Westward movement of eddies into the Gulf of Aden from the Arabian Sea

Sea level anomalies ( SLA) from satellite altimetry ( 1993 - 2003) reveal the westward movement of mesoscale eddies in the Gulf of Aden. Inside the gulf the eddies move at a speed of similar to 6.0 - 8.5 cm s(-1), comparable to the first- mode baroclinic Rossby wave speed of 7.2 cm s(-1). We show that the eddies, which enter the gulf from the Arabian Sea, owe their existence to more than one mechanism. Local Ekman pumping in the western Arabian Sea is important during the summer monsoon ( June September). In May and during the latter half of the summer monsoon ( late July to September) and the fall intermonsoon ( October), the dominant mechanism is the generation of eddies by the instabilities in the Somali Current and the large eddies associated with it ( Great Whirl and Socotra eddy). During the winter monsoon ( November - April) the dominant mechanism involves the westward propagating Rossby waves generated either in the Arabian Sea by Ekman pumping or along the west coast of India by poleward propagating Kelvin waves. These Rossby waves from the Arabian Sea propagate slower on entering the gulf because of a shallower thermocline in the gulf. Analysis shows that the SLA signal consists of low ( annual and subannual) and high ( similar to 100 - 180 d) frequencies. The low- frequency signal ( mainly annual) shows a discontinuity between 52 degrees E and 60 degrees E. Though the high- frequency signal is seen at all longitudes, a wavelet analysis shows that it was significant only west of 60 degrees E. An energy analysis, based on model simulations, suggests that barotropic instabilities are important during the entire year and that baroclinic instabilities are also important during the summer monsoon.