Contribution of atmospheric forcing to cooling of the Arabian Sea during onset of the southwest monsoon

Abstract

The rapid cooling of the mixed layer in the Arabian Sea during the southwest monsoon onset is examined. The atmospheric forcing fields over the Arabian Sea during the onsets in 1979, 1984, and 1985 are analyzed. Realistic wind speed and evaporative heat flux increases occur soon after onset. The solar radiation flux analyses suggest that both the Navy Operational Global Atmospheric Prediction System and the National Aeronautics and Space Administration four-dimensional data assimilation are too simplistic for accurate solar radiation flux calculations in the tropics. The solar radiation fluxes appear to be too low during the pre-onset periods and too high during the post-onset periods. The atmospheric forcing fields are then applied to a one-dimensional, ocean mixed layer model at four locations. The magnitude of the predicted sea-surface temperature decrease is 0.5 to 1.0°C during the 1984 and 1985 monsoon onsets. The results suggest that local atmospheric forcing accounts for most of the mixed layer cooling on a time scale of less than ten days and away from any major ocean current regions. The direct heat loss to the atmosphere contributes more to cooling of the mixed layer than does entrainment mixing at the base of the layer.