Northwest Indian Ocean’s Spring Cooling

A major cooling down of the northwestern Indian Ocean’s surface, including the Arabian Sea, starts in May, according to a well-known world atlas of SSTs. This is before the southwest monsoon which usually begins in June. Also within one year, there are two surface temperature maxima and two minima, which is not typical for the northern hemisphere. A surface current, cooler than the surrounding water, crosses the equator in April and May heading north and east on the western side of the ocean. That proposal is consistent with the given SST information. The warmer surrounding water is then moved to east and south as a consequence. Since wind driving is not available for initiation, the relatively cool northeastward current is thought to be caused by a thermohaline force related to the unstable northward temperature gradient in the west, which is of constant sign right across the equator beginning in May: cool in the south monotonically increasing to warm in the north.