Oceanography for Meteorologists

Citation preview

iiiiiiiiilii METEORQiOGISIS BY

SVERDRUP

OCEANOGRAPHY FOR

METEOROLOGISTS

FOR OCEANOGRAPHERS The Oceans: Their General Biology,

Physics, Chemistry, and

by H. U. Sverdrup,

Johnson, and R. H. Fleming

M. W.

OCEANOGRAPHY FOR

METEOROLOGISTS

BY

H. U.

SVERDRUP

Professor of Oceanography, University of California Director, Scripps Institution of Oceanography

H.

Soo.

.^1

New

York

PRENTICE-HALL, INC, 1942

Copyright, 1942, by

PRENTICE-HALL, INC. 70 Fifth Avenue, New York All rights reserved. No part of this book may be reproduced in any form, by mimeograph or any other means, without permission in writing from the publishers.

fNTED IN THE UNITED STATES OF AMERH'A

Preface

Up

to the present time, no adequate

book has existed from which a

meteorologist could readily obtain information as to findings in physical

oceanography that have bearing upon problems of the atmosphere. Nor has any text been available that, besides describing the methods used in physical oceanography, also contained a summary of our present knowledge of the current systems of the oceans and of the processes that maintain the currents.

The

It is to

fill

these needs that this

book has been

wTitten.

processes of the sea surface are of the most direct meteorological

significance.

Consequently, the temperature, the salinity, the currents

upper layers of the oceans, and the factors that control the existing conditions are the features which have been emphasized. Since these cannot be fully understood without taking the subsurface conditions into account, the deep-water circulation of the oceans has also been discussed. The theories of the large-scale ocean currents and of the wind-driven currents have been included, because in oceanography the application of hydrodynamics is of fundamental importance and because the treatment is similar to that of corresponding problems in the atmosphere. Tides and tidal currents have not been discussed, because the tides have not been shown to be of meteorological significance. Similarly, internal waves in the oceans have not been dealt with, but wind waves have been of the

included.

The

dynamics of the ocean currents and ocean areas are as yet incomplete, and be premature to generalize. Nevertheless, it has been

theoretical discussion of the

the factual information from therefore

it

may

many

attempted to overcome difficulties arising from differences in interpretation of incomplete data by placing emphasis on application of the equation of continuity in the description of the ocean circulation. No references to original papers are given, but several standard works are listed, some of which contain numerous references to meteorological and oceanographic literature. I am much indebted to my colleagues, Messrs. Johnson and Fleming, for their permission to make use of many parts of our common book, now in press: The Oceans: Their Physics, Chemistry, and General Biology. I am greatly obliged to Mr. John A. Fleming, Director of the Department of Terrestrial Magnetism, Carnegie Institution of Washington, for allow-

PREFACE

vl

unpublished data from the last cruise of the Carnegie. obliged to Mr. W. C. Jacobs for allowing me to use I several figures from his as yet unpublished paper on the exchange of heat and water vapor between the oceans and the atmosphere. Valuable assistance has been received from Mr. L. Lek, who has carried out a large number of calculations of transport by ocean currents and of net heat received by the oceans in the Northern Hemisphere. Most of the illustrations have been prepared especially for either the larger book or for this one, and credit for them is due Mr. E. C. La Fond. The manuscript has been critically examined by Mr. R. H. Fleming, and parts have been read by Messrs. J. Holmboe, of the University of California at Los Angeles, C.-G. Rossby, of the University of Chicago, and C. L. Utterback, of the University of Washington, all of whom have made helpful suggestions. To all these I extend my sincere thanks. Last, but not least, I wish to acknowledge the help received from my secretary. Miss Ruth Ragan, in correcting the manuscript, in reading

me free use of am also greatly

ing

proof,

and

in

preparing indices.

H. U. SVERDRUP Scripps Institution of OceanographyUniversity of California La Jolla, California

Literature

Most

of the following selected

books in oceanography and meterology

contain extensive bibliographies. Bjerknes, V., and

raphy.

Pt.

J.

W. Sandstrom.

Dynamic meterology and hydrog-

(Carnegie Institution, Publ. no. 88.)

I.

Washington.

146 pp., 1910. Bjerknes, V.,

J.

Bjerknes, H. Solberg, and T. Bergeron.

Hydrodynamik.

Berlin.

Julius Springer.

David. Physical and dynamical MacMillan, 2nd ed., 428 pp., 1939.

Brunt,

Physikalische

797 pp., 1933.

meteorology.

New

York.

Ocean waves and kindred geophysical phenomena. Cambridge University Press. 164 pp., 1934.

Cornish, Vaughan.

London.

Dorsey, N. Ernest.

(Amer. Reinhold Pub.

Properties of ordinary water-substance.

Chem. Soc, Monograph

Ser. no. 181.)

New

York.

Corp., 673 pp., 1940.

Dynamic meteorology. New York. Book Company, 365 pp., 1941. Krlimmel, Otto. Handbuch der Ozeanographie. Stuttgart.

Haurwitz, Bernhard.

McGraw-

Hill

horn.

Vol.

1,

526 pp., 1907;

vol. 2,

764 pp., 1911.

J.

Engel-

PREFACE

Marmer, H. A.

The sea.

New York.

vii

D. Appleton-Century Company.

312 pp., 1930. National Research Council. Physics of the earth. Vol. 5, Oceanography (National Research Council Bulletin no. 85), 581 pp., 1932.

Weather analysis and forecasting. Sverre. McGraw-Hill Book Company. 505 pp., 1941.

Petterssen,

New

York.

Sverdrup, H. U., M. W. Johnson, and R. H. Fleming. The oceans: Their physics, chemistry, and general biology. New York. PrenticeHall.

1942.

U. S. Weather Bureau.

Atlas of climatic charts of the oceans.

Bureau No. 1247), 130

pp., 1938.

(Weather

IVo

9o

Table of Contents

CHAPTER I.

PAGE

Introduction The Heat Budget

1

of the

Earth

Black-Body Radiation Selective Radiation and Absorption. Radiation from the Sun

1 1

Long-Wave Radiation to Space The Heat Budget of the Atmosphere The Atmosphere as a Thermodynamic Machine The Oceans as Part of the Subsurface of the Atmosphere 11.

2 3 4 4 6

.

....

Physical Properties of Sea Water. Salinity,

8 8 8 10 10 10 12 12 12

Temperature, and Pressure

Salinity

Temperature Pressure

Density of Sea Water Computation of Density and Specific Volume in Situ Thermal Properties of Sea Water Thermal Expansion Thermal Conductivity

Heat Latent Heat of Evaporation Adiabatic Temperature Changes in the Sea Freezing-Point Depression and Vapor-Pressure Lowering Other Properties of Sea Water

13 13 13 14

Specific

Maximum

....

Density

Compressibility Viscosity Diffusion Surface Tension Refractive Index Electric Conductivity

Eddy

Viscosity, Conductivity, and Diffusivity General Character of Eddy Coefficients Influence of Stability on Turbulence Numerical Values of the Vertical and Horizontal Eddy Viscosity and Eddy Conductivity Absorption of Radiation Absorption Coefficients of Distilled Water and of Pure Sea

14 15 15 15 15 16 16 17 17 17 17

20 22 26 26 27

Water Extinction Coefficients in the Sea ix

7

^

r>

TABLE

X

OF CONTENTS PAGB

CHAPTER II.

Physical Properties of Sea

Water

Influence of the Altitude of the

{Con't.)

Sun upon the Extinction

30 30

Coefficients

Large Extinction Coefficients in the Sea The Color of Sea Water Sea Ice Freezing and Melting of Ice Properties of Sea Ice

Cause

III.

of the

31

32 32 33

Observations in Physical Oceanography

35 35 37 37 38 40 40 41 43 44 44 44 46

Oceanographic Expeditions and Vessels Temperature Observations. Surface Thermometers Protected Reversing Thermometers

Unprotected Reversing Thermometers Thermographs Water-Sampling Devices Treatment and Analysis of Serial Observations Current Measurements Units and Terms Drift Methods Flow Methods IV.

The Heat Budget OF THE Oceans

49

Radiation

51 51

Incoming Radiation; Effect of Clouds; Reflection Absorption of Radiation Energy in the Sea Effective Back Radiation from the Sea Surface The Radiation Budget of the Oceans Exchange of Heat Between the Atmosphere and the Sea. Evaporation from the Sea The Process of Evaporation Observations and Computations of Evaporation Average Annual Evaporation from the Oceans Evaporation in Different Latitudes Annual Variation of Evaporation Diurnal Variation of Evaporation V.

General Distribution of

54 58 60

...

67 67 68 69

Salinity, Temperature,

and

Density

71

Salinity of the Surface Layer

Surface Salinity Periodic Variations of the Surface SaUnity Temperature of the Surface Layers Surface Temperature Difference Between Air- and Sea-Surface Temperatures

Annual Variation Annual Variation

61

62 62 65

71 71 73

....

Temperature of Temperature in the Surface Layers. ... Diurnal Variation of Surface Temperature Diurnal Variation of Temperature in the Upper Layers .... of Surface

Distribution of Density Subsurface Distribution of Temperature and Salinity

74 74 75 77 78 80 82 82 85

TABLE

OF CONTENTS

xi

PAGE

CHAPTER

V.

General Distribution of

Salinity, Temperature,

and

Density {ConH.) Water Masses The T-S Diagram Water Masses and VI.

their

Formation

Ocean Currents Related to the Distribution of Mass. Introduction

Equations of Motion AppHed to the Ocean General Equations

Motion

in the Circle of Inertia

SimpUfied Equations of Motion Practical Application of the Hydrodynamic Equation for the Computation of Ocean Currents

The Fields of Pressure and Mass in the Ocean The Field of Mass The Field of Pressure Currents in Stratified Water Practical Methods for Computing Ocean Currents "Relative" Currents Slope Currents Actual Currents Bjerknes' Theorem of Circulation Transport by Currents

VII.

Wind Currents and Wind Waves

86 86 88 92 92 93 93 93 95 97 99 100 101 103 105 105 108 109 112

114 117

117 118 121 PiUng Up of to the Stress of the Wind Wind Currents in Homogeneous Water 123 Wind Currents in Water in which the Density Increases with Depth 128 Secondary Effect of Wind in Producing Ocean Currents 129 Origin of Wind Waves 133 Form and Characteristics of Wind Waves 134 Relations Between Wind Velocity and Waves 142 146 Waves near the Coast. Breakers 147 Destructive Waves Frictional Forces

The

VIII.

Wind Water Due

Stress of the

Thermodynamics of Ocean Currents Thermal Circulation Thermohaline Circulation Vertical Convection Currents

IX.

Water Masses and Currents of the Oceans Water Masses

Oceans Currents of the North Atlantic Ocean The North Equatorial Current The Gulf Stream System The Florida Current The Gulf Stream The North Atlantic Current Transport of the

150 150 152 153 155

155 163 163 163 164 166 168 171

TABLE

xli

OF CONTENTS

CHAPTER

IX.

PAGB

Water Masses and Currents of the Oceans

(ConH.)

Currents of the Adjacent Seas of the North Atlantic Ocean The Norwegian Sea and the North Polar Sea The Labrador Sea and Baffin Bay The Mediterranean and the Black Sea The Caribbean Sea and the Gulf of Mexico Currents of the Equatorial Part of the Atlantic Ocean Currents of the South Atlantic Ocean Currents of the Indian Ocean The Red Sea Currents of the South Pacific Ocean Currents of the Equatorial Pacific Currents of the North Pacific Ocean The North Equatorial Current

...

The The The The The

Kuroshio System Kuroshio Kuroshio Extension North Pacific Current Aleutian (Subarctic) Current The Eastern Gyral in the North Pacific Ocean The California Current Transport Currents of the Antarctic Ocean The Deep-Water Circulation of the Oceans Ice in the Sea Ice Ice

and Icebergs and Icebergs

X. Interaction

in the Antarctic in the Arctic

Between the Atmosphere and the Oceans.

Character of Interaction The Oceans and the Climate

The Oceans and the Weather

Index

174 174 177 177 180 181

185 187 188 189 193 196 196 197 197 198 199 200 200 201 204 205 211 217 217 220

223 223 224 226 237

List

of Illustrations

PAGB

FIGURB 1.

2.

3. 4. 5.

Energy emitted by a black body Freezing point and temperature of maximum density as functions of chlorinity and salinity Refractive index and specific conductance of sea water .... Protected and unprotected reversing thermometers The Nansen reversing water bottle

6.

Determination of surface currents

7.

The Ekman current meter

8. 9.

Energy spectrum of radiation from sun and sky Energy spectra in different types of water

10.

Percentages of total energy reaching different depths in various

11.

Effective back radiation from the sea surface to a clear sky

12.

Temperatures, prevailing wind directions, and frequency of fog over the Grand Banks of Newfoundland •. Annual evaporation from the Atlantic Ocean Annual and diurnal variation in the amount of heat given off to

types of water

13. 14.

2 14

16

38 42 45 47 54 55 57

.

.

.

.

.

59

65 68

the atmosphere 69 71 Average values of surface salinity for all oceans 77 16. Average annual ranges of surface temperature and radiation 17. Annual variation of temperature in Monterey Bay and in the 78 Kuroshio 80 18. Annual variation of temperature off the Bay of Biscay .... 87 19. Temperature-salinity diagram 88 20. Results of vertical mixing of water types 90 21. The formation of a water mass 94 22. Rotating currents in the Baltic 104,105 23,24. Isobaric surfaces and currents in stratified water. 25. Geopotential topography of the sea surface off southern Cali15.

.

.

.

.

.

107

fornia 26.

Temperatures and sahnities ties of

in the Straits of Florida

and

veloci-

Ill

current

Location of a curve in the pressure field 28. Determination of the depth of no motion 27.

xiii

113 115

LIST

xiv

OF

ILLUSTRATIONS

FIQURB

29.

30. 31.

32. 33. 34.

35. 36.

body

water 122 125 128 current in shallow water 129 Effect of wind in producing a homogeneous surface layer 130 Effect of wind in producing currents parallel to a coast Geopotential topography of the sea surface off southern CaU132 fornia 137 True dimensions of steepest possible wave 140 Record of waves at the end of the Scripps Institution pier Stresses acting at the upper

Wind Wind

and lower surfaces

Ekman

current in deep water (the

of a

of

spiral)

.

.

.

....

.

.

.

Schematic picture of short-crested waves 38. Interference between a long swell and a much shorter wave 39. Types of circulation induced in water by different placement

.

141

37.

.

40.

warm and cold sources Approximate boundaries

41.

Temperature

151 of the

Upper Water Masses and tem-

perature-salinity relations of the principal water masses

42. 43. 44. 45.

46. 47.

48.

144

of

profiles across the

158, 159

.

Gulf Stream and the North

Atlantic Current Transport of Central and Subarctic Water in the Atlantic Ocean Approximate directions of flow of the Intermediate Water Masses Surface currents of the Norwegian Sea Surface currents of the Labrador Sea Temperature and saUnity in the Strait of Gibraltar Surface and intermediate currents in the Mediterranean. Surface currents in the Caribbean Sea and the Gulf of Mexico. .

.

169 171

173

175 177

178

.

180

.

181

discontinuity surface in the equatorial region of the Atlantic 182 50. Vertical circulation within the equatorial region of the Atlantic 184 51. The discontinuity surface in the equatorial region of the Pacific 193

49.

The

52.

Temperature,

and computed velocity in a vertical sec194 Equator in the Pacific the Temperatures and salinities in a vertical section across 199 Kuroshio salinity,

tion across the

53.

202 temperatures along the coast of California 1000the 55. Geopotential topography of the sea surface relative to 203 decibar surface off the American west coast 205 North Pacific 56. Transport chart of the 206 Antarctic Continent 57. Transport lines around the 54. Surface

58.

Temperature and

salinity in a vertical section

from Cape Leeu208

win, Australia, to the Antarctic Continent 59.

Currents, water masses, and distribution of temperature of the

60.

Distribution

61.

213 Western Atlantic Ocean Distribution of temperature, salinity, and oxygen in the Pacific

210

Antarctic regions

Ocean

of

temperature,

salinity,

and

oxygen

in

the

216

LIST

OF ILLUSTRATIONS

xv PAGE

PIGUBB

62.

Average northern

limits of

pack

ice

around the Antarctic Con218

tinent 63.

64.

65.

66. 67.

Annual variation of air- and sea-surface temperatures at Yokohama and San Francisco Total amount of energy given off in summer from the North Pacific and North Atlantic Oceans Total amount of energy given off in winter from the North Pacific and North Atlantic Oceans Net annual surplus of radiation penetrating the water surface. Total annual surplus of energy received by the ocean water when the exchange of energy with the atmosphere is considered Energy exchange between the sea surface and the atmosphere in lat. 35° to 40°N in the Pacific and Atlantic Oceans Energy exchange between the sea surface and the atmosphere in the Western and Eastern North Pacific Ocean Energy exchange between the sea surface and the atmosphere in the Western and Eastern North Atlantic Ocean .

68.

69.

70.

The

following charts follow p. 235:

Chart Chart Chart Chart

1.

Surface temperatures of the oceans in February.

2.

Surface temperatures of the oceans in August.

3.

Surface salinity of the oceans in northern summer.

4.

Surface currents of the oceans in February-March.

.

225 229 229 229

230 231

232 233

CHAPTER

I

Introduction

The Heat Budget of the Earth

The importance of the oceans to the circulation of the atmosphere is best understood by considering briefly the heat budgets of the earth as a whole and of the atmosphere. The earth as a whole receives and loses energy by processes of radiation. Radiation is described, in general, as a wave motion which, when absorbed in a medium, is transformed into some other form of energy, such as heat. The wave length of radiation varies within very wide limits, but the radiation that

the energy balance of the earth

The

is

of

importance to

within a relatively narrow range. wave lengths wdll here be given in the unit fi, which is equal to all falls

10~^ mm. In this unit the wave lengths of the visible Ught fall approximately within 0.38 /x and 0.7 /z, and the radiation that is of importance to the heat budget of the earth falls within the hmits 0.15 ju and about

120

m.

The energy which the earth

receives

by radiation from the sun

part reflected from the earth's surface or from clouds and

is

is

in

in part

absorbed in the atmosphere or at the earth's surface. Only the absorbed amount is of importance to the heat budget of the earth. The earth loses energy by radiation to space, partly directly from the earth's surface and partly from the atmosphere. On an average, the amounts of incoming and outgoing energy are at balance, because otherwise the temperature of the earth would change, and no evidence exists for measurable changes of this kind. In order to study this balance and to examine the heat budget of the atmosphere, it is necessary to enter briefly upon certain characteristics of the radiation from different bodies. Black-body Radiation. A body of a given temperature emits radiation of different wave lengths, the amount of radiation from a unit surface depending upon the temperature and the character of the radiating body. According to experience, at every temperature there is an upper Hmit to the amount of radiation that is emitted. A body which at any given temperature emits the maximum amount of radiation at every wave length is called a ''black body." For a black body the total amount of energy radiated per unit time from a unit area equals aT,"^ 1

INTRODUCTION

2

T is the absolute This relation is known as Stefan's law. If radiation is measured in gram calories per square centimeter per minute, the numeriThe energy of radiation of different wave cal value of 0- is 82 X 10"^^. by Planck's law, which can be written is expressed lengths, X, where

o-

is

a constant (Stefan's constant) and where

temperature.

Y^

-

~ji

jy^^Jy

where E\ is the energy emitted per unit area and unit time within a unit range of wave length and where ci and C2 are constants. The function and X = oo and is at a maximum when the value / (XT') is zero for X = of

XT

-3x10"'^

equals 2940.

Therefore the wave length of

maximum

radiation,

INTRODUCTION

3

temperature, in accordance with Planck's law, whereas the emissive of a gas may correspond to that of a black body of the same temperature at a few wave lengths only and may be nearly nil at other

power

wave

lengths.

The

rate at which radiation

the "absorptive power,"

that

is

is

is

absorbed per unit area, power in a manner

related to the emissive

expressed by Kirchhoff's law: At a given temperature the ratio

between the absorptive and emissive power for a given wave length

is the

This law implies that, if a body emits radiation of a given wave length at a given temperature, it will also absorb radiation Therefore, any gas which emits only a radiaof that same wave length. tion of certain wave lengths also absorbs only radiation of these wave lengths, and this type of radiation and absorption is called selective. Radiation from the Sun. The radiation from the sun is being examined by the Smithsonian Institution at high-altitude stations in California, Chile, and Sinai. By measurements at high altitudes, with the sun at different distances from zenith, it is possible to draw con-

same for

all bodies.

clusions as to the solar radiation that reaches the limit of the atmosphere. It has been found that at wave lengths greater than 0.3 /i this radiation has nearly the character of radiation from a black body at an absolute temperature of 5600° to 6000°. The maximum intensity lies close to a wave length of 0.5 n, and at wave lengths greater than about 2.6 /i the

energy

is

negligible.

The

radiation from the sun

is

therefore called

few bands that show the effect of absorption in the sun's atmosphere. The intensity of normal incident radiation is, on an average, 1.94 g cal/cmVmin, and this intensity short-wave radiation.

is

It is continuous except for a

called the ''solar constant."

Evidence has been accumulated which indicates that in the ultrawave lengths shorter than 0.3 m, the sun does not radiate as a black body but emits a greater and possibly variable amount of energy. This energy is absorbed in the very highest atmosphere and is not measurable. In the following sections it will therefore be necessary to disregard this component of the solar radiation, although it may contribute significantly to the heat balance of the earth and the heat budget of the atmosphere. Of the incoming radiation a considerable fraction is reflected from the atmosphere, the earth's surface, and the upper surfaces of the clouds. According to Aldrich (Brunt, 1939) the average fraction for the whole earth is 0.43. This figure therefore represents the albedo of the earth. The reflected portion of the solar radiation plays no part in the heat budget of the earth. During one year the average solar radiation that reaches the limit of the earth's atmosphere is SirR^/iTrR^ = S/i, where S violet, at

the solar constant and R is the radius of the earth. With S = 1.94 g cal/cmVmin, one obtains S/i = 0.485 g cal/cmVniin, of which 0.209 is lost by reflection and 0.276 is lost by back radiation to space. is

INTRODUCTION

4

According to Fowle, on a clear day with the sun at zenith, 6 to 8 per cent of solar radiation of wave length greater than 0.3 m is absorbed in the atmosphere, primarily by the water vapor, which shows some narrow

absorption bands in the vicinity of wave length 1.0 mLong-wave Radiation to Space. The loss of heat by radiation

is

from the water vapor in the atmosphere and in part to radiation from the earth's surface, which escapes without being absorbed in the atmosphere. At the prevailing temperature on the due

in part to radiation

earth the

maximum

lengths 10 M to 15

ju,

intensity of the outgoing radiation lies at

and

wave

this radiation is therefore called long-wave.

The water vapor is the only component in the atmosphere which absorbs or emits appreciable amounts of radiation at the temperatures that prevail in the atmosphere, but the radiation and absorption are Radiation of certain wave lengths is completely highly selective. absorbed in a small mass of water vapor, whereas for other wave lengths Studies of the mechanism of the the water vapor is nearly transparent. vapor in the atmosphere are in rapid characteristics of water radiation the present would lie beyond the scope summary of status progress, but a of this brief discussion.

It

must

suffice to

owing to the

state that,

selective absorption of the water vapor, a certain part of the outgoing long-wave radiation comes directly from the earth's surface. This part is

of great

importance to the heat budget

of the earth as

a whole, but

does not enter in the heat budget of the atmosphere.

The Heat Budget of the Atmosphere

Accepting the above values of the solar constant, 1.94 g cal/cm^/min, and the earth's albedo, 0.43, one obtains that, of the incoming radiation from the sun, an annual average amount of 0.276 g cal/cm^/min reaches the surface of the earth or is absorbed in the atmosphere. The same average amount must be lost by radiation to space from the water vapor and clouds in the atmosphere or directly from the earth's surface. As yet it is not possible to separate the amounts that are lost by these two processes, but it is possible to state certain limits between which the direct loss from the earth's surface must lie. The surface of the earth radiates nearly as a black body and emits therefore an amount which approximately equals aTe^, where Te is the temperature of the surface. The earth's surface also receives radiation from the water vapor and the clouds in the atmosphere, but this amount, which may be called R, is nearly always smaller than o-T"/. The difference, cTe^

—

R, represents the ''effective radiation of the earth's surface,

or the "nocturnal radiation"

(p.

58).

The

greatest possible loss of

radiation directly from the earth's surface would occur

if

the effective

radiation escaped without being absorbed in the atmosphere,

smallest possible loss would take place

if

'^

and the

the effective radiation were

INTRODUCTION

5

completely absorbed in the atmosphere. It is probable that the greater part of the effective radiation escapes because the water vapor is nearly transparent for radiation of wave lengths between 8.5 and 11 n, at

which the black-body radiation

of the earth's surface is nearly at a

maximum. The average value

of the effective radiation over all oceans between about 0.090 g cal/cmVmin (p. 59). The effective radiation from the land areas between the same latitudes is probably greater, whereas that from the polar regions is smaller. The above value may therefore be taken as a fair estimate of the average value for the earth. Accordingly the radiation from the atmosphere must lie between the limits 0.186 g cal/cm^/min and 0.276 g cal/cm'Vmin probably closer to the former. Since the heat gained by the atmosphere by

70°N and 70°S

is

—

absorption of radiation or by other processes must equal the loss by radiation, the gain must also lie between the same limits. (1)

This gain can be further analyzed, since it may be brought about by condensation of water vapor in the atmosphere, (2) conduction of heat

from the earth's surface, (3) absorption of short-wave radiation from the sun, and (4) absorption of long-wave radiation from the earth's surface. The amount under item (1) could be computed if one knew approximately the total precipitation on the earth. According to Wiist the total precipitation on the whole earth is 396,000 km^/year, corresponding to an average amount of heat released by condensation equal to 0.086 g cal/cmVmin. Under item (2) the average amount of heat conducted from the oceans to the atmosphere between latitudes 70°N and 70°S has been estimated at 0.013 g cal/cm^/min. The conduction from the land areas between the same latitudes is probably smaller, and in the polar region heat is conducted from the atmosphere to the earth's surface. On an average for the whole earth, item (2) can therefore be estimated at 0.010 g cal/cm^/min. The amounts of absorbed radiation, items and have not been estimated, but the limits of the amounts can (3) (4), be found because the sum of all items must lie between 0.186 and 0.276 g cal/cm^/min. On this basis the heat budget of the atmosphere can be summarized as follows: Minimum Maximum Heat gained by Condensation of water vapor Conduction of heat from the earth's surface. Absorption of radiation

Heat

lost

by radiation

(g

.

cal/cmVmin) .086 .010 .090

(g

cal/cmVmin) .086 .010 180

.

TT86

7276

.186

.276

probable that the true values lie closer to those listed under the heat budget. All numerical values given here are subject to revision, but such revision is not likely to reverse the feature which the available data bring It

is

minimum

INTRODUCTION

6 out

—namely,

that the condensation of water vapor in the atmosphere

accounts for nearly half the amount of energy which the atmosphere receives.

The Atmosphere as a Thermodynamic Machine

So far, we have considered only the average heat budget of the atmosphere and have not examined the geographic and seasonal distribution Such an examinaof the regions in which heating and cooling take place. tion cannot be made in detail because our knowledge of the processes of radiation in the atmosphere and of the regions where condensation of water vapor takes place is inadequate. It appears certain, however, that the greater heating takes place in the Tropics and the greater cooling in the high latitudes, so that on an average heat must be transported from the Tropics to the polar regions in order to maintain an approximately stationary distribution of temperature.

It is also

probable that this

by air currents, which implies that the atmosphere is a thermodynamic machine that transforms heat into This kinetic energy is dissipated by friction and transkinetic energy. formed back again into heat that is lost by radiation. If the machine runs at a constant speed, the amount of heat which in unit time is transformed into kinetic energy must equal the amount of heat which in unit time is generated by dissipation of kinetic energy. In a thermodynamic machine that produces kinetic energy the heating must take place, according to a theorem formulated by V. Bjerknes (Bjerknes et al, 1933), under a higher pressure and the cooling under a transport of heat takes place mainly

Applied to the atmosphere the theorem states that heatat a lower altitude and cooling at a higher altitude. This condition is fulfilled in the atmosphere because condensation of water vapor does take place at a relatively low altitude; absorption of long-wave radiation from the earth also takes place at relatively low altitudes, whereas radiation to space from the water vapor and the upper surfaces of clouds takes place from high altitudes. Thus, the general features of the thermodynamic machine represented by the atmosphere can be recognized, but the details are not understood. A discussion of the interaction between the oceans and the atmosphere will supply some of the details that have to be considered and is therefore necessary for the better comprehension of the atmospheric circulation. Several attempts have been made at an examination of the general character of the atmosphere as a thermodynamic machine, the latest and most widely quoted being that by Simpson (Bjerknes et al, 1933; Brunt, Simpson considers only the distributions with latitude of the 1939). incoming short-wave radiation and the total outgoing long-wave radiation, making certain simplifying assumptions as to the absorption spectrum of the water vapor. He finds that the earth receives a surplus of radiation up to latitude 35° and concludes that this surplus must be lower pressure.

ing

must take place

INTRODUCTION

7

transported by winds to higher latitudes in order to counterbalance the radiation deficit existing there. Simpson does not, however, examine the manner in which the incoming radiation becomes available for heating

He is therefore not concerned with the important part the atmosphere. played by condensation of water vapor in the atmosphere. Taking this into account and considering also that part of the outgoing radiation from the earth's surface escapes without influencing the heat budget of the atmosphere, this heat budget becomes much more complicated, and Simpson's numerical values for the heat transport by the atmosphere across parallels of latitude lose

much

of their significance.

The Oceans as Part of the Subsurface of the Atmosphere Since the absorption of long-wave radiation from the earth's surface and the condensation of water vapor that is supplied to the atmosphere from the earth's surface are factors of dominant importance to the heat

budget

of the

atmosphere,

it is

evident that the thermal characteristics

must be examined when studying the atmospheric circulaOver the land areas, an interaction between the surface and the

of the surface tion.

circulation of the of

snow greatly

atmosphere evidently takes place.

Thus, a covering

increases the reflection of incoming short-wave radiation,

dense clouds alter the radiation phenomena, heavy rainfalls that moisten the ground change the possibilities for evaporation, and so on, and every change must be reflected in a change in the pattern of atmospheric circulation.

The character

ocean surface

very different from that of the The heat absorbed near the surface is distributed over a relatively deep water layer through the stirring by waves and wind currents and it is transported over long distances by ocean currents that are more or less directly produced by the prevailing winds. Heat which in one season is absorbed in one part of the sea may therefore in another season be used for evaporation in a different part of the sea, depending upon the types of currents that the general circulation of the atmosphere induces. The oceans, as far as their interaction with the atmosphere is concerned, are a flexible medium that responds to changes in the circulation of the atmosphere. Inasmuch as the greater part of the water vapor in the atmosphere is supplied by evaporation from the oceans, the relation between the atmosphere and the circulation of the oceans becomes important, because this relation will determine the seasons and the regions from which excessive evaporation takes place. Thus the heat budget of the oceans and of the ocean currents is of the greatest importance to the atmosphere. In order of the

is

land, mainly because the ocean water can be set in motion.

to discuss this heat budget

and the currents

it is

with some of the physical properties of sea water.

necessary

first

to deal

CHAPTER

II

Physical Properties of

Sea Water

The physical properties of pure water depend upon two variables, temperature and pressure (Dorsey, 1940; International Critical Tables), whereas for sea water a third variable has to be considered namely, the Some salinity of the water, which will be defined and discussed below. of the properties, such as compressibility, thermal expansion, and refrac-

—

tive index, are only slightly altered

by the presence

of dissolved salts,

but

other properties that are constant in the case of pure water, such as freezing point

and temperature

of

the case of sea water, on salinity.

new

maximum

density, are dependent, in Furthermore, the dissolved salts add

properties, such as osmotic pressure

When

and

electrical conductivity.

it must also be taken into account that important characteristics are greatly modified by the presence of minute suspended particles or by the state of motion. Thus, the absorption of radiation in the sea is different from the absorption of radiation in pure water or in ''pure" sea water because the waters encountered in nature always contain suspended matter that causes increased scattering of the radiation and, consequently, an increased absorption in layers of similar thickness. The processes of heat conduction, chemical diffusion, and transfer of momentum from one layer to another are completely altered in moving water, for which reason the coefficients that have been determined under laboratory conditions must be replaced by corresponding eddy coefficients. Some of the physical properties of sea water, therefore, depend only upon the three variables, temperature, salinity, and pressure, which can all be determined with great accuracy, whereas others depend upon such variables as amount of suspended matter or character of motion, which at present cannot be accurately determined.

dealing with sea water as

Salinity,

it

occurs in nature,

Temperature, and Pressure

Salinity. Salinity is commonly defined as the ratio between the weight of the dissolved material and the weight of the sample of sea water, the ratio being stated in parts per thousand or per mille.

oceanography another definition of sea water, it is

In

used because, owing to the complexity impossible by direct chemical analysis to determine the is

8

PHYSICAL PROPERTIES

OF SEA WATER

total quantity of dissolved solids in a given sample.

impossible to obtain reproducible results

Furthermore,

9 it is

by evaporating sea water

to

dryness and weighing the residue, because certain of the material present, chiefly chloride, is lost in the last stages of drying. These difficulties can be avoided by following a technique yielding reproducible results which,

although they do not represent the total quantity of dissolved solids, do represent a quantity of slightly smaller numerical value that is closely related and by definition is called the salinity of the water. This technique was established in 1902 by an International Commission, according to which the salinity is defined as the total amount of solid material in grams contained in one kilogram of sea water when all the carbonate has been converted to oxide, the bromine and iodine replaced by chlorine, and all organic matter completely oxidized.

The determination Commission

is

rarely

if

of salinity

by the method

of the International

ever carried out at the present time because

too difficult and slow, but, inasmuch as

it is

has been established that the dissolved solids are present in constant ratios, the determination of any of the elements occurring in relatively large quantities can be used as a measure of the other elements and of the salinity. Chloride ions make up approximately 55 per cent of the dissolved solids and can be determined with ease and accuracy by titration with silver nitrate, using potassium chromate as indicator. The empirical relationship between salinity

and

chlorinity, as established

Salinity

The

=

0.03

+

it

by the International Commission,

1.805

X

is

Chlorinity.

and sample of sea water. Both salinity and chlorinity are always expressed in grams per kilogram of sea water that is, in parts per thousand, or per mille, for which the symbol °/oo is used. In practice, salinity is determined with an accuracy chlorinity that appears in this equation

does not represent the actual

amount

is

also a defined quantity

of chlorine in a

—

of

+0.027oo.

The

water sample can also be found by determining the known temperature, because empirical relationships have been established by which the density can be represented as a function of salinity (or chlorinity) and temperature. Other indirect methods for determining salinity are based on measurements of the electrical conductivity or the refractive index at a known temperature. In order to obtain the needed accuracy, the difference is measured in electrical conductivity or refractive index between the sample and a sample of known salinity that lies fairly close to that of the unknown salinity of a

density of the sample at a

sample.

The salinity in the oceans is generally between 33°/oo and 37°/ooIn regions of high rainfall or dilution by rivers the surface salinity may be considerably less, and in certain semi-enclosed areas, such as the Gulf of

PHYSICAL PROPERTIES

10

OF SEA WATER

Bothnia, it may be below 5 °/oo. On the other hand, in isolated seas in intermediate latitudes where evaporation is excessive, such as the Red As the range in the open oceans Sea, salinities may reach 40 °/oo or more. is rather small, it is sometimes convenient to use a salinity of 35.00 °/oo as

an average for

all

oceans.

In oceanography the temperature of the water is In the oceans the temperature ranges from stated in degrees centigrade. about —2° to +30°C. The lower limit is determined by the formation

Temperature.

and the upper limit is determined by processes of radiation and exchange of heat with the atmosphere (p. 74). In landlocked areas the surface temperature may be higher, but in the open ocean it rarely of ice,

exceeds 30°C.

Pressure. Pressure is measured in units of the c.g.s. system, in which the pressure unit is 1 dyne/cm^. One million dynes/cm^ was designated as 1 bar by V. Bjerknes. The corresponding practical unit used in physical oceanography is 1 decibar, which equals }{o bar. The pressure exerted by 1 m of sea water very nearly equals 1 decibar; that is, the hydrostatic pressure in the sea increases by 1 decibar for approximately every meter of depth. Therefore the depth in meters and the This pressure in decibars are expressed by nearly the same numerical value. rule

is

sufficiently accurate

when

considering the effect of pressure on the

physical properties of the water, but details of the pressure distribution must be computed from the density distribution (p. 99).

and salinity and the quantities these between in the oceans, some oceans of the water and bottom the deep of The temperature pressure. 4° — are high pressures and and 1°C, is always low, varying between of salinity the Similarly, temperatures. low with therefore associated — limits, 35.0 within narrow 34.5 varies °/oo, °/oo deep and bottom water and high pressures are therefore associated with salinities between these Exceptions are found in isolated seas in intermediate latitudes limits. such as the Mediterranean and Red Seas, where water of high temperature and high salinity is found at great depths that is, under great pressures.

Owing

to the character of the distribution of temperature

relationships exist

—

Density of

The Thus,

The

density of any substance

is

Sea Water defined as the mass per unit volume.

in the c.g.s. system, density is stated in

grams per cubic centimeter.

specific gravity is defined as the ratio of the density to that of

water at a given temperature and under atmospheric pressure. system the density of distilled water at 4°C is equal to unity. In oceanography specific gravities are now always referred to distilled water at 4°C and are therefore numerically identical with densities. The term density is generally used, although, strictly speaking, specific gravity is always considered. distilled

In the

c.g.s.

1

PHYSICAL PROPERTIES

The

density of sea water depends

OF SEA WATER

upon three

1

variables: temperature,

These are indicated by using for the density the symbol Ps.t?,p, but, when dealing with numerical values, space is saved by introducing the symbol ,p = 1.02575, °C the corresponding quantity is simply written > O O

23

(

PHYSICAL PROPERTIES

24

O I— H
o Q W

H^ >g OH o>^

q5 ^

.

f^p4

few

W H O

OF SEA WATER

CO CO 05 CO r-H CO

PHYSICAL PROPERTIES OF SEA WATER

1