264 265 (10)
METEOROLOG!' FOR MARINERS
264
speed at the locality in qucstion but also upon the difference bctwccn the humidity of the air at the sca surface and the humidity of the air measured, for cxample, at the level of the ship’s bridge. This difference becomes largest when cold and dry air lies ovcr a rclatively warm sea surface.
Accordingly, the ocean regions in the northern hemisphere which are subjccted to the most intense evaporation lie a short distance to the cast of the American and Asian continents between latitudes 30°n and 40°n and not, as might have been expectcd, in the tropics. The rcason is that powerful outbrcaks of very cold Continental air frcąucntly overrun the ocean ofTJapan and castern North America in the regions of the Kuro Shio and the Gulf Strcam. In winter thcsc are also the regions where the greatest total energy exchange (i.e. the sum of the energy provided by evaporation and convcction) is occurring, whilc in summer the regions where the total energy exchange is at a maximum are found in the tropical Atlantic and Pacific. The winter regions of greatest total energy cxchange bctwccn sca and atmosphere coincide with the area where the largest proportion of temperate latitudc depressions are initiated, while the summer regions broadly coincide with those where most tropical revolving storms begin their existcnce.
EfFect of the Sea upon the Atmosphere
The effects may be summarized as follows:
(a) Direct heating or cooling of the lowcst layers of the atmosphere according to whether the sca is warmer or cooler than the overlying air.
(b) The addition or rcmoval of water vapour to or from the atmosphere by evaporation or condensation at the sca surface. Heat is extracted from the sea surface during evaporation and later supplied to the atmosphere at a higher level when condensation occurs.
The processcs referred to in (a) and (b) above form the basis of most mctcorological activity. Chapter 9 dcscribed how the dramatic manifestations of the common depression are powered by the contrasting properties of the participating air masses. Thcsc air masses are created by heat and moisture cxchange with the oceans over protracted periods during which the air acquires temperaturę and humidity appropriate to a particular region. Later, the contrasting air masses are brought together so that their potcntial energy can be convertcd into the dynamie energy of a cyclonic depression. This bringing together depends upon the action of winds whose cxistence can be traccd back to difTcrential heating and cooling of the atmosphere in different regions.
A further effcct of the sea upon the atmosphere becomcs apparent when we consider the movement of an air mass over an ocean whcrcin the surface water temperaturę is progrcssively incrcasing. In the North Atlantic, for example, it is common for cold air, originating over the polar seas or the Canadian Arctic Archipelago, to be brought by a north-westerly gradient over Newfoundland and then towards the Azores. After passing over the Gulf Strcam this cold air becomcs rapidly heated in its lowcr layers, and progrcssivcly morę so as it moves to lower latitudes. Ultimatcly it will become convectively unstable {su Chapter 1) and as a rcsult, heat and humidity originally supplied to the lower layers only, become distributed throughout the troposphcrc. Thus the sea can dcterminc the dcgrcc of stability or instability of an air mass.
265
Turning to item (b) abovc, apart from the heating efTect on the atmosphere due to the liberation of latcnt heat released when water vapour derivcd from the sca condcnscs, another important cffcct of the occans consists of providing the water supply (via evaporation) for the formation of the clouds which furnish the world’s rainfall.
Largcly bccause of the ocean currcnts, the geographical distribution of mcan sca surface temperaturę departs considcrably from the simplc latitudinal distribution wherein temperaturę dccrcases with incrcasing latitudc. Bccause large arcas of ocean havc temperatures appreciably above or below the latitudinal mean, the climatc of places lying downwind from such arcas is corrcspondingly modificd. An obvious example is the climate of north-west Europę which is so much warmer and morę cąuablc than that ofeastern Canada in the same latitudc. This is largely due to the efTect of the warm water supplied to the Atlantic in these latitudes by the Gulf Strcam.
Apart from such climatic variations from one region to another, temporary changes in sea temperaturę distribution ovcr a period of a few days may causc corresponding variations in the wcather in a given place.
The EfTect of the Atmosphere upon the Sea
One of the most obvious of these eflects is the wavcs on the sea surface produced by wind. (Wavcs are discussed in Chapter 3.) When one considers that the winds can be traccd back to temperaturę diflerences in the atmosphere caused by corresponding diflerences in the undcrlying surface temperatures, the intricacy of the intcrdcpcndcnce between the atmosphere and the sca can be apprcciatcd. Apart from inducing this small-scalc wavc motion, the winds are also responsible for some of the large-scale drifts of water from one region to another which constitutc ocean currcnts. Thcsc are dcscribcd in Chapter 16. Considcration of currents also indicates the close interrelation between meteoro-logical and occanographic proccsscs. This is because not only do temperaturo diflerences in the water lead directly to water movement, but also these temperaturę diflerences are communicated to the air and thcrc give rise to winds which aflect the sca surface and in tum give rise to current. For this rcason most ocean currents must be regardcd as bcing complex in origin.
Bcsides the above mechanical eflects of the atmosphere upon the sca, the sca surface is heated in some regions and cooled in others by the efTect of warm or cold winds. In the winter months cold winds blowing from the contincnts over the rclatively warm waters of the oceans in middlc latitudes cause con-siderablc cooling of the water surface, partly by dircct heat transfer, and partly as a result of the loss of heat by cvaporation. In summer, in regions where the winds blow over the sea from the heated land, the surface water may be heated by the air in cases where the direct heat transfer is not outwcighed by the cooling produced by cvaporation, but this cffcct is of much less significancc in terms of heat transfer bccause, with the air being cooled from below, the conditions in the air are thermally stable (see Chapter 1) so that only a thin layer of air is aflectcd and this is soon cooled to a temperaturę ncar to that of the sea surface.
Another, less direct, eflcct of the winds is to givc rise locally to the pheno-menon of‘upwclling* (dcscribcd in Chapter 16). By this mcans water is caused to rise to the surface from lower levels and by bringing with it temperatures appropriate to those levels, effectivcly reduces the sca surface temperaturę in the area.
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