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CHAPTER 19

SOME INTERRELATIONSHIPS BETWEEN OCEANOGRAPHY AND METEOROLOGY

General Remarks

Oceanography is thc study of the oceans and mctcorology is the study of thc atmospherc. It is obvious that thc two studics arc intimatcly linkcd—one has only to think of such a phenomenon as sca \vaves to realize this, but thc dcgrec and complexity of thc linkagc is not always apprcciated. Mariners arc adviscd to study textbooks on oceanography not only because of its dircct importancc to navigation but also bccausc an understanding of thc physical proccsscs occurring within thc occans is necessary for a proper apprcciation of the proccsscs occurring in thc atmospherc. Some aspects of oceanography, for exatnple sca surface temperaturę, are so vital to thc study of meteorology that they arc commonly discussed at length in mctcorological writings. A fu ller understanding of such matters, ho\vcver, is obtained by studying them in the contcxt of oceanography. In thc casc of sea temperaturę, not only must one consider heat exchangc between ocean and atmospherc, but also that bctwccn thc surface laycr and the lower layers within the ocean. This invo!ves a study of the thermal structurc of thc ocean in thc vcrtical as wcll as the horizontal, and the movements occurring within the water mass.

In mcteorological proccsscs the prime mover is solar radiation. As the atmosphere is largely transparent to the incoming short-wave radiation from thc sun, thc greater part of this radiation passes through thc atmospherc and is absorbed, in thc form of heat (both latent and sensible) at thc carth’s surface. The sensible and latent heat passes to the atmosphere and so sets in motion most of thc mctcorological proccsses. Thcsc, thcrcforc, although dcriving their energy from the sun, do so largely by way of the earth’s surface. Since about seven-tcnths of this surface is water, thc importancc of thc occans bccomes apparent.

The part played by thc occans in thc global heat budget is, howcver, far in excess of what might be expcctcd from the simple rado of sca surface to land surface. Bccausc of a number of properties which arc pcculiar to water and difTer markedly from the corresponding properties of the matcrials which constitutc thc land surface, thc sca plays a disproportionatcly largc part in controlling thc heat exchange between the earth’s surface and the atmosphere. Firstly, water has a largc heat capacity. It rcquires about fivc times as much heat to produce a given rise of temperaturc in a specified mass of water as it docs to produce thc same rise in thc same mass of thc land surface. Incidentally, the amount of heat reąuired to raise the temperaturę of a given volumc of water is about 5000 times as much as that reąuired to produce the same temperaturc changc in the same volume of air. Sccondly, water allows radiation to pass through it and so to warm the water at depths within thc penetration region. Thirdly, thc mobility of water in the vertical piane mcans that heat can be exchangcd between thc surface and lower lcvcls. Fourthly, thc mobility of water in thc horizontal piane mcans that vast ąuantitics of heat received and stored in one region, for cxample the tropics, can be transported to higher latitudes by the

OCEAj\OGRAPHT AND METEOROLOGT    263

agcncy of ocean currents and can there serve to heat the atmospherc long after thc original water heating took place.

In contrast, the land has a rclativcly Iow heat capacity. Its thcrmal con-ductivity is also Iow and it is ncithcr mobile nor permcablc to radiation. This means that solar radiation afTccts only a thin surface layer of thc land in comparison with thc much greater depth of thc affectcd layer of the sea. The land surface accordingly rises to higher temperatures by day than docs the sca surface, and falls to lower temperatures by night. Since thc quantity of radiation from a surface varies as thc fourth power of the absolute temperaturc of thc surface, considcrably morę cnergy is rc-radiatcd from thc land surface, than from the sca surface, so that the sea retains a greater amount of heat than does thc land.

Because of the special properties of water, thc oceans servc a dual purposc, namcly to accumulate solar cnergy and to redistributc it, in the form of heat, in time and space.

Owing to the greater heat capacity of water, thc cxtreme rangę of sea surface temperaturc is only about onc-third of that of the air at a few metres above thc surface. This rangę is from about —2°c in polar seas to about 35°c in regions such as the Persian Gulf, a differcnce of about 37⁰. The extrcmc rangę of air temperaturc is from about —68°c to about 58°c, a differencc of about 126°.

H. Pcttcrson has comparcd the oceans to a kind of ‘savings bank’ for solar energy, in that they rcceive deposits during cxccssive insolation and pay them back in scasons of want.

The Energy Exchange between Sea and Atmosphere

In Chaptcr 1 it was stated that the mean temperaturę of the earth’s surface varies little from year to year, implying that a balancc must cxist between the magnitudc of the incoming and outgoing streams of radiation ovcr the whole carth. An understanding of the factors involved in rcaching this balancc is nccdcd in order to apprcciatc the problcms of the energy exchange between thc atmospherc and thc sca.

Ali thc incoming solar radiation arrives as short-wave radiation of which about 35% is rcflectcd back to space and lost. (This figurę expresscs thc loss as an avcrage over thc whole surface of the earth and is known as the earth’s ‘albedo’.) Of thc rcmaining 65%, about a ąuarter is absorbed by the earth’s atmospherc and the remainder is absorbed by thc carth’s surface. Of the latter, some is returned to space in thc form of long-wave radiation and some, in the form of heat, establishes a mcridional circulation in the atmosphere which hclps to reduce the excess of heat which would otherwise accumulate in eąuatorial regions, by transferring some of it to higher latitudes where there is a heat deficit.

Although heat passes from the sea to thc atmosphere by way of conduction and convection, a large proportion of the atmospheric warming occurs as a result of the liberation of latent heat when moisture derived from cvaporation of the sea surface condenscs in the atmosphere. This proportion has been claimed to be around 50%.

The fact that thc amount of evaporation from the sea surface depends upon the wind speed is further evidencc of the intimate interdepcndence which exists between atmospheric and oceanographic proccsscs.

The ratę of evaporation from the sca surface depends not only upon the wind