A comparison of mixed-layer dynamics between the Arabian Sea and Bay of Bengal: One-dimensional model results

Abstract

Using a one-dimensional turbulent closure model, we investigate the physical mechanisms governing the seasonal evolution of mixed-layer depth (MLD) and sea surface temperature (SST) along two meridional sections; 64.5°E and 88.5°E, which are representative of the open-ocean conditions of the Arabian Sea and the Bay of Bengal. Several perturbation experiments have been performed to isolate and judge exactly the relative importance of wind stress and buoyancy forcing to the evolution of MLD and SST. The wind stress forcing is found to be significant during the summer monsoon, while buoyancy flux plays a significant role during the winter monsoon. During the summer monsoon (June-September), the MLD in the Arabian Sea deepens to 80 m owing to strong wind-driven vertical mixing, but the weaker winds over the Bay of Bengal restrict the turbulent mixing to a shallow depth of ∼30 m. As a result, SST in the Arabian Sea (Bay of Bengal) decreases to ∼27°C (remains high ∼29°C). It is thus the asymmetry in the wind field that is responsible for the difference in MLD and SST between the Arabian Sea and the Bay of Bengal. During the winter monsoon (November-February), the MLD in the Arabian Sea penetrates to deeper than 80 m by convective vertical mixing because of strong negative buoyancy flux (net heat loss), but it is restricted to 40-50 m in the Bay of Bengal owing to reduced net heat loss from the ocean. It is the asymmetry in the buoyancy forcing caused by the difference in latent heat flux that is responsible for the difference in MLD and SST between the Arabian Sea and the Bay of Bengal. The atmospheric parameter that controls this buoyancy difference is the difference in humidity between the two basins. Thus it is the differences in surface forcing, rather than differences in the vertical salinity stratification, that are key in explaining the observed and modeled differences between the Arabian Sea and Bay of Bengal.