TOPOGRAPHIC EFFECTS ON THE SEASONAL CIRCULATION OF THE INDIAN-OCEAN

The relative effects of islands and topography on the seasonal circulation of the southern Indian Ocean are examined. The numerical simulations use a one-active layer reduced gravity model and a two-layer model with full bottom topography forced by the Hellerman and Rosenstein wind stress climatology. The reduced gravity experiments show that the absence of a seasonal cycle of transport off the north coast of Madagascar is not caused by the presence of islands to the east. The two-layer experiments indicate that the presence of abrupt topography associated with the Mascarene Ridge and the Providence Bank north of Madagascar decrease the annual barotropic signal of the North Madagascar Current and increase the seasonal variation of the Southeast Madagascar Current. Furthermore, the two-layer experiments predict the existence of a significant seasonal variation of transport north of the Saya de Malha Bank along the Mascarene Ridge; the signal (12-16 Sv in amplitude) is sufficiently large that the model results may be tested by future observations. Finally, the differences between the seasonal variation of the Indian and North Atlantic oceans are discussed.