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266 METEOROLOG!' FOR MARINERS

An cfTcct which is sometimes ovcrlookcd is that of atmosphcric pressure on the level of thc sea surface. Very intcnse atmosphcric dcprcssions producc a lifting of thc water level in thc central region relative to that in thc outer regions whcrc the air pressure is higher. The incrcascd water lcvcl movcs with thc depression and produces what is known as a ‘storni surge’ which on occasions can cause serious flooding. Flooding duc to local incrcase in sea lcvel is also produccd when storm winds blow on to a coastline, particularly if this is so shaped as to present a concave profile relative to thc wind dircction. Then the water displaccd by the action of the wind has no ready escape and a low-lying coastline can be inundated (see Ghapter ii—Bay of Bcngal).

The DifTerence between Air and Sea Temperaturę

The averagc temperaturę of the surface sea water is slightly greater than that of the overlying air. In the tropics the mean value of this excess is about o*8°c. In middle latitudes there are large scasonal and regional diffcrences. In the western North Atlantic and North Pacific the surface water in winter is, as a rulc, wariner than thc air by about 3 to 4^cc. In the eastern parts of these occans thc diffcrcnce is smali. In spring and summer the sea surface is colder than the air in many regions, and on the Grand Banks of Newfoundland the difference may amount to 1 or 2°c.

The air at the sea surface quickly adopts a temperaturę near to that of the sea; thus thc air temperaturę over thc ocean will show on thc averagc much thc same distribution as thc sea surface temperaturę, but comparativcly slight diffcrences between water and air are of great importancc to the atmosphcric proccsscs, and are particularly significant for the purposes of weather fore-casting. If thc sea is warmer than the air instability occurs, whereby hcat and moisture are readily propagated upwards into the atmosphere. The generał slight excess of sea surface over air temperaturę therefore means that in most regions and during most of thc ycar thc sea is energizing atmosphcric processcs by giving up hcat and water vapour to the air. The bulk of the heat transfer, however, takes place in winter ovcr rather wcll-dcfined arcas as described above.

We havc seen that the opposite cffect of cooling of thc air by thc sea has little quantitativc cfTect upon the atmosphere. Accordingly thc main influence of thc sea upon climatc is to produce a lcvelling upwards, i.e. the cffect is to producc a higher avcrage temperaturę (as wcll as a smaller rangę of variation) and a greater lcvcl of precipitation.

Atmosphcric conditions tend to be uniform where there are only slight diffcrences between the air and sea temperatures. These diffcrences are liable to becomc greatest in regions of rapid occanographical changes. Such a region occurs along thc northern border of the Gulf Strcam. Herc becausc of thc large gradient of sea surface temperaturę a comparatively smali horizontal displace-ment of air can rapidly changc thc air/sea temperaturę difference and tłiis, no doubt, is linkcd with the high incidcncc of eyelogenesis in this region. A similar arca of high cyclonic activity occurs in the western North Pacific where cold Continental air ovcrruns thc warm Kuro Shio.

The moderating effect of the Oceans

Having discussed the close interdependence which exists between thc occans and thc atmosphere it rcmains to considcr thc overall cffcct of this linkage upon

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thc climatc. The main eflcct is a moderating one in the sense that it tends to suppress thc development of extreme conditions. Long-term (i.e. multi-year) averagcs of thc main climatic clcmcnts show a considerable degree of constancy. This is becausc thc supply of energy from thc sun is comparativcly constant and thc occan/atmosphcrc linkage operates in such a way as to minimizc departures from average conditions.

As an cxamplc, let us assumc that an anomaly develops in thc form of abnormally Iow cloud amounts in Iow latitudcs. This will lead to incrcascd sca temperatures in these latitudcs. This will rcsult in incrcascd air temperatures and so to an incrcascd latitudinal air temperaturę gradient which will causc incrcascd winds. These will lead to incrcascd evaporation from the sca surface and so to a reduction in sea surface temperaturę through thc cxtraction of latcnt hcat. In addition thc incrcascd cvaporation leads to incrcascd cloud which reduces the amount of solar energy reaching the sca surface and so further rcduces the sea temperaturę.

If, on thc other hand, one starts with exccssivc cloud amounts, these will rcducc thc incoming solar radiation reaching thc sca surface and so rcducc both sca surface temperaturę and evaporation leading in time to reduced cloud amounts. In this way thc oceans act as a brakc and prevent the devclopment of cxtreme conditions.

Bibliography

Among the many sources studied in the course of preparing this cdition, thc following were found to be of spccial interest and arc recommended for further rcading:

ATKINSON, Major G. D.	*97 *
CALLACHAN, W. 0.	>975
COLE, F. W. DARBYSIIIRE, M. and	'975 '9*>3
DRAPER, L.
DRAPER, L.	*97*
DONN, W. L. CENTRY, R. C.	'975 *973
KOREVAAR, C. O.	'974
NEUMANN, O.	'968
WOLF-HODECK, F.	»972

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Ocean waves. Dublin, Meteorological Scrvicc, Technical Report No. 39.

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