Ocean Forcing and the Surface Mixed Layer

The ocean is forced by radiation from and to the atmosphere, fluxes across the air-sea interface, and by freshwater input from precipitation and rivers. They drive circulations in the surface mixed layer (ML) of the ocean, which in turn force deeper circulations. We review each of these forcings, and discuss their impacts on ML properties in both the real ocean and ML models. One impact of evaporation and freshwater input is that the ML thickness differs markedly in the northern areas of the Bay of Bengal and Arabian Sea. In the northern Bay, high freshwater input decreases near-surface salinity and density, and the resulting increase in near-surface stratification ensures that the ML remains relatively thin. Conversely, in the northern Arabian Sea high evaporation increases near-surface salinity and density, decreasing the near-surface stratification and allowing the ML to thicken to larger values. During the summer monsoon, the thinner ML in the northern Bay leads to sea-surface temperature being warm enough to support atmospheric convection, making the northern Bay one of the rainiest regions in the global tropics.