The Great Whirl: Observations of its seasonal development and interannual variability

In situ measurements are used, together with sea surface height data, to study the development and variability of the Great Whirl (GW), a large quasi-stationary anticyclone that appears off the coast of Somalia during the southwest monsoon season. We find that anticyclonic circulation indicative of the GW appears on average in April, almost two months before the onset of the southwest monsoon winds. This early initiation is coincident with the arrival of annual Rossby waves at the western boundary. With the onset of the monsoon winds in early June, the GW-proper intensifies quickly, remaining at its peak throughout July, August, and September, and dissipating about one month after the winds have died. The GW is present for an average 166 +/- 30 days per year and the position of its northern flank, close to 9 degrees N, coincides with the latitude of zero wind stress curl. The intraseasonal variability of the GW is intense as a result of mutual advection with one to three flanking cyclones, which accompany the GW 70% of the time and tend to circulate clockwise around it. There is no consistent seasonal pattern for the development or dissipation of the GW, although movement to the southwest is common toward the end of the season. The GW of 1995 deepened from 200 m in June to over 1000 m in September, and strengthened from a swirl transport of 10 to 60Sv. Cool waters in its core resulted from advection via the Somali Current and some local vertical mixing.