Annual cycle of the South Indian Ocean (Seychelles‐Chagos) thermocline ridge in a regional ocean model

Abstract

The presence of an upwelling “dome”‐like feature in the thermocline depth at 55°E–65°E, 5°S–12°S in the southwest tropical Indian Ocean (SWTIO) has been suggested in earlier work. However, the position, shape, and forcing mechanisms behind this upwelling region are not well understood. In this study, a regional ocean model is applied to the tropical South Indian Ocean. Experiments with monthly climatological winds from both Quick Scatterometer (QuikSCAT) and the National Centers for Environmental Protection (NCEP) reanalyses are performed. An annual and semiannual signal is present in the depth of the model thermocline. The model results suggest that SWTIO upwelling is focused in the west during austral spring and summer and forms a zonally elongated ridge during austral autumn and winter, termed here the Seychelles‐Chagos thermocline ridge. Although the large‐scale wind stress curl plays a major role in maintaining this upwelling ridge, local divergence between the southeasterly trade winds and the monsoon westerlies are shown to impact this region, as well as remote forcing through the arrival of both upwelling and downwelling annual Rossby waves from the eastern Indian Ocean.