Modeling salinity distributions in the Indian Ocean

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Abstract
A 4½‐layer model with active thermodynamics and mixed‐layer physics is used to isolate effects on salinity distributions in the Indian Ocean that result from various forcing mechanisms. These forcings include evaporation (ε) and precipitation (P) through the ocean surface and inflows across basin boundaries by river runoff in the Bay of Bengal, the Indonesian Throughflow, the Persian Gulf, and the Red Sea. A suite of solutions is obtained in which each forcing is added sequentially. In the solution forced only by P−ε, salinity patterns in the upper three layers agree qualitatively with the observations, but values tend to be higher throughout most of the basin. When river inflow into the Bay of Bengal is included, salinity values are significantly improved in the upper three layers, especially in the northern Bay and along the east and west coasts of India. In addition, solutions suggest that during the Northeast Monsoon part of the river water flows out of the Bay in the shallow channel between Sri Lanka and India: Only when this channel is opened in the upper layer do solutions develop a strong, across‐shelf salinity gradient along the west Indian coast, consistent with the observations. When the Indonesian Throughflow is added, salinities are lowered in all four layers, especially in the southern tropical ocean. Most of the Throughflow eventually flows out of the Indian Ocean along the western boundary and near Madagascar, but some is advected across the equator by the East African Coastal Current, where it is carried eastward and northward into the central Arabian Sea. Saltier Persian Gulf Water is introduced into the model in layer 3. Some of it subsequently entrains into the surface mixed layer, increasing sea surface salinity by 0.1–0.2 practical salinity units (psu) in a broad region of the Arabian Sea. Saltier Red Sea Water enters the basin in layer 4. It increases layer 4 salinity values throughout much of the Indian Ocean, particularly in the Somali Basin, the interior Arabian Sea, and the central and western equatorial ocean.