Abstract
The evolution of surface circulation, salinity budget, and processes at intermediate depth in the northwestern Indian Ocean were studied with the open boundary version of the Miami Isopycnic Coordinate Ocean Model (MICOM). Under climatological wind and thermodynamic forcing, the model develops solutions that are in good agreement with global MICOM results and with observations. When the observed winds (Legler et al., 1989) force the model, interannual variability of the surface fields increases significantly. However, coalescence of the two large eddies in the end of the summer monsoon, which was observed in some years, does not occur in the model. To identify what processes facilitate the merger, a series of experiments was performed with modified model parameters and forcing fields.The eddies coalesced when half-slip, rather than no-slip, boundary conditions were used. In this case, less positive vorticity was produced at the coast, resulting in reduced blocking effect on the propagation of the southern eddy. The Socotra Island, which is submerged in the standard model, hinders movement of the northern anticyclone, leading to stronger interaction between the eddies and their subsequent merging. A more realistic coalescence occurs in an experiment where winds are held constant after reaching the peak summer value.Freshwater fluxes from the east and south were previously considered important for the salinity budget in the Arabian Sea, where evaporation exceeds precipitation; however, the model demonstrated that only cross-equatorial transport of low-salinity water in the upper 400 m is essential. About 86% of this water is advected below the surface layer at the western boundary. The strongest interaction between the mixed layer and the oceanic interior occurs during the summer in the coastal upwelling regions off Somalia. 43% of all upwelled water comes from depths between 100 m and 200 m, thus signifying the importance of mid-depth circulation and water mass distribution for the surface processes.Both observations and model solutions demonstrate strong annual cycle and alongshore variability of coastal subsurface circulation. A cross-equatorial northward flow below the surface, which exists throughout the year, is disconnected from currents farther north. A southward undercurrent north of 5N is present during the winter monsoon. Water for the current is supplied by flows from the north and northeast. The former originates in the Persian Gulf and carries higher-salinity water, while the latter contribution is mainly from the Gulf of Aden. Separation of the southward undercurrent near 4N is not caused by its interaction with a topographical ridge, as was previously suggested. Agreement with the observations north of 5N improves when the Socotra Island is present in the model.