Modelling the Variability in the Somali Current

A numerical model of the wind driven circulation in the Indian Ocean is used to study the variability of the circulation on seasonal and interannual time scales. The model is a nonlinear reduced gravity model driven by observed winds. Model simulations use a monthly mean climatology of ships' winds as forcing and the 23 year long monthly mean Cadet and Diehl winds as forcing. The model is very successful in simulating the observed features of the circulation in this region, such as the formation and decay of the two-gyre system in the Somali Current during the southwest monsoon and the formation of the eddies off the coasts of Oman and Yemen. Examination of model statistics from many years of simulation using climatological monthly mean winds shows that the model fields are exactly repeating from one year to the next over most of the basin, even in the highly nonlinear eddies like the great whirl. Exceptions occur in the smaller scale eddies that form in the strong shear zones around the great whirl and in the southern gyre recirculation region, where the flow field exhibits a more chaotic nature, but even these features are nearly repeating from one year to the next. When observed, interannually varying winds are used to drive the model, the variability from year to year increases dramatically. This indicates that interannual variability in the model fields is due solely to variability in the winds and not due to inherent variability in the model physics, as is seen in mid-latitude models of the oceanic general circulation.

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