Seasonal effects of Indian Ocean freshwater forcing in a regional coupled model
Seo, H., S.-P. Xie, R. Murtugudde, M. Jochum, and A. J. Miller
Journal of Climate in press

Abstract
Effects of freshwater forcing from river discharge in the Indian Ocean on oceanic vertical structure and the Indian monsoons are investigated using a fully coupled, high-resolution, regional climate model. The effect of river discharge is included in the model by restoring sea surface salinity (SSS) towards observations. The simulations with and without this effect in the coupled model reveal a highly seasonal influence of salinity and the barrier layer (BL) on oceanic vertical density stratification, which is in turn linked to changes in sea surface temperature (SST), surface winds and precipitation.

During both the boreal summer and winter, SSS relaxation leads to a more realistic spatial distribution of salinity and the BL in the model. In summer, BL in the Bay of Bengal enhances the upper ocean stratification and increases the SST near the river mouths where the freshwater forcing is largest. However the warming is limited to the coastal ocean and the amplitude is not large enough to give a significant impact on the monsoon rainfall.

The strengthened BL during the boreal winter, on the other hand, leads to shallower mixed layer. Atmospheric heat flux forcing acting on a thin mixed layer results in an extensive reduction of SST over the northern Indian Ocean. Relatively suppressed mixing below the mixed layer warms the subsurface layer, leading to a temperature inversion. The cooling of the sea surface induces a large-scale adjustment in the winter atmosphere with amplified northeasterly winds. This impedes atmospheric convection north of the equator while facilitating it in the austral summer inter-tropical convergence zone, resulting in a hemispheric-asymmetric response pattern. Overall, our results suggest that freshwater forcing from the river charges play an important role during the boreal winter by affecting SST and the coupled ocean-atmosphere interaction, with potential impacts on the broad-scale regional climate.