The Response of the Surface Circulation of the Arabian Sea to Monsoonal Forcing

Abstract

wo decades of drifter and satellite data allow the authors to describe the seasonal evolution of the surface circulation of the Arabian Sea, which reverses annually with the Indian monsoon winds. This study finds several features that advance current understanding. Most significantly, northward flow appears along the length of the western boundary, together with a weak anticyclone at 6°N (a precursor to the Great Whirl) as early as March or April, one or two months before the southwest monsoon winds. This circulation is driven by planetary waves, which are initiated by wind curl forcing during the previous southwest monsoon, leading the authors to speculate that there is an oceanic mechanism through which one monsoon may precondition the next. Second, the authors find that the eastward South Equatorial Counter Current (SECC) is present year-round, fed by the northward East African Coastal Current (EACC). During the southwest monsoon the EACC overshoots the equator and splits, feeding both northward into the Somali Current and eastward into the SECC by looping back across the equator. This retroflection of the EACC is what was previously known as the southern gyre. At the surface, this circulation is obscured by strong, locally wind-driven, cross-equatorial transport. The semiannual variability of the SECC is governed by Ekman pumping over the equatorial gyre. Finally, there is broad, strong eastward flow at the mouth of the Gulf of Aden throughout the southwest monsoon, coincident with alongshore winds and a switch in sign of the wind stress curl along the axis of the atmospheric monsoon jet.