Influence of Salinity on Dynamics, Thermodynamics and Mixed-Layer Physics in the Indian Ocean

A nonlinear, 4½-layer model with active thermodynamics and mixed-layer physics is used to examine salinity effects due to various forcings in the Indian Ocean. Theses forcings include: evaporation (ε) and precipitation (Ρ), river runoff in the Bay of Bengal, the Indonesian Throughflow, and the influx of salty waters from the Persian Gulf and the Red Sea. Solutions with P - ε forcing produce salinity patterns that agree qualitatively with the observations in the upper three layers. Quantitatively, however, salinity values tend to be higher than the observations in most of the basin. In regions where precipitation is strong (P - ε » 0), a thin surface mixed layer (layer 1), and thus a thicker seasonal thermocline (layer 2, a barrier layer), are formed due to decreased entrainment. In these regions, surface currents generally strengthen, T2 warms considerably and SST increases somewhat, resulting in temperature inversions at some locations of the southern Bay and the eastern equatorial ocean. Somewhat surprisingly, P - ε also causes large temperature changes in layer 3 (thermocline) and thickness changes in layers 3 and 4 (intermediate water). The Bay-of-Bengal river runoff improves salinity values significantly in the upper three layers, especially within the Bay and alongC the west coast of India. During the Southwest onsoon (SWM), coastal Kelvin waves driven by the Ganges-Brahmaputra river inflow suppress upwelling along the northeast coast of India, increasing SST by 1°C. During the Northeast Monsoon (NEM), fresh water from the rivers is carried southward by the East India Coastal Current (EICC), raising sea level and thus strengthening the EICGby 10 cm/s. This fresh water can flow directly through the India-Sri Lanka separation in the surface mixed layer, generating a strong salinity gradient along the west Indian coast during winter. The river water decreases entrainment around the perimeter of the Bay during winter, thereby producing a thin surface mixed layer, increasing T2 , and resulting in temperature inversions in the northwestern Bay. Like P - ε, the rivers cause significant thickness and temperature anomalies in layer 3. The Indonesian Throughflow improves salinities in all four layers of the model, especially in the southern tropical ocean. Consistent with previous studies, most of the Throughflow water flows out of the Indian Ocean along the western boundary and near Madagascar. A significant amount of water, however, is advected northward into the Somali basin and subsequently carried eastward into the ocean interior and northward into the Arabian Sea. The Throughflow increases SST primarily along the west Australian coast but warms the thermocline (layer 3) throughout the Indian Ocean, especially in the southern tropical ocean. As a consequence, sea level is raised in the entire basin. Warmer and saltier Persian-Gulf water (PGW) enters the Indian Ocean in layer 3, warming the northern Arabian Sea by 0.2-2°C and increasing the salinity by 0.1-0.6 psu through horizontal mixing. It increases sea-surface salinity (SSS) in a broad region of the Arabian Sea by 0.1- 0.2 psu because entrainment and, to a less extent, coastal upwelling bring PGW into the surface mixed layer, where it spreads over a large region due to advection. High-salinity and high-temperature Red-Sea water (RSW) warms layer-4 (upper intermediate layer) and increases its salinity by a significant amount in most region of the Indian Ocean, especially in the Somali Basin, the interior Arabian Sea, and the central and western equatorial ocean