Premonsoon shortwave aerosol radiative forcings over the Arabian Sea and tropical Indian Ocean: Yearly and monthly mean variabilities

Abstract

The yearly and monthly mean variations in the clear‐sky shortwave aerosol direct radiative forcings are estimated over Coastal India (CI), the Arabian Sea (AS), and tropical Indian Ocean (TIO) during winter monsoon seasons of 1996–2000. The 5‐year mean TOA forcings are estimated to be −10, −9, and −4 W m−2 over CI, AS, and TIO. Surface (SFC) forcings are estimated to be −29, −22, and −5 W m−2 over CI, AS, and TIO, respectively. Atmosphere absorptions are quite large over CI and AS at +19 and +13 W m−2, indicating a significant influence of soot emitted from fossil fuel, biomass burning, and mineral dust. The yearly and monthly mean aerosol forcings are found to exhibit variations. The large atmospheric absorption estimated over CI and AS is found to be a feature of the other polluted ocean and land regions. The influence of relative humidity on the aerosol forcing is estimated. It is shown that as the single scattering albedo increases, the TOA forcing increases while the SFC forcing decreases. With an increase in aerosol optical depths the forcings at TOA and SFC are found to increase substantially, while an increase in the asymmetry parameter is found to decrease both TOA and SFC forcings. The effective factors (forcing at 80% RH/forcing at 30% RH) for the surface are found to be about 1.2 over CI, AS, and 1.4 over TIO. The effective factors estimated over these oceanic regions are found to be smaller than those obtained over the southeastern United States and the mid‐Atlantic coast, indicating that the composition of aerosols found over these regions is different.