Aerosol optical depth, physical properties and radiative forcing over the Arabian Sea

Abstract

The Arabian Sea region $(4^{o} N-20^{o} N to 50^{o} E-78^{o} E)$ has a unique weather pattern on account of the Indian monsoon and the associated winds that reverse direction seasonally. The aerosol data, collected using ship-borne and island platforms (for 8 years from 1995 to 2002) along with MODIS (onboard TERRA satellite) data (from 2000 to 2003) have been used to evolve a comprehensive characterisation of the spatial and temporal variation in the physical, chemical, and radiative properties of aerosols over the Arabian Sea. The aerosol optical depth (AOD) was found to increase with latitude between the equator and $12^{o} N$. Over the northern Arabian Sea (regions lying north of $12^{o} N$), AODs do not show significant latitudinal variations; the average aerosol optical depth for this region was 0.29\pm0.12 during winter monsoon season (WMS; November to March) and 0.47\pm0.14 during summer monsoon season (SMS; April=May to September). The corresponding Angstrom exponents were 0.7\pm0.12 and 0.3\pm0.08, respectively. The low values of the exponent during SMS indicate the dominance of large aerosols (mainly dust particles > 1 \mu m). The latitudinal gradient in AOD in the southern Arabian Sea is larger during SMS compared to WMS. The size distribution of aerosols shows two well-defined modes, one in the accumulation size regime and the other in the coarse size regime. During WMS, a third mode (nucleation) also appears in the sub micron range below ~0.1 mm. The single scattering albedo does not show significant seasonal variations (remains within ~0.93 to 0.98 through out the year). During WMS (SMS), top of the atmosphere diurnally averaged aerosol forcing remains around -6.1 (-14.3)W$m^{-2}$ over the northern Arabian Sea up to around $12^{o} N$ and decreases southwards till it attains a value of -3.8 (-3.4)W$m^{-2}$ at the equator. The surface forcing remains around -16.2 (-15.2)W$m^{-2}$ over the northern Arabian Sea up to $12^{o} N$ and decreases southwards to a value of -5.5 (-3.5)Wm$m^{-2}$ at the equator. Over the north Arabian Sea, instantaneous forcing (flux change) at the surface can be as high as -50W$m^{-2}$. The instantaneous forcing decreases with latitude in the southern Arabian Sea at a rate of ~3W$m^{-2} deg^{-1}$.