The role of oceanic internal instabilities on the Great Whirl interannual variability

The Great Whirl (GW) is a quasi-permanent anticyclonic eddy that appears every summer monsoon in the western Arabian Sea off the horn of Africa. It generally forms in June, peaks in July-August, and dissipates afterward. While the annual cycle of the GW has been previously described, its year-to-year variability has been less explored. Satellite observations reveal that the leading mode of summer interannual sea-level variability in this region is associated with a typically ~100-km northward or southward shift of the GW. This shift is associated with coherent sea surface temperature and surface chlorophyll signals, with warmer SST and reduced marine primary productivity in regions with positive sea level anomalies and vice versa. Eddy-permitting (~25 km) and eddy-resolving (~10 km) ocean general circulation model simulations reproduce the observed pattern reasonably well, even in the absence of interannual variations in the surface forcing. This implies that the GW interannual variability partly arises from oceanic internal instabilities. Ensemble oceanic simulations further reveal that this stochastic oceanic intrinsic variability and the deterministic response to wind forcing each contribute to ~50% of the total GW interannual variability in July-August. The deterministic part appears to be related to the oceanic response  to Somalia alongshore wind stress and offshore wind-stress curl variations during the monsoon onset projecting onto the GW structure, and getting amplified by oceanic instabilities. After August, the stochastic component dominates the GW variability.