Oceans and climate
Solar heating is distributed unequally over the Earth’s surface. Oceanic motion makes an important contribution to the transport of heat and reduces the equator-pole temperature gradient over the Earth. On an average, the ocean transports as much heat to the higher latitudes as does the atmosphere. The larger heat capacity of water compared to air makes it possible to do this with currents that are much weaker than the winds. Oceanic circulation therefore has a close relation to climate and affects it on a range of space and time scales. On long time scales, the oceans participate in determining climate through the global conveyor belt (Fig. 15), which is affected by changes intrinsic to the atmosphere and the ocean and by the changes in solar heating due to variations in the Earth’s orbit around the Sun.
A more easily observable example of the ocean’s role in climate is El Niño, a phenomenon that occurs in the equatorial Pacific Ocean. Under normal conditions, the easterly (from the east) trade winds maintain a reservoir of warm water in the western Pacific off Indonesia and Papua New Guinea (Fig. 20). This warm water supports strong atmospheric convection. As a result, rainfall in this region is among the highest in the world. During the summer monsoon this band of high rainfall extends into the Indian Ocean and over the Indian subcontinent (Fig. 21). The rising air that is responsible for this rainfall moves across the Pacific basin and sinks over the cool waters off Peru on the eastern side. When an El Niño occurs, the waters off Peru warm, and these warm waters spread westward, increasing the sea surface temperature across the eastern and central Pacific. This suppresses convection over Indonesia and the western Pacific. The effect of El Niños is not restricted to the equatorial Pacific. The large expanse of the basin, which covers almost half the globe, ensures that El Niño has a global impact on climate. During El Niño, with the tmospheric convection over the western Pacific being suppressed and the band of high rainfall shifting eastward, there is a tendency for rainfall over India also to decrease.
Dramatic advances in satellite technology have led to the recent discovery of an El-Niño-like oscillation in the equatorial Indian Ocean. It has been called the Indian Ocean Dipole Mode. Under normal conditions, the band of warm waters in the western Pacific extends across the north Indian Ocean (Fig. 20). The eastern equatorial Indian Ocean is usually warmer than its western counterpart (Fig. 22). When the positive phase of the dipole occurs, as it did in 1997, sea surface temperature decreases in the east and increases in the west. Recent research suggests that the dipole has a significant influence on the rainfall over India.
The Arabian Sea and the Bay of Bengal 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. The depressions that form over the northern Bay of Bengal move northwestward across the Indo-Gangetic plains, bringing rain to most of northern India (Fig. 23). Over the Arabian Sea, rainfall is much less on
an average. The ocean plays a major role in keeping the Arabian Sea relatively dry. Recent research shows that there are two causes. First, the winds over the Arabian Sea are stronger because of the presence of the mountains of East Africa. These strong winds force a much more vigorous oceanic circulation and the heat received at the surface is transported southward and into the deeper ocean. The winds over the Bay of Bengal, in contrast, are more sluggish and the bay is unable to remove the heat received at the surface. Second, 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 the bay and stabilizes the water column, making it more difficult for the winds to mix the warm, stable surface layer with the cooler waters below (Fig. 14). In the Arabian Sea, there is no such stabilizing effect. As a consequence, the mixing with the cooler waters below is more vigorous. Since a sea surface temperature of about 28°C is ecessary for convection to take place in the atmosphere, this condition is satisfied in the Bay of Bengal but not in much of the Arabian Sea (Fig. 22). Thus, in spite of their geographical similarities, the two arms of the north Indian Ocean are strikingly different when it comes to climate.