A Coupled Ocean–Atmosphere System of SST Modulation for the Indian Ocean*

The net heat flux into the northern Indian Ocean is between 75 and 100 W m22 during the boreal spring and early summer but only 10–20 W m22 in the western Pacific during the entire year. The differences in the net surface heat fluxes arise from the radiation shielding by deep convective clouds over the western Pacific and the near absence of clouds over the northern Indian Ocean during spring. Despite the strong heat flux during the boreal spring and early summer, the northern Indian Ocean only warms by 38C over a 4-month period, much smaller than the 128C estimated using single ocean column models. Similarly, the cooling of the North Indian Ocean during the boreal winter is less than can be explained from thermodynamic processes. The tropical western Pacific Ocean sea surface temperature (SST), on the other hand, varies by only 18C throughout the year. Despite the different net surface fluxes of heat into the two warm pools, the magnitude of the annual cycle of SST in each basin is very small and is confined to lie between 268 and 308C. It is argued that SST regulation in the Pacific Ocean occurs as a combination of local thermodynamic balance with significant contributions from large-scale atmospheric circulations forced by SST gradients. On the other hand, the northern Indian Ocean SST is regulated by strong oceanic advection across the equator, southward during the boreal spring and early summer and northward during the boreal winter, and by changes in heat storage of the upper ocean. Cross-equatorial transports of heat vary from 0.5 PW southward in spring to 1.5 PW in summer. During the boreal winter, the heat fluxes reverse, averaging about 1 PW toward the north. Large oscillations of ocean heat transport are also noted on timescales of 20–50 days. It is concluded that ocean heat transports are an essential process in the annual cycle of the Indian Ocean heat balance. Last, the results of the study are used to propose a coupled ocean–atmosphere dynamic regulatory system for the annual cycle of the monsoon and the SST in the Indian Ocean.