c l a s s i c one. I t s values a r e surprinsingly high o f f Ushant Island. I t i s well known t h a t t h e wind e f f e c t on current i n t h a t area i s very g r e a t ... and d i f f i c u l t t o c a l c u l a t e . We may assume t h a t some variat i o n s of current with time have been supposed t o proceed from t h e wind e f f e c t , and a r e in f a c t due t o turbulence on t h e space s c a l e of 5 t o 10 km.

REFERENCES [l] Broche, P . ,

121 131 [4]

151 161 171 [8] [9]

1979. Sea s t a t e directional spectra observed by HF Doppler radar. Agard Conf. Proc., 263, 31.1-31.12. Parent, J . and Delloue, J . , 1982. Determination de l a d i r e c t i o n d u vent a l a surface de l a mer au moyen d ' u n radar a r e t r o d i f f u s i o n ionospherique. Ann. Geophys., t . 38, f a s c . 6, pp.863-873. Gay, H., Blanchet, C . , Nicolas, J . and Piau, P . , 1982. Determination of wind and s h o r t wave d i r e c t i o n a t g r e a t distances with OSUREM r a d a r . In Wave and Wind D i r e c t i o n a l i t y . Ed. Technip, P a r i s . Forget, P . , Broche, P . , De Maistre, J.C. and Fontanel, A . , 1981. Sea s t a t e frequency f e a t u r e s observed by ground wave HF Doppler r a d a r . Radio Science, Vol. 16, No 5, pp.917-925. Lipa, B . and Barrick D . , 1982. Codar measurements of ocean surface parameters a t ARSLOE. Preliminary r e s u l t s . Oceans 82. Janopaul, M.M. e t a1 ., 1982. Comparison of measurements of sea currents by HF radar and by conventional means. I n t . 3 . Remote Sensing, vol. 3, NO 4, p p . 409-422. Barrick, D. and Snider, J . B . , 1977. The s t a t i s t i c s of HF sea-echo Doppler s p e c t r a . I . E . E . E . Trans. on Antennas and Propagation, vol. AP-25, No 1. Service Hydrographique e t Oceanographique de l a Marine, 1968. Tome No. 550. Courants de maree dans l a Manche e t s u r l e s c6tes franGaises de 1 'Atlantique. Broche, P . , de Maistre, J . C . and Forget, P . , 1983. Mesure par radar decametrique coherent des courant5 s u p e r f i c i e l s engendres par l e vent. Oceanologica Acta, Vol. 6, n o 1.

275

[lo] Witte, H . e t al.,

1982. Small s c a l e d i s p e r s i o n measurements o f d r i f t e r buoys i n t h e N o r t h Sea. F i r s t i n t . cong. on m e t e o r o l o g y and a i r / s e a i n t e r a c t i o n o f t h e c o a s t a l zone. The Hague, May 10-14. I l l ] Cavani@, . A . , E z r a t y , R. and G o u i l l o n , J . P . , 1982. T i d a l c u r r e n t modulat i o n s o f wave d i r e c t i o n a l s p e c t r a parameters measured w i t h a p i t c h and r o l l buoy west o f Ushant i n w i n t e r , F i r s t i n t e r n a t i o n a l conference on m e t e o r o l o g y and a i r / s e a i n t e r a c t i o n o f t h e c o a s t a l zone. The Hague. May 10-14.

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277

A Q U A S I GEOSTROPHIC MODEL OF THE CIRCULATION OF THE MEDITERRANEAN S E A

Laurent LOTH (*) and Michel C R E P O N (**)

(*) I N R I A

-

Domaine de Voluceau - Rocquencourt - B.P. 1 0 5 - 78150 - L E CHESNAY -

France

(**) Laboratoire d'Oc6anographie Physique - Museum National d' Histoire Naturelle LA175

-

CNRS

-

-

43 Rue Cuvier - 75005 P A R I S - France

Abstract A quasi geostrophic model o f t h e Mediterranean sea i s solved b y using a f i n i t e element

method.

The barotropic

and baroclinic

mode are computed independently.

The Alboran Sea gyre i s observed i n both models but it i s less intense than i n nature. When penetrating t h e Mediterranean sea t h e Alboran sea current overshoots t o t h e North, then becomes trapped by t h e Algerian shore.

1.INTR 0 D U CTIO N The Mediterranean Sea is a concentration basin. Evaporation creates a mass d e f i c i t i n t h e whole basin which i s compensated by an inflow o f Atlantic water passing through the s t r a i t o f Gibraltar and through t h e s t r a i t o f Sardinia. The incoming A t l a n t i c water which i s l i g h t i s t r a n f o r m e d i n t o dense water by a complicated convective process (Gascard

-

1978). This dense water f o r m s a deep l a y e r which f l o w s out i n t o t h e A t l a n t i c

ocean.These fluxes strongly influence t h e general circulation of t h e sea. From a schemat i c "point o f view",

t h e Mediterranean sea can be considered as a , t w o l a y e r ocean.

In t h e subsequent we focus our i n t e r e s t on t h e barotropic and baroclinic circulation

o f t h e western basin f o r c e d b y t h e fluxes through t h e t w o straits.

2. T H E MODEL Since we are interested i n low frequency phenomena we deal with t h e quasi geostrophic version o f t h e shallow water equations. The governing equations are, i n a coordinate f r a m e with x positive east-ward and y positive northward.

where Y

i s t h e stream function (u=-%;,

v=E)

R i s t h e i n t e r n a l Rossby radius o f deformation. (1/R2 i s set equal t o zero t o obtain t h e barotropic mode) 5 t h e variation r a t e o f t h e Coriolis parameter P

( 5 = 2.10- 11s -1m -1)

t h e density

D t h e depth A t h e horizontal turbulent viscosity c o e f f i c i e n t E

t h e bottom f r i c t i o n parameter (

E

=

( A = 512.m2s-')

5.11)-~s-l)

We only study t h e motion generated by fluxes o f water f l o w i n g through t h e straits o f Gibraltar and Sardinia. I n t h i s study t h e f o r c i n g due t o t h e wind i s neglected. I n order t o satisfy t h e mass continuity (Pedlosky, 1979) it can be shown t h a t

Thus, t o solve (1) subject t o (2) we l e t (Holland, 1978) YJ = Y o + c(t) Y

where Y

1

i s a solution o f

a(2,

-+)

at

t h a t t h e t i m e independent f i e l d Y function Y

= 0 with

Yl

=

1 on boundaries. Note

needs t o be determined only once. The Stream

i s a solution o f (l), w i t h Y

= Y

0

= 0 on t h e south boundary ( A f r i c a n

- y o N i s equal t o t h e f l u x o f water f l o w i n g through t h e strait, with Coast) and Y N yo = Y on t h e northern boundary (European coast). Now, condition (2) determines c(t) a t each instant, i-e.

279

3. M E T H O D O F SOLUTION I n order t o approximate t h e coastline geometry as closely as possible, we have chosen a numerical f i n i t e element approach, using a triangular grid (Fig. 1). The model i s f o r c e d by imposing velocity profiles a t t h e t w o straits, t h e fluxes o f which are equal.

We s t a r t f r o m r e s t a t t = 0 and t h e t w o fluxes reach a constant

value i n one month. A t each strait, t h e boundary condition i s imposed a t t h e end o f a canal t h e length o f which i s f o u r grid size. This allows t h e f l u i d t o adjust i t s e l f before entering t h e sea and prevents unrealistic forcings i n t h e basin. The t i m e discretization i s a leap f r o g scheme with a Matsuno scheme every nine steps. The f i n i t e elements are interpolated by linear functions (Dumas e t a1

-

-

1982, Dumas

1982).

The bottom i s assumed t o be f l a t . The barotropic and baroclinic modes are solved separatly (1/R2 = 0 f o r t h e barotropic mode i n 1). This implies t h a t baroclinic unstabil i t y i s n o t taken i n t o account. The depth o f t h e upper layer i s 200 m and t h e reduced

g r a v i t y parameter g' i s 10-Zms-2 (g'=g AP / p ) i.e t h e i n t e r n a l radius o f deformation R i s equal t o 40 km. This value i s l a r g e r than t h e actual one, b u t allows us t o deal w i t h a minimum number o f triangles and t o respect t h e dynamical constraints between t h e i n t e r n a l radius o f deformation and t h e grid size which i s taken about h a l f o f t h i s value i.e. 20 km. A t t h e coast a f r e e slip condition i s used.

Fig. 1 : F i n i t e elements grid used f o r t h e Mediterranean sea.

280

Many runs were performed i n order t o check t h e sensitivity o f t h e model t o t h e width o f t h e s t r a i t o f Gibraltar, t o t h e velocity profile, t o t h e magnitude o f t h e incoming f l u x e and t o t h e eddy viscosity coefficient. The f i n a l runs were done with r e a l i s t i c parameters. The width o f t h e s t r a i t o f Gibraltar (Sardinia) was 20 km (160 km).

According t o Lacombe and Richez

t h e incoming (and out-going) f l u x was 0.32

Sverdrup f o r t h e barotropic

-

1982

-,

model and

1.6 Sverdrup f o r t h e baroclinic one. The viscosity c o e f f i c i e n t A was 512 m2s-'.

In

t h e s t r a i t o f Gibraltar t h e grid size i s 10 km. This makes it possible t o vary t h e velocit y p r o f i l e o f t h e forcing.

I n t h e following runs assym metric parabolic profiles are

used (Fig. 3). Equilibrium i s reached a f t e r 400 days i n t h e barotropic case, a f t e r 700 days i n t h e baroclinic case. The t i m e step i s 4 hours.

4. RESULTS I n both runs, t h e main c u r r e n t i s deviated t o t h e northern coast o f t h e Alboran sea (Fig. 2 and 4). A p a r t o f t h i s f l o w i s recycled southward and f o r m s one o r t w o anticyclonic eddies. I n t h e barotropic case one observes t h e f o r m a t i o n o f t w o weak anticyclonic eddies which are separated a t t h e l e v e l o f Cape Tres Forcas (Fig. 3). I n t h e baroclinic case, there is one strong anticyclonic eddy which extends through t h e whole sea (Fig. 5, 6). When penetrating t h e Mediterranean sea, t h e c u r r e n t overshoots t o North. This overshooting could be responsible f o r t h e f r o n t which extends between t h e Balearic Islands and Sardinia and which i s o f t e n observed on i n f r a - r e d s a t e l l i t e images (Fig. 7) (Deschamps e t a1

-

1984). Then t h e c u r r e n t bends southward

and f l o w s along t h e Algerian coast. The p a t t e r n o f t h e circulation i n t h e Alboran sea i s strongly dependent on t h e v o r t i c i t y o f t h e forcing. If t h e v o r t i c i t y i s positive, t h e Alboran gyre is enhanced, if t h e v o r t i c i t y i s negative, t h e main c u r r e n t i s n o t any more deviated t o t h e coast

o f Spain b u t f o l l o w s t h e coast o f Morocco and t h e gyre disappears. Following Holland (1978), several length scales o f i n t e r e s t are defined Wi = ( u / ~)'I2 = 70 km W

S

= E / B = 10 km

m = 2(+'13

=

60 km

1

where u i s a t y p i c a l velocity ( u = 0.1 ms- ) The length scales Wi,

W s and W m are respectively t h e width o f t h e western boun-

d a r y c u r r e n t when i n e r t i a l e f f e c t s dominate and when bottom f r i c t i o n dominates and when l a t e r a l f r i c t i o n dominates. These values show t h a t t h e circulation i n t h e Alboran sea i s strongly dependent on i n e r t i a l e f f e c t and l a t e r a l f r i c t i o n .

Thus, an analysis

o f t h e motion i n t e r m s o f v o r t i c i t y balance must include t h e f r i c t i o n term.

281

Fig.2. equal t o 0.32

I .\

Stream lines o f t h e barotropic Sverdrup.

model. The fluxes a t t h e straits are

The distance between t w o stream lines i s 0.032

Sverdrup.

m e t r i c parabolic

n the Strait

Fig.3.

Barotropic model. Enhanced p i c t u r e o f t h e stream lines i n t h e Alboran

sea. The distance between t w o stream lines i s 0 . 0 0 3 Sverdrup. The velocity p r o f i l e o f t h e f o r c i n g i n t h e Gibraltar s t r a i t i s shown i n t h e upper l e f t corner.

282

5 O

Fig.4.

O0

Stream lines o f t h e baroclinic

5 E

model. The fluxes a t t h e straits are

equal t o 1.6 Sverdrup. The distance between t w o stream lines i s 0.16 Sverdrup.

Fig.5. sea.

Baroclinic model. Enhanced p i c t u r e o f t h e stream lines i n t h e Alboran

The distance between t w o stream lines i s 0.05

of t h e f o r c i n g i s t h e same as i n Fig.3.

Sverdrup.

The velocity profile

283

ALGERIA

Fig.6. Baroclinic model. Velocity vectors in the Alboran sea.

Fig.7. Pattern of the thermal front between the Balearic Islands and Sardinia a t different months of the year 1978 observed from N O A A satellite (from Deschamps e t al. 1984) - (5 is May, 6 June, 7 July, 8 August).

284

5. CONCLUSION The f i n i t e element technics i s a valuable t o o l t o study t h e mediterranean circulation. The model supports r e a l i s t i c i n f l o w s o f A t l a n t i c water passing through t h e s t r a i t o f Gibraltar. Despite t h e over-simplification o f t h e model, many features o f t h e circulat i o n as t h e Alboran gyre (Lanoix - 1974) are reproduced. But it i s noted t h a t t h e circulat i o n i n t h e northern p a r t o f t h e Basin i s not obtained. I n particular, t h e strong cyclonic gyre (Crepon e t al. 1982) existing between France and Corsica i s n o t observed. The n e x t stage i s t o include t h e wind stress and t h e bottom topography by dealing with a t w o layer model which can t r i g g e r t h e Alboran sea gyre more intensely. The computations were done on t h e Cray 1 o f French Research.

A C K N 0 W L E D G E M E N TS This work was supported by f r e n c h c o n t r a t D r e t N081/1117 and by C N R S and C N E X O . The f i n i t e element model was k i n d l y provided by C. L e Provost. Discussion with P.

Delecluse and J.C.

Gascard,

C.

M i l l o t have been very helpful1 throughout

t h i s work.

REFER E N C ES Wald L. and Monget J.M. - 1982 - L o w frequency waves i n t h e Ligurian

Crepon M.,

sea during December 1977. J.G.R Oeschamps P.Y.,

Vol. 82 C 1 pp 595-600.

Frouin R. and Crepon M. - 1984

-

Sea surface temperature o f t h e

coastal zones o f France observed by t h e H C M M satellite - J.G.R ( i n Press) Dumas E. - 1982 - Modelisation des circulations oceaniques ge'nerales par des mgthodes aux 6 E m e n t s finis. These Dumas E.,

-

University o f Grenoble - June 1982.

L e Provost C. and Poncet A.

-

1982 - Feasibility o f f i n i t e element methods

f o r oceanic general circulation modeling. I n Proc. o f 4th. Int. Conf. on f i n i t e elements i n Water Res. H A N O V E R - 1982. Gascard

J.C.

-

1978 -

Mediterranean

deep

water f o r m a t i o n ; baroclinic

and ocean eddies - Oceanologica Acta - Vol. 1, NO3

- pp.

315-330.

instability

285

Holland B. ocean; VOl.

-

1978

numerical

-

The r o l e of Experiment

mesoscale eddies i n t h e general circulation of t h e

Using a wind-driven

quasi geostrophic

model.

J.P.O.

8 NO3 pp. 363-392.

Lacombe ti. and Richez C.

-

1982 - The regime o f t h e s t r a i t o f Gibraltar i n hydrody-

namics o f semi-enclosed seas. I n hydrodynamics o f semi-enclosed seas by J.C.J

Nihoul

(Editor), Elsevier, Amsterdam, pp. 13-73. Lanoix F. - 1974 - Project Alboran

-

Etude hydrologique e t dynamique de l a mer d'Albo-

r a n - N A T O technical r e p o r t 66, 39 p. Pedlosky J. - 1979 - Geophysical Fluid Dynamics - Springer Verlag - 624 p.

This Page Intentionally Left Blank

287

SOME APPLICATIONS OF REMOTE SENSING TO STUDIES I N THE BAY O F BISCAY, CELTIC SEA AND ENGLISH CHANNEL R.D.

PINGREE

I n s t i t u t e of Oceanographic S c i e n c e s , Wormley, S u r r e y , GU8 5UB, England

ABSTRACT Infra-red,

C o a s t a l Zone Colour Scanner and S y n t h e t i c A p e r t u r e

Radar images have been used t o i d e n t i f y s e a s u r f a c e s t r u c t u r e s i n t h e B i s c a y , C e l t i c Sea and E n g l i s h Channel r e g i o n s .

Attention

h a s been f o c u s s e d on s h e l f - b r e a k c o o l i n g , s h e l f - b r e a k c h l o r o p h y l l ' a ' , Biscay e d d i e s , i n t e r n a l waves and t u r b i d i t y s t r u c t u r e s i n t h e E n g l i s h Channel and extended where p o s s i b l e w i t h examples drawn from work a t s e a .

INTRODUCTION

One of t h e most i m p o r t a n t c o n t r i b u t i o n s of remote s e n s i n g t o oceanographic and s h e l f s t u d i e s i n t h e B i s c a y , C e l t i c Sea and E n g l i s h Channel h a s been t o p r o v i d e c l e a r i l l u s t r a t i o n s of a v a r i e t y of p h y s i c a l and b i o l o g i c a l phenomena.

With a c l e a r

p i c t u r e i n mind of t h e p r o c e s s and i t s g e o g r a p h i c a l limits it becomes a r e l a t i v e l y s i m p l e m a t t e r t o i n v e s t i g a t e t h e p r o c e s s e s f u r t h e r w i t h measurements a t s e a .

For example t h e r e were no

r e p o r t s of t h e e x t e n s i v e s h e l f - b r e a k c o o l i n g i n t h i s a r e a u n t i l it had been f i r s t n o t e d i n t h e i n f r a - r e d s a t e l l i t e imagery.

The

widespread o c c u r r e n c e and p e r s i s t e n c e of t h e s h e l f - b r e a k c o o l i n g s t i m u l a t e d models of b o t h t h e M2 b a r o t r o p i c t i d a l c u r r e n t s and t h e i n t e r n a l t i d e f o r t h i s area.

I n t h i s paper some examples of

t h e k i n d s of s t r u c t u r e s t h a t can be observed u s i n g remote s e n s i n g t e c h n i q u e s a r e drawn from t h e B i s c a y , C e l t i c Sea and E n g l i s h Channel and i l l u s t r a t e s h e l f - b r e a k c o o l i n g and a s s o c i a t e d phytoplankton blooms, Biscay e d d i e s , s h e l f t i d a l f r o n t s , c o a s t a l u p w e l l i n g , f r o n t a l e d d i e s and i n s t a b i l i t i e s , i n t e r n a l waves and turbidity structures.

Some s u p p o r t i n g s e a t r u t h i s a l s o p r e s e n t e d

b u t it is clear t h a t r e a l i s t i c models f o r t h e s e p r o c e s s e s i s a s u b j e c t of f u t u r e r e s e a r c h .

288

F i g . 1. I n f r a - r e d s a t e l l i t e image (1339 GMT, 26 August 1 9 8 1 ) i l l u s t r a t i n g shelf-break cooling. The U s h a n t , S c i l l y I s l e s , Lands End, and C e l t i c S e a t i d a l f r o n t s c a n a l s o b e i d e n t i f i e d . In a d d i t i o n , c o a s t a l t i d a l f r o n t s o c c u r a l o n g t h e Armorican s h e l f . Cool w a t e r due t o p r e v i o u s wind i n d u c e d u p w e l l i n g a l s o o c c u r s o f f S o u t h w e s t I r e l a n d and t h e S p a n i s h C o a s t . High p r e s s u r e , calm wind c o n d i t i o n s e x i s t e d on 2 6 August 1981 and sea s u r f a c e t e m p e r a t u r e 'hot-spots' (see F i g . 4 ) a s s o c i a t e d w i t h w i n d l e s s h i g h p r e s s u r e c o n d i t i o n s c a n b e s e e n i n t h e C e l t i c S e a and w e s t e r n E n g l i s h Channel.

289

SURFACE TEMPERATURE STRUCTURE

1.

1.1.

She,lf T i d a l f r o n t s , banded s t r u c t u r e s and u p w e l l i n g f r o n t s

I n f r a - r e d s a t e l l i t e imagery h a s r e v e a l e d c l e a r l y t h e t i d a l f r o n t s i n t h e E n g l i s h Channel, C e l t i c Sea and Armorican s h e l f ( F i g s . 1 and 2 ) .

These t r a n s i t i o n s between t i d a l l y mixed and

s t r a t i f i e d w a t e r (Simpson and Hunter, 1 9 7 4 ; P i n g r e e and G r i f f i t h s , 1978) p e r s i s t f o r about

%

1 0 0 days o v e r t h e summer months J u n e ,

J u l y , August. The b o u n d a r i e s between mixed and s t r a t i f i e d w a t e r s appear t o be u n s t a b l e and a r e c h a r a c t e r i s e d by i r r e g u l a r e d d i e s

Fig. 2. A s k e t c h of some f e a t u r e s observed i n t h e i n f r a - r e d s a t e l l i t e imagery ( d o t t e d l i n e s ) . Also shown a r e some c u r r e n t measurements (see t e x t f o r e x p l a n a t i o n s ) . A d o t s i g n i f i e s t h e p o s i t i o n of t h e c u r r e n t meter mooring and on t h e s h e l f o n l y t h e measurements r e f e r t o t h e upper p a r t of t h e w a t e r column. A wavy arrow r e p r e s e n t s flow i n f e r r e d from s a t e l l i t e images. An arrow w i t h o u t a d o t i n d i c a t e s t h e movement of a s u r f a c e d r i f t i n g buoy. The numbers used g i v e t h e mean speed i n c m s-l. ~ l s o shown a r e t h e 1 0 0 fm and 1 0 0 0 fm c o n t o u r s .

290

which o c c a s i o n a l l y show a tendency t o be c y c l o n i c ( P i n g r e e e t a l . , 1 9 7 9 ) , as e x e m p l i f i e d by t h e Lands End f r o n t a l zone ( F i g . 1).

The

i n s t a b i l i t i e s have time s c a l e s of o r d e r 1 day and l e n g t h s c a l e s of a b o u t 2 0 km.

Long ( 3 0 km) i n t r u s i v e f i n g e r s w i t h s e p a r a t i o n s of

10-20 km c a n a l s o b e observed on t h e Ushant f r o n t and t h e s e f e a t u r e s t e n d t o be p a r t i c u l a r l y conspicuous i n September a s t h e mixed r e g i o n i n c r e a s e s i n a r e a and e x t e n d s a c r o s s t h e mouth of t h e E n g l i s h Channel.

A t t h i s t i m e of y e a r t h e Ushant t i d a l f r o n t can

show a marked s p r i n g - n e a p v a r i a t i o n i n g e o g r a p h i c a l e x t e n t . F r o n t a l i n s t a b i l i t i e s a r e t h o u g h t t o r e p r e s e n t an i m p o r t a n t agency i n t h e c r o s s f r o n t a l t r a n s f e r of w a t e r p r o p e r t i e s . T w o i n t e r e s t i n g f e a t u r e s t h a t have a l s o shown up w i t h i n f r a - r e d

imagery (which s t i l l r e q u i r e s e a t r u t h t o confirm t h a t t h e y a r e indeed r e a l f e a t u r e s of t h e sea s u r f a c e t e m p e r a t u r e s r a t h e r t h a n a t m o s p h e r i c e f f e c t s ) a r e t h e bands of a p p a r e n t l y c o l d w a t e r t h a t a p p e a r o n - s h e l f e x t e n d i n g a p p r o x i m a t e l y normal t o t h e s h e l f - b r e a k i n May and June and p a r a l l e l t o t h e s h e l f - b r e a k i n J u l y and August. The normal bands e x t e n d f o r

Q,

100-200 lan and have a wavelength

2.

15 km and p r o b a b l y r e s u l t from mixing o r i n t e r n a l t i d e s a s s o c i a t e d w i t h t h e l i n e a r t i d a l sand r i d g e s t h a t o c c u r i n t h e C e l t i c Sea on similar scales

Q,

1 5 km, f i g . 3 ( a ) .

The p a r a l l e l bands have

wavelengths of 20-30 km and a r e a l s o r e l a t i v e l y s t a t i o n a r y .

It

might be argued t h a t t h e y occur where t h e c u r r e n t s of t h e b a r o t r o p i c t i d e a r e i n phase w i t h t h e c u r r e n t s of a p r o g r e s s i v e i n t e r n a l t i d e r e s u l t i n g i n i n c r e a s e d l o c a l mixing. I n f r a - r e d s a t e l l i t e imagery h a s a l s o i d e n t i f i e d c l e a r l y t h e marked s e a s o n a l u p w e l l i n g t h a t o c c u r s a l o n g t h e Spanish and P o r t u g u e s e c o a s t and shown t h a t u p w e l l i n g a l s o o c c u r s o c c a s i o n a l l y o f f Southwest I r e l a n d and on t h a t p a r t of t h e French c o a s t which

i s a d j a c e n t t o t h e Armorican and A q u i t a i n e s h e l f . 1.2.

S h e l f Break c o o l i n g and Biscay mesoscale e d d i e s

W h i l s t some of t h e g r o s s f e a t u r e s l i s t e d above w e r e known b e f o r e t h e widespread u s e of i n f r a - r e d s a t e l l i t e imagery t h e r e

w e r e few r e p o r t s of s h e l f - b r e a k c o o l i n g and none showing t h e c h a r a c t e r i s t i c a l l y deep Biscay eddy s t r u c t u r e .

Shelf-break cooling

e x t e n d s t y p i c a l l y f o r 300 km a l o n g t h e s h e l f - b r e a k and s l o p e r e g i o n s and p e r s i s t s from l a t e May t o l a t e September and l i k e t h e t i d a l f r o n t s i s c h a r a c t e r i s e d by i r r e g u l a r s m a l l e r s c a l e structures.

S h e l f - b r e a k c o o l i n g h a s n o t been observed i n w i n t e r

291

F i g . 3 ( a ) . I n f r a - r e d s a t e l l i t e image ( 2 7 . 5 . 8 3 ) showing a p p a r e n t l y c o o l b a n d s e x t e n d i n g a p p r o x i m a t e l y normal t o t h e s h e l f - b r e a k and a s s o c i a t e d w i t h t h e s a n d r i d g e s which h a v e s i m i l a r t r a n s v e r s e wavelengths 10-15 km. The w h i t e r f e a t u r e s a r e c l o u d s and s h o u l d be i g n o r e d . The i s l a n d on t h e l o w e r r i g h t i s Ushant and t h e s t a r t of t h e Ushant f r o n t i s i n d i c a t e d by t h e w i s p y f e a t u r e s s t r e t c h i n g from t h e F r e n c h c o a s t . The p a s s c o r r e s p o n d s t o s p r i n g t i d e s and h i g h p r e s s u r e a t m o s p h e r i c c o n d i t i o n s and s e a m i s t may p e r h a p s s e r v e t o a c c e n t u a t e some of t h e s e f e a t u r e s .

t h o u g h s h e l f - b r e a k warming h a s b e e n n o t e d i n J a n u a r y 1 9 7 9 and 1982 ( a s f a r n o r t h as 4 7 O N ) and p r e s u m a b l y a s s o c i a t e d w i t h a d v e c t i o n and s p r e a d i n g of w a r m w a t e r from t h e S p a n i s h c o a s t .

Shelf-break

c o o l i n g h a s b e e n a t t r i b u t e d t o r e s u l t from m i x i n g by i n t e r n a l t i d e s simply because i t s p o s i t i o n corresponds approximately t o t h e r e g i o n where t h e M2 t i d a l c u r r e n t s have maximum v a l u e s and t h i s a s p e c t i s d i s c u s s e d i n more d e t a i l l a t e r .

However u p w e l l i n g

p r o c e s s e s , m i x i n g by t r a p p e d waves and i n e r t i a l c u r r e n t s a r e a l l thought t o play a contributing r o l e i n shelf-break cooling. C o o l e r w a t e r from t h e s l o p e s h a s been o b s e r v e d s p r e a d i n g o n t o t h e shelf f o r limited distances and Ode1 canyons.

(%

1 5 km) from Penmarch, G u i l v i n e c

292

F i g . 3b. I n f r a - r e d s a t e l l i t e image showing a n e v o l v i n g B i s c a y vortex pair. The lower images ( 3 . 6 . 3 2 and 3 0 . 6 . 2 2 ) f i t i n t o t h e B i s c a y r e g i o n i n t h e same manner a s t h e u p p e r images ( 2 0 . 4 . 8 2 and 25.5.82).

I n common w i t h most o c e a n i c r e g i o n s , t h e d e e p B i s c a y shows l a r g e ( 1 0 0 km) i n t e r c o n n e c t e d e d d i e s t h a t are g e n e r a l l y c o n f i n e d t o t h e a b y s s a l p l a i n by t h e s l o p e s .

I n d i v i d u a l e x a m p l e s of s u c h

e d d i e s from i n f r a - r e d images h a v e b e e n g i v e n by F r o u i n ( 1 9 8 1 ) and D i c k s o n and Hughes ( 1 9 8 1 ) .

A t

sea t h e y h a v e b e e n s t u d i e d u s i n g

293

d r i f t i n g buoys (Madelain and K e r u t , 1978) and i n t h e T o u r b i l l o n e x p e r i m e n t ( L e Groupe T o u r b i l l o n ,

1983).

They have t h e

c h a r a c t e r i s t i c s t r u c t u r e s k e t c h e d i n F i g . 2 and can l a s t f o r a week o r more.

The a n t i c y c l o n i c s t r u c t u r e of t h e v o r t e x p a i r

i l l u s t r a t e d i n F i g . 3 ( b ) a p p e a r e d t o p e r s i s t i n one form o r a n o t h e r o v e r a p e r i o d of two months a p p a r e n t l y f e d by ( o r drawing i n ) a c o o l s t r e a m of water f l o w i n g a l o n g t h e b a s e of t h e s l o p e s . Although t h i s v o r t e x p a i r a p p e a r s i n t h e c e n t r a l B i s c a y p o i n t of view of t h e 3 0 0 0 - 4 0 0 0

from t h e

m topography it i s p r e s s e d up

a g a i n s t t h e b a s e of t h e s l o p e i n t h e S . E .

c o r n e r of t h e Biscay

and seems t o s u b s e q u e n t l y p u t some c o o l e r w a t e r up on t h e S p a n i s h slope.

B i s c a y e d d i e s a l s o a p p e a r t o be a b l e t o draw w a t e r o f f

t h e s h e l f and c o o l plumes can sometimes be o b s e r v e d e x t e n d i n g from t h e r e g i o n of s h e l f - b r e a k c o o l i n g f o r d i s t a n c e s of 100-200km, Fig.

2.

I n addition t o t h e interconnecting vortex p a i r s t r u c t u r e s , c y c l o n i c e d d i e s w i t h wave l e n g t h s of 1 0 0 km have been observed i n the S.E.

Biscay which a p p e a r t o be c o n f i n e d t o t h e lower p a r t of

t h e s l o p e s and a r e a l s o s k e t c h e d i n F i g . 2 . 1.3.

Hot s p o t s

Cloud-free i n f r a - r e d

s a t e l l i t e images g e n e r a l l y o c c u r under

high atmospheric p r e s s u r e conditions. show " h o t - s p o t s ' '

The daytime p a s s e s t h e n

a s t h e sea s u r f a c e warms up i n l o c a l i s e d p l a c e s

where c o n d i t i o n s a r e r e l a t i v e l y w i n d l e s s .

Measurements a t sea

and from d r i f t i n g buoys have shown t h a t under such w i n d l e s s conditions

( < F o r c e 2 ) t h e t o p m e t r e can w a r m up by

(Fig. 4 ) .

Such e f f e c t s have a l l o w e d s t r u c t u r e s t o be observed i n

2-3OC

t h e g e n e r a l l y t i d a l mixed c o n d i t i o n s of t h e c e n t r a l and e a s t e r n r e g i o n s of t h e E n g l i s h Channel, f o r example, f r e s h e r w a t e r s p r e a d i n g from t h e Bay of S e i n e r e g i o n p a r t i c u l a r l y a t neap t i d e s , and t h e e f f e c t s of t i d a l mixing and topography i n t h e Channel

Isles a r e a .

2. 2.1.

SURFACE CHLOROPHYLL STRUCTURES S h e l f - b r e a k f r o n t a l F l u o r e s c e n c e and R e f l e c t a n c e Measurements a t sea have shown t h a t b o t h t h e Ushant f r o n t and

t h e s h e l f - b r e a k c o o l i n g r e g i o n c a n show i n c r e a s e s o f c h l o r o p h y l l ' a ' a t t h e s u r f a c e (Pingree e t a l . ,

1982).

T h i s i s t h o u g h t t o be

294

201 18

OC 16

1 , 1 , , 1 19821 , , , , , , , 198

211

F i g . 4. S u r f a c e t e m p e r a t u r e r e c o r d from s u r f a c e d r i f t i n g buoy (which f o l l o w e d t h e 2 0 0 0 m c o n t o u r northwestward i n t h e v i c i n i t y of 9OW a t about 5 c m s - 1 ) showing marked d i u r n a l t e m p e r a t u r e variations.

due t o t h e f a v o u r a b l e n u t r i e n t and l i g h t regime a f f o r d e d by t h e p h y s i c a l mixing p r o c e s s e s .

I n June a band of i n o r g a n i c n u t r i e n t s

occurs along t h e shelf-break with n i t r a t e values t y p i c a l l y 2,

1 ug a t 1-1 N-NO3

(Fig. 5 ) .

I n J u l y , August i s o l a t e d , h i g h e r

t h a n background, n i t r a t e - n i t r o g e n

p a t c h e s occur w i t h g e n e r a l l y

c o o l e r w a t e r showing t h a t t h e r e i s , i n d e e d , on o c c a s i o n s , a n i t r a t e s o u r c e a t t h e s u r f a c e t h a t can be u t i l i s e d by phytoplankton p h y t o p l a n k t o n growing n e a r t h e s u r f a c e ( F i g . 6 ) .

Whilst t h e

v a l u e s of f l u o r e s c e n c e a t t h e s h e l f - b r e a k a r e v e r y v a r i a b l e w i t h e x c e p t i o n a l l y h i g h v a l u e s a s s o c i a t e d w i t h some nannoplankton communities ( f o r example t h e Prasinophycean f l a g e l l a t e Micromonas s p (1-211 d i a m e t e r ) t o g e t h e r w i t h t h e Chrysophycean f l a g e l l a t e

P s e u d o p e d i n e l l a s p ( 6 d~i a m e t e r ) ) , t h e c h l o r o p h y l l ' a ' v a l u e s a r e t y p i c a l l y only

Q ,

1 mg c h l ' a ' m-3,

an o r d e r of magnitude l o w e r

t h a n t h e v a l u e s t h a t a r e commonly a s s o c i a t e d w i t h blooms i n t h e v i c i n i t y of t h e s h e l f - t i d a l f r o n t s o r which o c c u r d u r i n g t h e s p r i n g bloom i n t h e C e l t i c Sea.

However mackerel eggs o c c u r i n

maximum number a t t h e s h e l f - b r e a k i n May-June

(Coombs e t a l . ,

1981) and it may be t h e l a r g e g e o g r a p h i c a l e x t e n t of t h i s r e g i o n

of i n c r e a s e d l e v e l s of s u r f a c e c h l o r o p h y l l ' a ' and t h e a s s o c i a t e d

296

49

N

4; 45

N

4;

F i g . 5. ( a ) S u r f a c e t e m p e r a t u r e (OC); ( b ) s a l i n i t y (o/oo); ( c ) c h l o r o p h y l l ' a ' (mg m-3) and ( d ) i n o r g a n i c n i t r a t e (pM) (3-6 J u n e 1 9 8 3 ) . 200 m c o n t o u r shown by d o t t e d l i n e .

p r o t r a c t e d p r o d u c t i v e s e a s o n of b o t h primary and secondary p r o d u c t i o n which p r o v i d e t h e e c o l o g i c a l a d v a n t a g e s t h a t f a v o u r t h i s spawning a r e a . The C o a s t a l Zone Colour Scanner (C.Z.C.S.)

imagery h a s shown

more c l e a r l y t h a n e v e r b e f o r e t h e g e o g r a p h i c s c a l e and p e r s i s t e n c e of t h e s h e l f - b r e a k blooms ( F i g . 7 ) .

Chlorophyll

a b s o r b s more s t r o n g l y a t t h e b l u e end of t h e v i s i b l e spectrum t h a n i n t h e y e l l o w p a r t and i n broad t e r m s a measure of t h e c h l o r o p h y l l from C.Z.C.S.

d a t a c a n be o b t a i n e d from t h e r a t i o of t h e

r e f l e c t a n c e s from c h a n n e l 1 ( b l u e , 443 nm) o r c h a n n e l 2 ( g r e e n , 5 2 0 nm) t o c h a n n e l 3 ( y e l l o w , 550 nm) a f t e r a p p l y i n g an a t m o s p h e r i c c o r r e c t i o n t o e a c h u s i n g c h a n n e l 4 ( r e d , 670 nm). Some of t h e s p e c t a c u l a r blooms t h a t have been observed a t t h e s h e l f - b r e a k a r e comprised mainly of c o c c o l i t h o p h o r e s ( H o l l i g a n

et al.,

1983) which g i v e a c h a r a c t e r i s t i c milky appearance t o t h e

water.

The c a l c i t e p l a t e s of t h e c o c c o l i t h o p h o r e s a r e s t r o n g l y

r e f l e c t i n g and t h e s t r u c t u r e of t h e s e blooms can b e s e e n i n t h e raw c h a n n e l 3 d a t a . C.Z.C.S.

I n common w i t h t h e i n f r a - r e d

imagery t h e

imagery h a s shown t h i n plumes e x t e n d i n g o u t from t h e

296

0 4

a

Temperatureloc)

,

CI

I

I

(0.4

48O

'ablr 48"

-

\

3 0

50 47O

Chlorophyll 'a'

(mg m?)

5"

7'

8-

.

6"

6. ( a ) S u r f a c e t e m p e r a t u r e ( O C ) and s h i p ' s t r a c k : s a l i n i t y (O/oO): ( c ) c h l o r o p h y l l ' a ' (mg m-3) and i n o r g a n i c n i t r a t e ( p M ) (August 1 9 8 0 ) . Bottom topography i s g i v e n i n metres.

s h e l f - b r e a k and e d d i e s i n t h e d e e p e r Biscay r e g i o n s .

The plumes

of p h y t o p l a n k t o n drawn o f f from t h e s h e l f - b r e a k s l o p e r e g i o n a r e f u r t h e r e v i d e n c e of p h y s i c a l p r o c e s s e s ( i n t h i s c a s e B i s c a y e d d i e s ) and may be i m p o r t a n t i n t h e development and s u b s e q u e n t decay o f s h e l f - b r e a k blooms.

2.2.

F l u o r e s c e n c e Along S h e l f t i d a l f r o n t s

I n c r e a s e s i n r e f l e c t a n c e a l s o o c c u r a l o n g t h e Ushant t i d a l f r o n t ( F i g . 7 ) where p h y s i c a l mixing p r o c e s s e s a g a i n c o n t r o l t h e

291

F i g . 7. C . Z . C . S . ( C o a s t a l Zone C o l o u r S c a n n e r ) image (22 J u n e 1 9 8 1 ) showing r e g i o n s of r e l a t i v e l y h i g h s u r f a c e c h l o r o p h y l l i n t h e v i c i n i t y of t h e s h e l f - b r e a k and t o t h e s t r a t i f i e d s i d e of t h e Ushant f r o n t .

a v a i l a b i l i t y of n u t r i e n t s and l i g h t .

A s t h e season progresses

t h e p h y t o p l a n k t o n c o m p o s i t i o n c h a n g e s from a dominance of d i a t o m s t o a dominance of d i n o f l a g e l l a t e s which t e n d t o o c c u r i n t h e s t r a t i f i e d waters adjacent t o t h e t i d a l f r o n t s .

Spectacular

s u r f a c e blooms of d i n o f l a g e l l a t e s (Gyrodinium a u r e o l u m ) h a v e b e e n o b s e r v e d i n 1975, 1 9 7 6 , 1978, 1981 whicn e x t e n d from t h e f r o n t a l boundary w e l l a c r o s s i n t o s u r f a c e w a t e r s of t h e s h a l l o w thermocline

(%

2 0 m ) r e g i o n s of t h e w e s t e r n E n g l i s h Channel where

v a l u e s of c h l o r o p h y l l ' a ' a s h i g h a s recorded.

'~r

1 0 0 mg c h l ' a ' m-3

h a v e been

The p r e c i s e r o l e of w a t e r movement, n u t r i e n t f l u x e s and

v e r t i c a l m i g r a t i o n of t h e d i n o f l a g e l l a t e s i n m a i n t a i n i n g t h e s e

298

surface d i s t r i b u t i o n s i n t h e shallow s t r a t i f i e d waters adjacent to

t h e Ushant t i d a l f r o n t i s a s u b j e c t of c o n t i n u i n g r e s e a r c h . INTERNAL WAVES AND TIDES

3.

(i)

Surface radar s t r u c t u r e s .

Whilst s h e l f - b r e a k c o o l i n g may

be c o n s i d e r e d a s p o s s i b l e i n d i r e c t e v i d e n c e f o r i n t e r n a l t i d e s , i n f r a - r e d s a t e l l i t e imagery h a s n o t y e t p r o v i d e d c l e a r examples o f internal tides.

T h i s i s h a r d l y s u r p r i s i n g s i n c e measurements a t

t h e s h e l f - b r e a k n e a r 48ON have shown t h a t a l t h o u g h t h e t h e r m o c l i n e may o s c i l l a t e by more t h a n 50 m a t s p r i n g t i d e s ( F i g . 8 ) t h e r e may b e no s u r f a c e t e m p e r a t u r e e x p r e s s i o n of t h e internal tide.

The s y n t h e t i c a p e r t u r e r a d a r (S.A.R.) on board t h e

SEASAT on t h e o t h e r hand h a s p r o v i d e d s t r i k i n g examples of i n t e r n a l waves i n t h e Biscay r e g i o n and a l l o w e d an e s t i m a t e t o be made f o r t h e phase speed f o r t h e i n t e r n a l t i d e p r o p a g a t i n g

HOURS 6

12

E

r I-

n W

n

F i g . 8. I s o t h e r m s ( O C ) o b t a i n e d from r e p e a t e d S.T.D. p r o f i l e s ( e v e r y 1 0 mins) n e a r t h e s h e l f - b r e a k i n about 250-350 m d e p t h f o l l o w i n g a d r i f t i n g dahn whose approximate mean p o s i t i o n was 48O08'N 8O11'W. The h i g h e r f r e q u e n c y o s c i l l a t i o n s of a b o u t 15 min p e r i o d c o u l d b e c o n t o u r e d w i t h o u t a l i a s i n g u s i n g t h e echo sounder t o monitor t h e a c o u s t i c s c a t t e r i n g l a y e r s i n t h e thermocline. The t i m e of maximum o f f - s h e l f t i d a l s t r e a m i n g is i n d i c a t e d by an arrow. The t i d a l o s c i l l a t i o n s of t h e t h e r m o c l i n e (14°C c o n t o u r ) i n t h i s r e g i o n have a peak t o t r o u g h d i s p l a c e m e n t o f a b o u t 50 m a t s p r i n g t i d e s . There i s a l s o a marked second b a r o c l i n e mode. The 1 6 O C c o n t o u r n e a r t h e s u r f a c e i s n o t shown.

299

on-shelf. V a r i a t i o n s i n sea s n r f a c e roughness due t o t h e i n t e r a c t i o n of t h e i n t e r n a l t i d a l c u r r e n t s w i t h t h e s u r f a c e waves a l l o w s t h e r a d a r t o r e v e a l t h e i n t e r n a l waves c l e a r l y ( F i g . 9 ) . Such waves c a n , i n f a c t , be observed a t s e a u s i n g s h i p ' s r a d a r

(see f o r example Haury e t a l . ,

1983) o r even v i s u a l l y a s a r e s u l t

of t h e i n c r e a s e d number of b r e a k i n g waves a s s o c i a t e d w i t h t h e i n t e r n a l wave t r a i n s .

On o c c a s i o n s p a r a l l e l " w a l l s o f white''

w a t e r ( b r e a k i n g s u r f a c e waves) s e p a r a t e d by a b o u t 1 k m can be s e e n s t r e t c h i n g f o r s e v e r a l m i l e s i n d i c a t i n g t h e p r e s e n c e of l a r g e i n t e r n a l waves p r o p a g a t i n g o n - s h e l f . image f o r t h i s r e g i o n ( P i n g r e e and M a r d e l l , 1981)

The S . A . R .

i n d i c a t e s t h a t a l t h o u g h t h e r e a r e many s o u r c e s f o r t h e i n t e r n a l waves t h e y mainly o r i g i n a t e a t t h e s h e l f - b r e a k from l o c a l i s e d sources.

I n t e r n a l waves appear t o move on-shelf

about

Q

30 km i n

what i s assumed t o be a t i d a l p e r i o d g i v i n g a phase speed of 6 7 c m s-1.

They a l s o p r o p a g a t e o f f - s l o p e and o u t i n t o t h e B i s c a y .

The i n t e r n a l waves i l l u s t r a t e d i n F i g . 9 have wavelengths of o r d e r 1 km and n o n - l i n e a r e f f e c t s a r e i m p o r t a n t i n t h e i r g e n e r a t i o n and s u b s e q u e n t p r o p a g a t i o n . Such images have s t i m u l a t e d t h e development of n u m e r i c a l models and measurements of t h e i n t e r n a l t i d e u s i n g t h e r m i s t o r c h a i n s and c u r r e n t meter moorings.

S i n c e t h e i n t e r n a l t i d e s a r e t h o u g h t t o r e p r e s e n t one

of t h e main c a n d i d a t e s c a u s i n g s h e l f - b r e a k c o o l i n g and t h e a s s o c i a t e d s h e l f - b r e a k f l u o r e s c e n c e it i s of i n t e r e s t t o c o n s i d e r some of t h e p o s s i b l e c h a r a c t e r i s t i c s of t h e i n t e r n a l t i d e s i n t h i s region. (ii) Numerical models

The f o l l o w i n g s i m p l e n u m e r i c a l model n e g l e c t s r o t a t i o n , assumes t h e s h e l f - s l o p e r e g i o n h a s a r e g u l a r geometry and i s o n l y v a l i d f o r long waves (so lee wave f o r m a t i o n where n o n - h y d r o s t a t i c p r e s s u r e becomes i m p o r t a n t i s n o t t a k e n i n t o a c c o u n t ) .

Although

i n i t s p r e s e n t form t h e model may n o t b e v e r y r e a l i s t i c it d o e s show t h a t long wave i n t e r n a l t i d e s might be f o r c e d by t h e b a r o t r o p i c t i d e a s t h e t i d a l c u r r e n t s move up and down t h e s l o p e t h e r e b y c a u s i n g o s c i l l a t i o n s of t h e t h e r m o c l i n e .

I n t h i s model

a c r e s t i s formed n e a r t h e s h e l f - b r e a k j u s t a f t e r on-shelf s t r e a m i n g , whereas a t r o u g h forms j u s t a f t e r o f f - s h e l f streaming.

tidal

tidal

The crests and t r o u g h s d i v i d e i n t h e s l o p e r e g i o n

n e a r t h e s h e l f - b r e a k and p r o p a g a t e a s f r e e waves b o t h on-shelf and o f f - s h e l f

towards t h e ocean.

S i n c e t h e t r o u g h formed d u r i n g

300

Fig. 9 . A d i g i t a l l y p r o c e s s e d p o r t i o n of t h e s y n t h e t i c a p e r t u r e r a d a r (S.A.R.) p a s s on 2 0 August 1978 showing i n t e r n a l waves i n t h e s h e l f - b r e a k r e g i o n w i t h w a v e l e n g t h s t y p i c a l l y o f o r d e r 1 km. The image c e n t r e i s l o c a t e d a t 46°51'36"N, 5O9'58"W. The p a s s c o r r e s p o n d s t o t i d a l c o n d i t i o n s 1 h o u r a f t e r maximum o f f - s h e l f t i d a l streaming a t spring t i d e s .

off-shelf

t i d a l streaming i s propagating on-shelf

against the

t i d a l c u r r e n t and a marked s t e e p e n i n g of t h e i n t e r n a l wave p r o f i l e o c c u r s which p r o p a g a t e s a s an i n t e r n a l t i d a l b o r e . The model i s a v e r t i c a l s e c t i o n normal t o t h e s h e l f - b r e a k spanning oceanic ( 4 0 0 0 m ) ,

s l o p e and s h e l f r e g i o n s ( 2 0 0 m ) .

The

t h e r m o c l i n e i s r e p r e s e n t e d by an upper l a y e r h ' of d e n s i t y p ' and The x a x i s i s c h o s e n p o s i t i v e a l o w e r l a y e r , h " , of d e n s i t y p " .

301

200 m

50 km

Fig. 1 0 . Schematic r e p r e s e n t a t i o n of t h e model showing t h e r m o c l i n e spanning o c e a n i c , s l o p e and s h e l f r e g i o n s . The g r i d s c a l e i s 500m.

i n t h e on-shelf

d i r e c t i o n and t h e g r i d s c a l e i s 500 m ( F i g . 1 0 ) .

F o r s i m p l i c i t y c o n d i t i o n s a r e t a k e n as uniform i n t h e a l o n g - s h e l f sense.

The i n t e r n a l t i d e U i s d e f i n e d by U = u'

- u"

where u ' i s

t h e c u r r e n t i n t h e upper l a y e r and u" i s t h e c u r r e n t i n t h e lower layer.

The b a r o t r o p i c t i d e , o r v e r t i c a l l y i n t e g r a t e d t i d a l

c u r r e n t , u, i s assumed t o be unmodified by t h e i n t e r n a l t i d e and i s s p e c i f i e d i n advance i n a c c o r d a n c e w i t h s h e l f - s l o p e geometry.

The e q u a t i o n of c o n t i n u i t y f o r t h e upper l a y e r can be transformed i n t o a n equation f o r t h e i n t e r n a l o s c i l l a t i o n

n

against t i m e t , t o give

a ax

(h'u)

a ( h ' h "7 + ax U) = a at

where H = h '

+

h" = h l

+

h2

and uH = u ' h '

+

u"h"

The f i r s t t e r m on t h e l e f t hand s i d e of e q u a t i o n (1) i s t h e s o u r c e t e r m f o r t h e i n t e r n a l d i s p l a c e m e n t of t h e t h e r m o c l i n e

n.

I t a l s o a l l o w s t h e b a r o t r o p i c t i d e t o move t h e i n t e r n a l t i d e back

and f o r t h on t h e s h e l f s i n c e h ' = h l

-

q.

Variations i n surface

e l e v a t i o n are neglected with r e s p e c t t o t h e i n t e r n a l o s c i l l a t i o n . The b a r o t r o p i c t i d e , u , i s p r e s c r i b e d a c c o r d i n g t o t h e non-divergent equation

302

a ax (Hu) =

0

Thus the source term for the internal tide varies as -('/HI

2

3

ax

and has a maximum value just at the top of the slopes when ax

is constant.

A simplified momentum equation is obtained by subtracting the momentum equations for the upper and lower layers to give

= (uu) = at + ax a

B - an + K V U 2

- dlP

where B = g(l

(3)

ax

is the reduced gravity and g is the "

acceleration due to gravity. More complete forms for the term arising from advection a (uU) gave qualitatively similar results and in this simple treatment 2 are not further discussed. The term KV U represents attenuation by diffusion, with coefficient K, and also assists with numerical stability. The mean depth of the upper layer was taken as hl = 30 m and the slope region had a uniform gradient of 1 in 10 from H = 200 m to H = 4000 m in 38 km. Thus in a linear model the phase speed is

f

%

50 cm

s",

with B

%

1 cm sec-2 , and h2 = 170 m,

considerably less than that suggested by the S . A . R . image. The 2n corresponding wavelength X = -2T I;- for M2 tidal frequency, u = T is X = 23 km. K was chosen such that Kk2 % 2 / r so free waves in a linear model would decay to l/e of their amplitude after a time T and T was set T = 4T where T is the M2 tidal period, thus 2 2 -1 K % 1.4 x 10 m s The amplitude of the oscillating barotropic tide was taken as 75 cm s-l which represents a peak spring tide condition for a % 100 km stretch along the shelf-break in the Celtic Sea Armorican Shelf region. (iii) Long waves without rotation. The structure of the internal tide is illustrated by hourly sequences of the displacement of the thermocline. The linear model where all non-linear terms were neglected is shown in Fig. 11 and the results of the non-linear model using the full equations (l), (2) and (3) is shown in Fig. 12. In both models a trough occurs at the shelf-break just after maximum of€-shelf tidal streaming.

.

-40 m

6

12

5

11

3

9

2

a

303

7

F i g . 11. I n t e r n a l t i d a l d i s p l a c e m e n t s of t h e t h e r m o c l i n e e v e r y l u n a r hour u s i n g l i n e a r i s e d e q u a t i o n s . The s l o p e r e g i o n e x t e n d s from 0 t o S where S i s t h e s h e l f - b r e a k ( 2 0 0 m ) and 0 i s t h e s t a r t of t h e o c e a n i c r e g i o n ( 4 0 0 0 m ) . D i s an on-shelf A vertical p r o p a g a t i n g t r o u g h and C i s an ocean-going t r o u g h . s c a l e of 4 0 m i s shown a t hour 3 . Hour 3 c o r r e s p o n d s w i t h maximum o n - s h e l f t i d a l s t r e a m i n g and maximum o f f - s h e l f t i d a l s t r e a m i n g o c c u r s a t h o u r 9 ( d e p i c t e d by a r r o w ) .

304

1 2 q

5

4

IA

1

YA

2

8

F i g . 1 2 . I n t e r n a l t i d a l d i s p l a c e m e n t of t h e t h e r m o c l i n e e v e r y l u n a r hour using t h e f u l l y non-linear e q u a t i o n s . The s l o p e r e g i o n e x t e n d s from 0 t o S where S i s t h e s h e l f - b r e a k ( 2 0 0 m ) a n d 0 i s t h e s t a r t of t h e o c e a n i c r e g i o n ( 4 0 0 0 m ) . B i s an o c e a n g o i n g t r o u g h and A i s a n o n - s h e l f p r o p a g a t i n g t r o u g h . Hour 3 c o r r e s p o n d s w i t h maximum o n - s h e l f t i d a l s t r e a m i n g .

305

However i n t h e n o n - l i n e a r model t h e l e a d i n g edge o f t h e on-shelf p r o p a g a t i n g t r o u g h i s u n a b l e t o move o n - s h e l f

against the

b a r o t r o p i c t i d a l c u r r e n t s u n t i l t h e t i d a l streams s l a c k e n .

This

r e s u l t s i n a v e r y d i s t o r t e d and s t e e p e n e d t r o u g h f o r t h e i n t e r n a l t i d e which s u b s e q u e n t l y p r o p a g a t e s r a p i d l y a c r o s s t h e s h e l f when t h e t i d a l streams are o n - s h e l f

and i s h a l t e d and momentarily

r e v e r s e d i n d i r e c t i o n d u r i n g maximum o f f - s h e l f (iv)

E f f e c t due t o r o t a t i o n .

t i d a l streaming.

When r o t a t i o n i s t a k e n i n t o

a c c o u n t and c o n d i t i o n s are a g a i n uniform i n t h e a l o n g - s l o p e s e n s e r o t a t i o n i n c r e a s e s t h e p r o p a g a t i o n speed.

In addition r e l a t i v e l y

more energy i s a s s o c i a t e d w i t h t h e c u r r e n t s r a t h e r t h a n t h e i n t e r n a l d i s p l a c e m e n t s of t h e t h e r m o c l i n e .

L i n e a r t h e o r y and

n u m e r i c a l model g i v e t h e p r o p a g a t i o n speed f o r long waves w i t h h o r i z o n t a l crests a s

c

2

h h

p' - -$iH

= g(l

1 2)

(1

-

fL u2

--)-I

(4)

hl+h2

where f i s t h e C o r i o l i s p a r a m e t e r and a i s t h e t i d a l f r e q u e n c y . For f/o

%

0.77 appropriate for these l a t i t u d e s , t h i s w i l l r e s u l t

i n a n i n c r e a s e i n p h a s e s p e e d and wavelength f o r t h e p r o g r e s s i v e i n t e r n a l t i d e of a b o u t x 1 . 6 . The waves a r e now d i s p e r s i v e and t h e group v e l o c i t y , c l i n e a r long waves i s d e f i n e d a s ao

%==-

-

c(1

and w i t h f / o

%

-

g'

for

2 2 f /u ) 0.77,

a s b e f o r e , t h i s g i v e s a group v e l o c i t y of

a b o u t 1 . 6 t i m e s s m a l l e r t h a n t h e p h a s e speed of waves i n t h e a b s e n c e of r o t a t i o n .

T h i s i m p l i e s t h a t any m o d u l a t i o n of t h e

long-wave i n t e r n a l t i d a l s i g n a l a t t h e s h e l f - b r e a k due t o t h e s p r i n g - n e a p c y c l e of t h e b a r o t r o p i c t i d e w i l l t r a v e l o n l y s l o w l y on s h e l f o r o f f s h e l f .

t e r m s i n e q u a t i o n s (1) and ( 3 ) w e r e i n c l u d e d t h e d i s t o r t i o n s of t h e i n t e r n a l t i d e on t h e s h e l f w e r e When t h e n o n - l i n e a r

no l o n g e r a s i n d i c a t e d i n F i g . 1 2 b u t had d e e p l y p e n e t r a t i n g t r o u g h s and t h e wave p r o f i l e on t h e s h e l f a l s o t e n d e d t o be symmetric w i t h r e s p e c t t o t h e t r o u g h s . (v)

S h o r t e r wavelengths.

The waves so f a r c o n s i d e r e d assume

t h a t t h e i n t e r n a l t i d a l c u r r e n t s a r e uniform i n t o p and bottom layers.

C l e a r l y t h i s i s not v a l i d f o r s h o r t e r wavelengths a s

306 e x e m p l i f i e d by t h e S.A.R.

image which i n d i c a t e s n o n - l i n e a r

i n t e r n a l wave t r a i n s or p a c k e t s of i n t e r n a l s o l i t o n s .

More

r e a l i s t i c models would have t o make allowance f o r t h e v e r t i c a l s t r u c t u r e of t h e c u r r e n t s i n t h e upper and lower l a y e r f o r t h e h i g h e r wave numbers a s i n t h e Korteweg and d e V r i e s (1895) f i r s t approximation.

S o l i t a r y waves and s o l i t o n s t r a v e l a t s p e e d s i n

e x c e s s of t h a t g i v e n by s m a l l a m p l i t u d e l i n e a r t h e o r y .

Their

f r a c t i o n a l i n c r e a s e i n phase speed i s v e r y a p p r o x i m a t e l y

%

f n/h,

( A l p e r s and S a l u s t i , 1983) and s o t h e f i n i t e a m p l i t u d e of s h o r t e r waves may produce i n c r e a s e s i n phase speed t h a t c o u l d match t h e Thus nonv a l u e ( % 67 c m s-l) i n f e r r e d from t h e S.A.R. image. l i n e a r e f f e c t s of f i n i t e a m p l i t u d e f o r t h e s h o r t e r waves o r t h e l i n e a r e f f e c t s o f r o t a t i o n f o r t h e l o n g e r waves s i g n i f i c a n t l y i n c r e a s e s t h e p r o p a g a t i o n s p e e d s of t h e i n t e r n a l waves. ( v i ) Measurements a t sea. An extreme example of t h e s t r u c t u r e of t h e i n t e r n a l t i d e a t s p r i n g t i d e s o b t a i n e d from a t h e r m i s t o r c h a i n mooring p l a c e d on t h e s h e l f i n t h e r e g i o n of maximum M2 t i d a l c u r r e n t s a t 47°40.0'N

6O19.1'W

from t h e s h e l f - b r e a k ) i s shown i n F i g . 13

.

( a b o u t 2 0 km

A t spring t i d e s t h e

b a r o t r o p i c t i d a l c u r r e n t s r e a c h a l m o s t 2 knot a t t h i s p o s i t i o n and a l t h o u g h t h e t i d a l c u r r e n t s a r e reduced a t t h e s h e l f - b r e a k t h e y a r e s t i l l comparable w i t h t h e phase speed of t h e i n t e r n a l tide.

The t r o u g h formed d u r i n g o f f - s h e l f

p r o p a g a t i n g on-shelf

t i d a l streaming i s t h u s

a g a i n s t t h e t i d a l c u r r e n t and a t s p r i n g

t i d e s t h i s w i l l r e s u l t i n a marked s t e e p e n i n g of t h e i n t e r n a l tide. Measurement made from f i x e d moorings w i l l need c o r r e c t i n g f o r t h e d i s t o r t i o n s t h a t occur a s t h e t i d a l c u r r e n t s a d v e c t t h e i n t e r n a l t i d e p a s t t h e mooring.

C u r r e n t measurements made n e a r

t h e t h e r m i s t o r c h a i n mooring showed t h a t t h e l e a d i n g edge of t h e t r o u g h of t h e i n t e r n a l t i d e p a s s e d t h e t h e r m i s t o r c h a i n mooring when t h e on-shelf

t i d a l c u r r e n t was a b o u t 1 . 5 k n o t ( a b o u t 1 . 0

h o u r s a f t e r maximum on-shelf

t i d a l streaming).

Thus some of t h e

s t e e p e n i n g a s s o c i a t e d w i t h t h e t r o u g h of t h e i n t e r n a l t i d e i s a p p a r e n t and due t o making measurements a t a f i x e d p o i n t r a t h e r t h a n f o l l o w i n g t h e o s c i l l a t i n g b a r o t r o p i c t i d a l flow.

The

14O-15OC i s o t h e r m s descend below 50 m f o r a b o u t 2 0 % of t h e wave

period.

During t h i s t i m e a l o c a l w a t e r column would move

on-shelf

about 1 - 2 k m which i s o n l y a s m a l l f r a c t i o n

t h e wavelength of t h e i n t e r n a l t i d e

(%

30 k m ) .

(%

5 % ) of

So it a p p e a r s

307

F i g . 13. I s o t h e r m s (OC) from t h e t h e r m i s t o r c h a i n mooring 0 6 9 (47O41.8" 6018.2'W). The measured s t r u c t u r e of t h e i n t e r n a l t i d e p r o p a g a t i n g on-shelf i s h i g h l y d i s t o r t e d w i t h d e e p l y p e n e t r a t i n g t r o u g h s . There i s a l s o a n o t i c e a b l e second b a r o c l i n i c mode. Some smoothing of t h e d a t a was n e c e s s a r y t o produce a c l e a r e r i l l u s t r a t i o n . The p e r i o d i l l u s t r a t e d corresponds t o s p r i n g - t i d e conditions with semi-diurnal c u r r e n t s typically 80-90 c m s-l ( v e r t i c a l l y i n t e g r a t e d ) . Q

t h a t a t spring t i d e s , a t l e a s t , t h e i n t e r n a l t i d e is d i s t o r t e d i n t h i s p a r t i c u l a r r e g i o n w i t h more d e e p l y p e n e t r a t i n g t r o u g h s . A c l o s e r i n s p e c t i o n of Fig.

1 3 shows t h a t t h e t r o u g h s a r e

g e n e r a l l y composed of two l a r g e a m p l i t u d e waves a t t h i s s i t e . A t some p l a c e s n e a r t h e s h e l f - b r e a k t h e i n t e r n a l t i d a l s i g n a l

t a k e s on t h e form of a g r o u p of s h o r t wavelength i n t e r n a l waves p r o p a g a t i n g o n - s h e l f .

< 1

(%

km)

An example i s i l l u s t r a t e d

i n P i g . 1 4 which shows a l a r g e a m p l i t u d e wave f o l l o w e d by s m a l l e r waves and such waves a r e b e l i e v e d t o c a u s e t h e s u r f a c e f e a t u r e s s e e n i n t h e S.A.R.

image ( F i g . 9 ) .

C u r r e n t measurements have a l s o been made a t t h e s h e l f - b r e a k n e a r 47O30'N t o see whether t h e s h e a r produced by t h e i n t e r n a l t i d e is s u f f i c i e n t t o c a u s e mixing i n t h e t h e r m o c l i n e and c o n t r i b u t e t o t h e s h e l f - b r e a k c o o l i n g observed i n t h e i n f r a - r e d s a t e l l i t e imagery. G r a d i e n t Richardson numbers of measured by c u r r e n t meters s e p a r a t e d v e r t i c a l l y by

%

1 have been

%

84 m w i t h

t e m p e r a t u r e d i f f e r e n c e s a c r o s s t h e b a s e of t h e t h e r m o c l i n e of It i s hard not t o Q l 0 C f o r p e r i o d s of % 1 hour a t s p r i n g t i d e s . draw t h e c o n c l u s i o n t h a t a c l o s e r s e p a r a t i o n of c u r r e n t meters

308

' > i g . 14. Ecno sound.er t r a c e from 47052.5" 6 O 2 9 ' W ( 2 0 n.m from t n e s h e l z - b r e a k ( 2 0 0 m c o n t o u r ) on 2 7 . 7 . 8 3 ) showing l a r g e a m p l i t u d e waves on t h e t h e r m o c l i n e p r o p a g a t i n g o n - s h e l f which p r o d u c e t h e f e a t u r e s s e e n i n t h e S.A.R. image ( F i g . 9 ) . (Near v e r t i c a l l i n e s show C . T . D . d i p s ) .

would p r o d u c e e v e n lower R i c h a r d s o n number v a l u e s . p r o f i l e s h a v e a l s o shown small-scale

(
.02 a W

g -.02 0

-.06 cn I-

z W z .06 0 IL

W

a

a H 0 0

0

z

5

-.06 3

a

(3

a K

.O2kzZzd

w -.02

650

700

750

WAVELENGTH (nm)

F i g . 9 . R e c o n s t r u c t i o n of an o b s e r v e d r e f l e c t a n c e s p e c t r u m ( f o r a d i a t o m p o p u l a t i o n ) t h r o u g h t h e u s e o f G a u s s i a n s h a p e d components. The b o t t o m p a n e l shows t h e d i f f e r e n c e between t h e measured and reconstructed spectra.

329

1.0 *

0.8 cl

9

lx

w

V)

m

0

I

I

I

I

I

I

I

I

I

I

I

-

-

0.4 0.2 0.6

0W

0

u)

a

g

z

IZ

l0

0

W -I LL

w

a

g

0.3 0.2 0.1

0

*

z

o

a

ijj -0.1 cn 3 a -0.2 0

a 0 a a

0.2

0

W

0.2 650

700

750

WAVELENGTH ( n m )

F i g . 1 0 . R e c o n s t r u c t i o n of an o b s e r v e d r e f l e c t a n c e s p e c t r u m ( f o r a v i s u a l l y d i s c o l o u r e d bloom of Mesodinium rubrum) t h r o u g h t h e u s e of Gaussian shaped components. The lower p a n e l i l l u s t r a t e s t h e d i f f e r e n c e between t h e measured and r e c o n s t r u c t e d s p e c t r a .

330 f l a g e l l a t e s o r Plesodinium rubrum. V7e f i n d t h a t e a c h d i f f e r e n t a l g a l g r o u p h a s d i f f e r e n t o p t i c a l

properties.

F o r example, even though t h e e x t r a c t e d c h l o r o p h y l l

c o n t e n t i s t h e same f o r two g r o u p s , t h e most s i g n i f i c a n t f l u o r e s c e n c e l i n e may b e l o c a t e d a t d i f f e r e n t w a v e l e n g t h s . i l l u s t r a t e s t h i s point.

Table 2

The c h l o r o p h y l l 5 c o n c e n t r a t i o n was 6 . 0

mg/m3 a t b o t h s t a t i o n s 1 0 and 34.

However, f o r s t a t i o n 1 0 , t h e

main f l u o r e s c e n c e l i n e i s a t 682 nm, w h i l e f o r s t a t i o n 3 4 , it i s l o c a t e d a t 6 9 2 nm.

TABLE 2

Comparison o f G a u s s i a n a m p l i t u d e s f o r s t a t i o n s h a v i n g sin?ilar e x t r a c t e d c h l o r o p h y l l 5 concentration b u t d i f f e r e n t phytoplankton.

Dominant PhytoChlorophyll 5 A 6 8 2 ( ~l o 3 ) Station Plankton concentration 10 34 3

33 2

38

diatoms Mesodinium flage 1l a t e s Mesodinium dinoflagellates Mesodinium

A

~

~

l o~ 3 )(

Ax7 1 0 ( ~

6.0 6.0

1.26 0.23

0.33 0.94

-0.05 0.49

14.1

1.46

1.28

0.33

14.0

0.19

1.01

0.54

2.1

0.43

2.8

0.29

-0.6

0.25

lo3)

-0.51 -0.60

F i g u r e 11 i l l u s t r a t e s t h e r e l a t i o n s h i p between t h e a m p l i t u d e s a t 7 1 0 nm and 682 nm f o r t h e f o u r g r o u p s .

F o r a l l b u t t h e Mesodinium

s p e c t r a t h e a m p l i t u d e a t 710 nm i n c r e a s e s a t a b o u t one h a l f t h e r a t e of t h e 682 nm a m p l i t u d e .

Note t h a t t h e a r t i f i c i a l l i n e a r b a s e l i n e

i s r e s p o n s i b l e f o r t h e n e g a t i v e 7 1 0 nm a m p l i t u d e s .

W e are not

i n t e r p r e t i n g t h e s e a s a b s o r p t i o n and v i s u a l i n s p e c t i o n and t h e l i t e r a t u r e supports t h i s . S i m i l a r l y , t h e c h o i c e of a l i n e a r b a s e l i n e c o l o u r s o u r i n t e r p r e t a t i o n o f t h e a p p a r e n t s l o p e o f t h e Mesod i n i u m d a t a i n F i g u r e 11.

I n s p e c t i o n of t h e s p e c t r a from t h e

v i s u a l l y d i s c o l o u r e d s t a t i o n s ( F i g . 3) shows t h a t t h e s h o r t wavel e n g t h s e n d of t h e b a s e l i n e i s b e i n g l i f t e d by p h y c o e r y t h r i n f l u o r e s c e n c e n e a r 6 0 0 nm.

W e c o n c l u d e t h a t Mesodinium rubrum h a s

o n l y a s m a l l and c o n s t a n t amount of c h l o r o p h y l l 5 f l u o r e s c e n c e a t 682 nm ( a b s o l u t e c o n c e n t r a t i o n s are d i f f i c u l t t o c a l c u l a t e a t t h i s

331 T h i s is i n t e r e s t i n g from a b i o l o g i c a l

s t a g e i n our a n a l y s i s ) .

p o i n t o f view b e c a u s e a l t h o u g h Mesodinium rubrum i s c a p a b l e of p h o t o s y n t h e s i s i t i s n o t a p l a n t , b u t a p r o t o z o a n c o n t a i n i n g what a r e r e g a r d e d as " i n c o m p l e t e s y m b i o n t s " - e s s e n t i a l l y j u s t c h l o r o p l a s t s ( T a y l o r , B l a c k b o u r n and B l a c k b o u r n , 1 9 7 8 ) .

The a b s e n c e of

f l u o r e s c e n c e a t 6 8 2 nm p r e s u m a b l y means t h a t t h i s o r g a n i s m l a c k s t h e form o f c h l o r o p h y l l 5 which n o r m a l l y c o n s t i t u t e s 80% of t h e c e l l t o t a l i n o t h e r k i n d s of p h y t o p l a n k t o n

(Prgzelin, 1981).

The many

forms of c h l o r o p h y l l 5 (which a r e n o t d i f f e r e n t i a t e d i n t h e r o u t i n e e x t r a c t i v e p r o c e d u r e s u s e d by o c e a n o g r a p h e r s ) a r e u n e q u a l l y d i s t r i b u t e d w i t h i n t h e p l a n t p h o t o s y n t h e t i c mechanism ( G o v i n d j e e and B r a u n , 1 9 7 4 ) and t h e r e a r e some i n d i c a t i o n s t h a t t h e r e l a t i v e amounts o f

f l u o r e s c e n c e c h a n g e s between a l g a l t y p e s ( G o e d h e e r , 1 9 7 2 ) o r i n a g e i n g o r l o w l i g h t a d a p t e d c e l l s (Brown, 1 9 6 7 ) .

3 .O

0

Mesodinium

A

dinof lagellotes

2.0

I .o

J

a

0

5

a - I .o

-0 5

0

0 5

10

15

A M P L I T U D E 68Znrn ( I O - I~ F i g . 11. The r e l a t i o n s h i p between t h e G a u s s i a n a m p l i t u d e s a t 6 8 2 nm and 712 nm i n 39 r e f l e c t a n c e s p e c t r a from B r Y t i s h Columbia c o a s t a l waters. S p e c t r a t r o m Mesodinium rubrum p o p u l a t i o n s show a s i g n i f i c a n t l y d i f f e r e n t signature, thus allowing t h i s species t o be r e m o t e l y d i f f e r e n t i a t e d from o t h e r forms of p h y t o p l a n k t o n w i t h i n t h e s t i p p l e d zone ( e x t r a c t e d c h l o r o p h y l l g c o n c e n t r a t i o n 4 199 mg/m3).

-

332 ESTIMATION OF CHLOROPHYLL TON POPULATIONS

A

CONCENTRATION FOR DIFFERENT PHYTOPLANK-

W e a r e c u r r e n t l y i n v e s t i g a t i n g w h e t h e r it i s p o s s i b l e t o u s e t h e

methods d e s c r i b e d h e r e t o r e m o t e l y d e t e c t e i t h e r g r o s s t a x o n o m i c o r p h y s i o l o g i c a l changes i n a p h y t o p l a n k t o n p o p u l a t i o n .

For t h i s d a t a

s e t w e c a n d i f f e r e n t i a t e Mesodinium rubrum p o p u l a t i o n s from t h o s e of d i n o f l a g e l l a t e s , d i a t o m s and f l a g e l l a t e s on t h e b a s i s o f t h e 7 1 0 nm e m i s s i o n . Where t h e a m p l i t u d e of t h e 7 1 0 nm G a u s s i a n i s nega t i v e o r t h e 6 8 2 nm G a u s s i a n i s g r e a t e r t h a n 0 . 0 0 0 5

(unstippled area

i n F i g . 11) w e c a n c a l c u l a t e t h e c h l o r o p h y l l c o n c e n t r a t i o n a c c o r d i n g t o t h e formula: mgChl a/m3 = 0 . 4

+

(28.34A682

+

77.66Asg2

-

16.48A710) x

lo2

(1)

F i g u r e 1 2 i l l u s t r a t e s t h e . a g r e e m e n t between t h i s c a l c u l a t i o n and e x t r a c t e d c h l o r o p h y l l g c o n c e n t r a t i o n f o r p o p u l a t i o n s dominated by d i n o f l a g e l l a t e s , diatoms o r f l a g e l l a t e s .

The c o r r e l a t i o n c o e f f i c -

i e n t i s 0 . 9 6 w h i l e t h e s c a t t e r a b o u t t h e 1:l l i n e i s a b o u t 2 1 . 0 mg/m3.

This i s b e t t e r than using t h e eigenvector a n a l y s i s

(Fig. 7 ) .

15

10 R= 0.963 AC = .f I MG/M3

5

0

EXCLUDING DISCOLOURED STATIONS

I . 5

10

15

20

EXTRACTED CHLOROPHYLL g (MG/M3)

F i g . 1 2 . The a g r e e m e n t between t h e e x t r a c t e d c h l o r o p h y l l a concent r a t i o n and t h a t c a l c u l a t e d on t h e b a s i s of t h r e e G a u s s i a n s i g n a l s a t 682 nm, 692 nm and 7 1 0 nm.

333 Where t h e a m p l i t u d e o f t h e 710 nm G a u s s i a n i s p o s i t i v e and t h e 6 8 2 nm G a u s s i a n less t h a n 0 . 0 0 0 5

( s t i p p l e d area i n Fig.

l l ) , w e can

r e m o t e l y c l a s s i f y t h e dominant o r g a n i s m as Mesodinium a n d c a l c u l a t e t h e c o n c e n t r a t i o n of c h l o r o p h y l l mgChl g/m3 = 5.19

+

( 5 . 6 5 A710)

2 from t h e 710 nm a m p l i t u d e .

x 1 02

(2)

F i g u r e 1 3 i l l u s t r a t e s t h e a g r e e m e n t between t h i s c a l c u l a t i o n and e x t r a c t e d c h l o r o p h y l l o v e r t h e r a n g e 4 t o 199 mg c h l o r o p h y l l a / m 3 . The c o r r e l a t i o n c o e f f i c i e n t i n t h i s case i s 0.95,

while t h e s c a t t e r

a b o u t t h e l i n e i s a s g r e a t a s f 4 mg/m3.

200 r

/

/@

'IONS

AMPLITUDE 710nm ( x 100) F i g . 1 3 . The r e l a t i o n s h i p between t h e a m p l i t u d e of t h e 710 nm G a u s s i a n e m i s s i o n and t h e e x t r a c t e d c h l o r o p h y l l g c o n c e n t r a t i o n f o r s e v e n s p e c t r a o b t a i n e d from v i s u a l l y d i s c o l o u r e d blooms o f M z d i n i u m rubrum. R e g r e s s i o n a n d c o r r e l a t i o n c o e f f i c i e n t s do n o t include highest point. ~~

334 SUMMARY AND CONCLUSIONS

The f l u o r e s c e n c e l i n e h e i g h t (FLH) method c a n b e s u c c e s s f u l l y employed t o r e m o t e l y measure t h e c h l o r o p h y l l g c o n c e n t r a t i o n i n many o c e a n i c a r e a s , however, where l a r g e blooms o f t h e c i l i a t e Mesodinium rubrum a r e e n c o u n t e r e d , t h e a c c u r a c y o f t h e FLH c a l c u l a t i o n i s s i g n i f i c a n t l y a f f e c t e d by an a p p a r e n t s h i f t i n t h e emission wavelength. From an a n a l y s i s of 56 r e f l e c t a n c e s p e c t r a o b t a i n e d i n c o a s t a l B r i t i s h Columbia w a t e r s , w e f i n d a t l e a s t t h r e e p r i n c i p a l G a u s s i a n s h a p e d f l u o r e s c e n c e l i n e s , l o c a t e d a t 682 nm, 6 9 2 nm and 7 1 0 nm. I n t h i s d a t a s e t s p e c t r a from v i s i b l y d i s c o l o u r e d blooms of Mesodinium rubrum c o u l d b e s t b e m o d e l l e d by a s s u m i n g l a r g e e m i s s i o n s a t 7 1 0 nm and 692 nm, w i t h v e r y s m a l l e m i s s i o n a t 682 nm. The c o n c e n t r a t i o n o f e x t r a c t a b l e c h l o r o p h y l l 5 ( a l l f o r m s ) f o r t h e s e p o p u l a t i o n s c o u l d b e a c c u r a t e l y e s t i m a t e d from t h e h e i g h t of t h e 7 1 0 nm G a u s s i a n .

F o r a l l o t h e r s p e c t r a , where t h e a p p a r e n t

f l u o r e s c e n c e l i n e i s l o c a t e d n e a r 685 nm, t h e e x t r a c t a b l e c h l o r o phyll

a

i s e s t i m a t e d e i t h e r b y t h e FLH method o r an e q u a t i o n

employing t h e a m p l i t u d e s a t 682 nm, 6 9 2 nm and 7 1 0 nm.

335 REFERENCES and Gower, J . F . R . , 1 9 8 1 . A i r b o r n e B o r s t a d , G . A . , Brown, R . M . , r e m o t e s e n s i n g o f sea s u r f a c e c h l o r o p h y l l and t e m p e r a t u r e a l o n g t h e o u t e r B r i t i s h Columbia c o a s t . P r o c e e d i n g s of t h e 6 t h Canadian Symposium on Remote S e n s i n g , H a l i f a x , N . S . , pp. 541-541. B o r s t a d , G.A. and Gower, J . F . R . , 1983. A s h i p and a i r c r a f t s u r v e y of phytoplankton c h l o r o p h y l l d i s t r i b u t i o n i n t h e e a s t e r n A r c t i c ( i n press). Canadian A r c t i c . Brown, J . S . , 1967. F l u o r o m e t r i c e v i d e n c e f o r t h e p a r t i c i p a t i o n of c h l o r o p h y l l 5 - 695 i n s y s t e m s of p h o t o s y n t h e s i s . Biochem. Biophys. A c t a . , 143:391-398. Clark, D.K., 1981. P h y t o p l a n k t o n a l g o r i t h m s f o r t h e Nimbus-7 CZCS. I n : J . F . R . Gower ( E d i t o r ) , Oceanography From S p a c e , Plenum P r e s s , N e w York, pp. 227-228. Doerf f e r , R. , 1 9 81. F a c t o r a n a l y s i s i n ocean c o l o u r i n t e r p r e t a t i o n . I n : J.F.R. Gower ( E d i t o r ) , Oceanography From S p a c e , Plenum P r e s s , N e w York, pp. 339-345. 1972. Fluorescence i n r e l a t i o n t o photosynthesis. Goedheer, J . C . , Ann. Rev. P l a n t P h y s i o l . , 23:87-112. Clark, D.K., Brown, J.W., Brown, O.B. , E v a n s , R . H . , Gordon, H . R . , Broenkow, W.W., 1983. P h y t o p l a n k t o n pigment c o n c e n t r a t i o n s i n t h e Middle A t l a n t i c B i g h t : a comparison of s h i p d e t e r m i n a t i o n s and CZCS e s t i m a t e s . Appl. O p t i c s , 2 2 :20-36. Gordon, H . R . , C l a r k , D . K . , M u e l l e r , J . L . and H o v i s , W . A . , 1980. P h y t o p l a n k t o n p i g m e n t s from t h e Nimbus-7 C o a s t a l Zone C o l o u r S c a n n e r : Comparisons w i t h s u r f a c e measurements. Science, 2 1 0 :6 3-6 6 . Gower, J . F . R . , 1980. O b s e r v a t i o n s o f i n s i t u f l u o r e s c e n c e of Boundary L a y e r M e t e o r o l o g y , chlorophyll 5 i n Saanich I n l e t . 1 8 : 235-245. 1981. U s e o f i n v i v o f l u o r e s c e n c e Gower, J.F.R. a n d B o r s t a d , G . A . , l i n e a t 6 8 5 nm f o r remote s e n s i n g s u r v e y s of s u r f a c e c h l o r o p h y l l a. I n : J . F . R . Gower ( E d i t o r ) , Oceanography From S p a c e , Plenum Fress, N e w York, pp. 329-338. Gower, J . F . R . , L i n , S . , and B o r s t a d , G . A . , 1983. The i n f o r m a t i o n cont e n t of d i f f e r e n t o p t i c a l s p e c t r a l r a n g e s f o r r e m o t e c h l o r o p h y l l estimation i n c o a s t a l waters. I n t . J . Remote S e n s i n g ( i n p r e s s ) . G o v i n d j e e and B r a u n , B . Z . , 1 9 7 4 . L i g h t a b s o r p t i o n , e m i s s i o n and S t e w a r t ( E d i t o r ) , A l g a l Physiology photosynthesis. I n : W.D.P. and B i o c h e m i s t r y , Univ. C a l i f o r n i a P r e s s , B e r k e l e y , pp. 346-390. 1979. G o v i n d j e e , Wong, D . , P r e z e l i n , B.B. and Sweeney, B.M., C h l o r o p h y l l 5 f l u o r e s c e n c e of G o n y l a u l a x p o l y e d r a grown on a Photochem. l i g h t - dark c y c l e a f t e r t r a n s f e r t o c o n s t a n t l i g h t . P h o t o b i o l . , 30:405-411. M u e l l e r , J . L . , 1913. The i n f l u e n c e of p h y t o p l a n k t o n on ocean colour spectra. PhD. T h e s i s , Oregon S t a t e U n i v e r s i t y , C o r v a l -

lis.

N e v i l l e , R.A. and Gower, J . F . R . , 1 9 7 7 . P a s s i v e remote s e n s i n g Of phytoplankton v i a c h l o r o p h y l l fluorescence. J . Geophys. R e s . , 82 :3487-3493. P r g z e l i n , B.B., 1981. Light reactions irl photosynthesis. In: T. P l a t t ( E d i t o r ) , P h y s i o l o g i c a l b a s e s of p h y t o p l a n k t o n e c o l o g y . Can. B u l l . F i s h . Aquat. S c i . , 2 1 0 :1 - 4 2 . S m i t h , R . C . and B a k e r , K.S., 1982. O c e a n i c c h l o r o p h y l l c o n c e n t r a t i o n s a s d e t e r m i n e d by s a t e l l i t e (Nimbus-7 C o a s t a l Zone C o l o u r S c a n n e r ) . M a r . B i o l . , 66:269-280.

336 Taylor, J.F.R., B l a c k b o u r n , D . J . a n d B l a c k b o u r n , J . , 1 9 7 1 . The red-water c i l i a t e Mesodinium r u b r u m a n d i t s “ i n c o m p l e t e symb i o n t s ” : a review i n c l u d i n g new u l t r a s t r u c t u r a l o b s e r v a t i o n s . J . Fish. R e s . B d . C a n a d a , 28:391-407.

337

SATELLITE REPRESENTATION OF FEATURES OF OCEAN CIRCULATION INDICATED BY CZCS COLORIMETRY C.S. YENTSCH Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, Main 04575, U.S.A.

ABSTRACT Coastal Zone Color Scanner (CZCS) images have been used to demonstrate that the major factors which influence the patterns of ocean color and hence the abundance of phytoplankton are associated with the density discontinuities of large scale ocean currents. This argues that variations in color in large scale patterns are reflecting phytoplankton growth. Pigment patterns, therefore, are not passive tracers of surface water movement.

INTRODUCTION There is now a considerable number of CZCS images which allow the biological oceanographer to visually see patterns of phytoplankton pigments over large regions of the earth's oceans. In examining these images, one's first impression is that the ocean is characterized by highly diverse patterns of pigment concentrations. It is also evident that the spatial magnitude of these patterns differ. Immediately we can ask : "Are these patterns the result of spatial movements of phytoplankton ? " "Can phytoplankton be considered a conservative tracer of the water masses, thereby producing patterns similarly seen by addina cream to a teacup ? " Or, "Are these patterns explainedinterms of factors other thanhorizontal transport, specifically those factors which we believe regulate the growth and abundance of phytoplankton in the oceans ? " These questions are important to oceanographers since the distribution of phytoplankton in time and space, and the mechanisms controlling this distribution, have been obtained largely by one-dimensional shipboard observations which are limited in coverage of both time and space : the need for remote sensing is driven by the desire to view the

enormity of ocean space in synoptic fashion and to test wether or not we have not biased our impressions by quasi-synoptic observation on ships. This paper has two main goals. First, in a general sense, to acquaint the uninitiated reader with some of the factors affecting large scale distribution of phytoplankton in the oceans and to demonstrate how the spatial distributions are viewed from space. The second goal of the paper is to demonstrate and interpret the large scale patterns of phytoplankton in terms of the major planetary inertial forces that are operatinq on water masses. I will argue that the spatial patterns are reflecting the degree of buoyant forces in the water mass. That is, spatial changes that one observes in these images are regulated by the intensity of vertical mixing throughout the water column. If correct, then the large scale patterns, and perhaps the small scale patterns as well, are reflecting the net growth of phytoplankton. In other words, the distribution of phytoplankton pigment abundance observed in the surface waters of the oceans is representing growth processes and not merely the redistribution of abundance. T o give substance to these goals, I will utilize satellite images and conceptual models as well as water column observations. I have specifically chosen regions of the oceans where fluid forces favor the destruction of buoyancy in the water column, thereby

promoting vertical effects associated the rotary motions the interaction of

mixing. These forces are derived from the shear with major frontal regions of ocean currents, of mesoscale ocean eddies, and friction from tidal flow across shallow waters.

Large scale features of phytoplankton distribution associated with general circulation. For phytoplankton, the extremes of poverty and luxury are defined by the oligotrophic central gyres of the ocean on one hand, and the eutrophic waters that lie adjacent to major continents on the other hand. Between these extremes are sharp gradients of phytoplankton abundance which are correlated with water masses which have an extreme baroclinicity and intense horizontal advection. The effects of ocean currents were seen from space first in satellite thermal images. However, more recently, CZCS colorimetry has demonstrated the sharp color discontinuities associated with ocean currents, thus delineating marked gradients in phytoplankton abundance. The question is : How does large scale flow change the

339

distribution of phytoplankton ? The density field of large ocean currents are "baroclinic analogues" of upwelling and represent the largest, perhaps most important, mechanism in the world's ocean of vertical transport of nutrients (Yentsch, 1974). Some of the best examples of these extensive color fronts representing discontinuities in phytoplankton abundance are found in regions occupied by the western boundary current systems. Figure 1 is CZCS Orbit 0 2 6 4 6 that features the Gulf Stream system from Cape Hatteras to Yarmouth, Nova Scotia. The reader's attention is called to the delineation by color of slope and Gulf Stream waters.

Fig. 1. CZCS Orbit 0 2 6 4 6 featuring the Gulf Stream system from Cape Hatteras to Yarmouth, Nova Scotia. The process which is responsible for the delineation of color concerns augmentation of phytoplankton growth which is directly associated with the geostrophic flow. The first fluid dynamic model of the Gulf Stream was produced by Carl Rossby. He considered the Stream a major jet driving into a non-rotating stratified fluid (Fig. 2 ) . When the earth's rotation (C ) was considered as a f

balance to the pressure gradient (P ) , the Rossby model predicted g that secondary cross-stream flow would be associated with the horizontal advection. This cross-stream flow was transported alonq lines of equal density from the Sargasso Sea into the slope and coastal waters off New England. The important aspect of this model

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Fig. 2 . The Rossby (1936) model of the Gulf Stream system off New England. horizontally and vertically - that is, from the deep waters of the Sargasso Sea to the surface waters of the euphotic zone in slope waters off New England. Examples of the effect of this transport can be seen by comparing density structure across the Gulf Stream (Fig. 3 ) with the distribution of a limiting nutrient such as nitrate-nitrogen in the same section (Fig. 4 ) . Facing into the picture, one sees line of equal density intersecting at station 9, which is referred to as the "cold wall", since cooler, deeper waters are elevated to that side of the Gulf Stream. The enrichment process is signaled by the fact that lines of equal density are mirrored by lines of equal distributions of nitrate which, as mentioned above, is the limiting nutrient for phytoplankton growth in these waters. It should be noted that the fluid dynamics behind movements of the water along the isopycnals, is still not well understood and the resultant magnitude of vertical transport is not well known. In general, the cause of the movement along isopycnals can be considered an imbalance between the pressure gradient (P ) and the Coriolis forces (C,) associated with the g mass transport of the Gulf Stream itself. Regardless of the cause, the fertility of the waters lying adjacent to the main thrust of the Gulf Stream (in the cold wall) can be traced along lines of equal density from the north central Sargasso Sea to the cold wall of the Gulf Stream (Fig. 5). The enriched water entering the eupho-

341

tic zone in the cold wall causes a marked discontinuity in the spatial abundance of phytoplancton chlorophyll. It is this variation that one clearly sees from space by way of CZCS colorimetry as a marked difference in water color which distinguishes the slope waters from that in the Gulf Stream. Other examples of phytoplankton augmentation associated with major ocean currents can be observed in CZCS images of Florida and the western Gulf of Mexico. In this region, the thermal loop current forms a front which is due to the entry of equatorial water into the western Gulf of Mexico through the Straits of Yucatan (Fig. 6). The equatorial water penetrates as far north as

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N and essentially encompasses most of the region of thewestern Gulf of Mexico. The CZCS colorimetric pattern of this image correlates with the general thermal pattern shown in the infrared image (Fig. 6). This correlation shows that warm equatorial waters are associated with low concentrations of phytoplankton pigment and

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343

Fig. 6 . NIMBUS-7 imagery from Orbit 1965 on 15 March 1979. CZCS Channel 1 ( 4 4 3 nm) image where light tone denotes high attenuation of blue due to phytoplankton chlorophyll. Channel 6 images of sea-surface temperature variation in which the dark tone depicts cold water. cooler waters with high levels of phytoplankton pigment. The nutrient enrichment process and increased phytoplankton abundance are a combination of the flow of the loop current and the constraints placed on that current by the shape of the Florida peninsula continental platform. The position of the pigment fronts outlined from the image follow the trend of the isobaths along both coasts of the Florida peninsual (Fig. 7). The general position of these fronts is interpreted to be associated with the mass transport on

344

Fig. 7. NIMBUS-7 CZCS image from Orbit 30 on 2 November 1980, of the Florida region showing chlorophyll concentration (dark) on the coastal shelf (upper) and the major bathymetric features of the region (lower).

345

either side of the peninsula. This is confirmed by comparing the dynamic topography on the western side of the Gulf (Fig. 8). The distributisn of sea level height shows that channel constraints of the mass flow by the Florida escarpment augments the horizontal velocity of the flow. Along with this augmentation of flow, an imbalance between the Coriolis and pressure forces create the isopycnal flow which causes the enrichment of the waters adjacent to the peninsula. Therefore, it is through these processes that

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346

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Fig. 9. Chart of the depth, in hundreds of meters, of the isothermal surface, showing the Gulf Stream, nine cyclonic rings, and three anticyclonic rings. Contours based on data obtained between 16 March and 9 July 1975.(From Richardson et al., 1978). t i m e s c l o s e ( p i n c h o f f ) p o r t i o n s o f w a t e r masses o n e i t h e r s i d e o f t h e G u l f S t r e a m . The e d d i e s f o r m e d b y " p i n c h i n q o f f " a w a r m c o r e

of S a r g a s s o S e a w a t e r a r e r e f e r r e d t o a s w a r m c o r e r i n s s a n d r e s i d e i n t h e s l o p e w a t e r t o t h e w e s t o f t h e Gulf S t r e a m ( F i p . 1 0 ) . Cold

c o r e r i n g s a r e t h e r e v e r s e i n t h a t by t h e " p i n c h i n a o f f " p r o c e s s , s l o p e w a t e r i s e n t r a i n e d i n t h e c e n t e r . These c o l d core r i n g s g e n e r a l l y move i n t o t h e S a r g a s s o S e a

( F i g . 1 1 ) . T h e s e rinus were

Fig. 10. Warrr core ring (center) and new ring forming on right.

347

Fig. 11. Cold core ring (CCR) off Cape Hatteras. first observed by Fritz Fuglister using shipboard temperature measurements, however, both warm and cold core rinqs, because of their sharp thermal gradients, are easily identified in satellite thermal imagery. Satellite observation by CZCS colorimetry has demonstrated that both warm. core and cold core rinqs are also well defined in terms of their differences in color

:

The sharp thermal

gradient as seen by the satellite, are mirrored by gradients in phytoplankton pigment (Gordon et al., 1982). The question we can now ask is

:

Why is this so ?

The rotary motion of ocean eddies to phytoplankton growth concerns changes in the vertical distribution of the density field within the eddy. If we assume that phytoplankton growth is nutrient limited and distribution of nutrients is reflected by the density field, then the following concepts (Fig. 12) influence spatial patterns of growth throughout the eddy. N

+

and N- represent

two water masses of nutrient-rich, cold, dense and nutrient-poor, warm buoyant water, respectively. These are enclosed in a cylinder which simulates the dimensions of an oceanic eddy. The two water masses are separated by the density nutrient boundary layer (Nb) which for this discussion we can refer to as the thernocline. In the non-rotational stationary Rode, the boundary between the two water masses is horizontal across the cylinder. However, when the cylinder is rotated with velocities in the surface beina somewhat greater than at depth, the Coriolis and other inertial forces will be balanced by the pressure gradient created by the aeostrophic flow within the eddy. In the anti-cyclonic mode, sea surface level domes up around the axis (warm core) while in the cyclonic mode,

348

STATIONARY

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Fig. 12. Geostrophic relationships in warm and cold core rings. Nb, nutrient boundary: Ze, euphotic zone: H and L are the high and low velocities.

it will be depressed in the axis (cold core). In the anti-cyclonic eddies, such as the warm core rina, the lighter water will accumulate at the center and the heavy water will be swept to the rim of the eddy. Assuming that the volume of the eddy is being maintained, the boundary dips downwardtowards the axis and upward towards the rim of the eddy. The reverse situation occurs in the cold core ring. If the eddy is illuminated from the surface, and the photic layer (Z ) resides at a comparable depth and boundary layer, we can see why productivity is enhanced due to the upward displacement of nutrient rich water. This upward displacement of the nutrient boundary layer allows vertical mixing to easily transport nutrients to the euphotic zone. Therefore, the spatial pattern of phytoplankton distribution reflects relative nutrient addition to the euphotic layer by the differences vertically in the level of the boundary between the two water masses. The explanation for the observed distribution of phytoplankton pigments in rings argues that geostrophic principles apply to these rings. Implicit to this nutrient enrichment hypothesis is the idea that the rotary motion induces nutrient transport along isopycnals and phytoplankton production occurs when these isopycnals intersect the euphotic layers. Coastal tidal processes Simpson and Hunter (1974), Pingree and Griffiths (1978), and Pingree et al. (1975) have pioneerd the approach of using satellite

349

imagery and modelling to the study of tidal frontal phenomena in the waters around Great Britain. Remote sensing was needed to obtain information on water mass structure and its pigment distribution and to obtain these parameters in a synoptic fashion over wide areas. In the final analysis, the concepts derived from either observation and/or numerical modelling were substantiated or reinforced by remote sensing capabilities. In this section, I will describe a similar study which essentially began in 1927 with a series of shipboard observations by H.B. Bigelow in the area of the Gulf of Maine and Georges Bank. His conclusions as the result of the observation are confirmed by satellite imagery taken in 1979. In the beginning, Bigelow measured the thermal structure of water masses of the Gulf of Maine and Georges Bank and computed the stability of these water masses to outline different regimes of vertical mixing. From this analysis, he concluded that the different regimes of temperature which outline the areas of vertical mixing were due to the intense tidal action throughout the area. More recently, Garrett et al. (1978) subjected this region (Fig. 13) to an analysis using a numerical model developed by Simpson and Hunter (1974). This model proposes that the difference between mixed and stratified waters is dictated by an index or ratio of the potential energy (required to thoroughly mix the water) to the rate of energy that is dissipated by the flow or tidal current across the bottom. The relevant parameter of index for separating mixed from stratified waters by tide is referred to as log H/U3, where the water, H, is divided by the tidal velocity frictional component, U3. Essentially, this numerical model (Fig. 13) confirmed Bigelow's original observations that tidally mixed areas were centered on Georges Bank and Nantucket Shoals. It also identified other tidal regions off Nova Scotia and in the Bay of Fundy. The question now asked is how real the model is and/or how accurate Bigelow's original observations are -this is where the satellite images can help us. Comparison of satellite thermal and colorimetric imagery (CZCS, Fig. 14) with the numerical model and Bigelow's observations, confirms that much of the mixing is tidally driven. In the thermal image, the light areas indicate warm water and the dark areas indicate colder waters. The region of Georges Bank and Nantucket Shoals clearly shows u p as well as the cold tidally mixed regions off Nova Scotia. Dark filamentous segments appear to be intrusions of either warm slope water of the Gulf Stream and

350

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Fig. 13. Numerical model of tidal mixing, log H/W3 (after Garrett et al., 1978); 1.5 indicates areas totally mixed by tides. other mixed areas that had not been identified by either observations or modelling. Satellite thermal imagery compared with the bathymetry of this area gives information with regard to the critical mixed depth for tidal activity. The mixed fronts around Georges Bank appear to center on the 60m isobath; this depth appears to be rather consistent for the entire region. The significance of tidal mixing on phytoplankton abundance is explained as follows : During the months when the water column is being heated in this region, the greatest buoyancy of surface waters tends to isolate the nutrient rich water from the euphotic zone. Therefore, the restoration of growth by vertical mixing of nutrients into the euphotic zone becomes crucial in regulating the rate of phytoplankton growth. The conceptual model of the density and nutrient distribution across Georges Bank explains why the Bank itself imparts color and temperature signatures on the water (Fig. 15). Nutrient rich water in deeper waters is brought up into the euphotic zone by the tidal action at the frontal edge on either side of the Bank. This water is mixed across the top of the Bank which is in the euphotic zone and promotes luxurious growth on top of the Bank. In summary, the CZCS colorimetry shows that high concentrations of phytoplankton pigment are located on the Bank and the other frontal regions which outline the areas of vertical mixing, such as Nantucket Shoals. The low phytoplankton pigment concentrations

351

Fig. 14. Orbit 3326 14 June 1 9 7 9 . Top image : sea surface temperature; dark, cold water; light area, warm water. Bottom image : phytoplankton pigment; dark, high pigment; light, low pigment. (Yentsch and Garfield, 1981). occur in the slope waters or in the central region of the Gulf of Maine where tidal mixing and bottom friction action is not effective. Passive tracer or growth There appear to be two obvious hypotheses to attribute to the patterns of ocean color. 1) Distribution of phytoplankton pigments are passive tracers to the movements of surface waters and, 2) Distribution of phytoplankton reflects the fluid aynamics of the

352

isotherms

Fig. 1 5 . Conceptual diagram of nutrient enrichment on Georges Bank. water masses which supplies nutrients for phytoplankton growth the hypothesis of nutrient enrichment for growth. The first hypothesis is unattractive because the satellite images that I have observed show a close correlation between the thermal signatures and the colorimetric signatures. If the low nutrient concentrations in the surface waters of the ocean are limitinq growth and hence, the abundance of phytoplankton, one would expect that the horizontal diffusion would progressively disperse t!ie phytoplankton. Any correlation between temperature and. c o l o r would come about almost by accident. The nutrient enrichment hypothesis argues that it is the vertical flux of nutrients which requlate phytoplankton abundance. It is through this process that one can account for the close correspondence between temperature and water color observed in satellite imagery. This hypothesis also argues that in order to have correspondence between temperature and color, growth must be in excess of that removed by grazing or sinking by the phytoplankton piqment, and is consistent with our concept of how productivity is requlatcd. In short, regulation of abundance is brouqht about by periodic injections of nutrients which change the growth rate in the surface waters of the ocean. The satellite imagery shown in this paper demonstrates tliat color chanaes are closely associated with vertical mixing. The CZCS color pigment patterns are not an undecipherable mix, but clearly reflect the role vertical mixing plays in nutrient supply. A paradox arises

:

the acquisition of buoyancy to water masses is the

antithesis to growth. But growth occurs throuqhout the oceans because

certain forces tend to override the buoyant forces.

These forces are largely associated with ocean currents and the vertical mixinq as a result of bottom friction, and/or the diffe-

353

rences between the density of the water masses. The definition that I have used here of large scale motion requires better definition. The scale of the motions I am discussing are those that are influenced by earth's rotation

-

that is

water motions whose Rossby number is characteristically very smallhence the large scale motions are comparatively slower than the velocity imposed by the earth's rotation. One imagines that in

-

water masses where the Rossby number is very large that is, the flow is large compared to the earth's rotation - color pigment relationships could probably be treated in a Lagrangian sense. Flow of this sort is uncharacteristic of the open ocean. Summary At the opening of this symposium, Jacques Nihoul stressed that remote sensing occupies a companion role with conceptual and numerical modelling. Both are the principle tools by which oceanographers can study their medium. The numerical models used in this text and reported elsewhere, document the interrelationship between modelling and remote sensing. In order for this approach to be successful the modeller has to acquire a mental picture of the pattern of events that will occur in the sea and have the capAbility of comparing these patterns to a satellite image. As more satell-iteimagery becomes available to the biological oceanographer, pattern recognition will become important. This recognition will depend on better measurements of motions in the ocean interior, as well as an appreciation of the size of these features

ACKNOWLEDGEMENTS The author greatly acknowledges the assistance of Pat Boisvert and Jim Rollins in preparing the manuscript. The work was funded by the National Aeronautics and Space Administration, the Office of Naval Research, the National Science Foundation and the State of Maine. REFERENCES

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