Aerosol radiative forcing over tropical Indian Ocean: Modulation by sea-surface winds

It is now clearly understood that atmospheric aerosols have a significant impact on climate because of their important role in modifying the incoming solar and outgoing IR radiation. Recent investigations over the tropical Indian Ocean have shown that the single largest natural contributor of the aerosol visible optical depth are sea-salt aerosols (more than 50% of the natural). It is well known that sea-salt aerosol concentration depends on the sea-surface wind speed(1). In this paper, the reduction of surface reaching solar flux and increase in the top of the atmosphere (TOA)-reflected solar flux due to the presence of sea-salt aerosols are estimated as a function of wind speed and their role on the radiative forcing and its implications are examined. It is shown that in cloudy conditions over the ocean, the effect of sea-salt is to partly offset the positive forcing (heating) by soot aerosol. The surface and TOA forcing by sea-salt aerosols are as high as - 6.1 W m(-2) and - 5.8 W m(-2) respectively (at high wind conditions), which are about 20% and 60%, respectively of the total aerosol forcing (for a mean aerosol optical depth of 0.4) over tropical northern Indian Ocean. Over pristine regions of the Southern Hemisphere where anthropogenic influence is minimal, forcing is mainly determined by the surface wind speed. Results show that the algorithms for the retrieval of aerosol properties and sea-surface temperature (SST) from satellite data should take into account the changes in aerosol chemical composition with changes in sea-surface wind speed.