Ocean and Climate 650 Solar heating is distributed unequally over the 's surface. Oceanic motion makes an 600 important contribution to the transport of heat 550 and reduces the equator-pole temperature 500 gradient over the Earth. On an average, the transports as much heat to the higher 450 latitudes, as does the atmosphere. 400

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El Niño Phenomenon 250

A more easily observable that occurs example 200 of the ocean's role in climate is El Niño, a phenomenon that occurs in the equatorial 150 . When an El Niño occurs, the 100 waters off Peru warm, and these warm waters 50 spread westward, increasing the surface temperature across the eastern and central 0 Pacific. This suppresses convection over 20 m s -1 and the western Pacific. The effect of El Niños is not restricted to the equatorial Rainfall (colour, mm/month) and (vectors, m/s) during January (left) and July (right). Note the band of high rainfall in the vicinity of the equator during January; in the Indian Pacific. The large expanse of the basin, which Ocean, this band migrates northward over the during July. Also note covers almost half the globe, ensures that El that the western Pacific and eastern are wetter than their eastern and western Niño has a global impact on climate. During El counterparts, respectively. Niño, with the atmospheric convection over the western Pacific being suppressed and the Role of and of band of high rainfall shifting eastward, there is a tendency for rainfall over also to in climate decrease. The Arabian Sea and the also exercise a profound influence on climate. Though both are located in the same latitude band and receive the same amount of solar radiation from the Sun, the Bay of Bengal is much warmer than the Arabian Sea and many more storms brew over the bay. Recent research shows that there are two causes.

-2 0 5 10 15 20 25 28 31 18 19 20 21 22 23 24 25 26 27 28 29 30 1) The winds over the Arabian Sea are stronger 80°N because of the presence of the mountains of 20°N 40°N 10°N East . These strong winds force a much 0° 0° more vigorous oceanic circulation and the heat 40°S 10°S received at the surface is transported 80°S 20°S southward and into the deeper ocean. The 50°E 150°E 110°W 10°W 40°E 60°E 80°E 100°E winds over the Bay of Bengal, in contrast, are Sea surface temperature (ºC) during Sea surface temperature (ºC) during July more sluggish and the bay is unable to remove May. Note the ‘warm pool’ that spreads in the Indian Ocean. Note that the the heat received at the surface. across the western Pacific and the north Arabian Sea cools during the , Indian Ocean. but the Bay of Bengal remains warm. 2) The bay receives more rainfall; it also receives more freshwater from the large rivers, especially the Ganga and the Brahmaputra, that empty into it. This freshens the surface of 0 20 40 60 80 100 120 140 160 180 200 220 240 0 20 40 60 80 100 120 140 160 the bay and stabilizes the water column,

30°N making it more difficult for the winds to mix the warm, stable surface layer with the cooler 20°N waters below. In the Arabian Sea, there is no

10°N such stabilizing effect. As a consequence, the mixing with the cooler waters below is more 0° vigorous. Since a sea surface temperature of 10°S about 28ºC is necessary for convection to take 30°E 50°E 70°E 90°E 110°E 30°E 50°E 70°E 90°E 110°E place in the atmosphere, this condition is

Cumulative rainfall (colour bar, cm) and storm tracks during June-September and October- satisfied in the Bay of Bengal but not in much January. Data for 1990-1997 have been used. The light blue (cyan) tracks are for of the Arabian Sea. Thus, in spite of their depressions, the red tracks for (storms), and the black tracks for severe cyclones. Note the large number of depressions that form in the northern Bay of Bengal during June- geographical similarities, the two arms of the September, when cyclones are fewer. These depressions move northwestward into the north Indian Ocean are strikingly different when Gangetic plains and bring rain to much of northern India. During October-January, there are fewer depressions, but the number of cyclones is larger. it comes to climate.