Modulation in Direct Radiative Forcing Caused by Wind Generated Sea-Salt Aerosols

ABSTRACT

Sea-salt aerosols, prominent natural aerosols over the ocean, play a vital role in direct and indirect radiative forcing. Since surface winds are the prime cause of sea-salt generation, we have developed an empirical relationship between aerosol optical depth (AOD) and sea-surface winds over the study region (60–70°E; 40°S–20°N). The latitudinal variation of background aerosol optical depth (τ₀) and the wind index (b) are estimated as they are essential inputs for the estimation of the spatial variation of sea-salt aerosols and are used over the Arabian Sea (AS) to generate spatial heterogeneity of sea-salt aerosols. The latitudinal variation of τ0 and b show a nearly exponential and linear increase, respectively, as we moved towards the north. We used an empirical-cum-model approach to construct an aerosol system to reproduce the observed AOD and aerosol optical properties. Utilizing this information as input to a radiative transfer model, we worked out direct radiative forcing (DRF) over the study region in the short wave (0.2–4 µm) and long wave (8–14 µm) region at the surface, top of the atmosphere (TOA) and in the atmosphere. Short wave cooling at the surface, TOA and heating in the atmosphere are estimated as 40 W m^–2, 32 W m^–2 and 8 W m^–2, respectively. Long wave heating due to the sea-salt aerosols estimated at the surface, TOA and in the atmosphere is about 9 W m^–2, 6 W m^–2 and 15 W m^–2 respectively. Long wave forcing (LWF) partly counterbalances the effect of short wave forcing (SWF) and the cooling at the surface and at TOA. The highest value of such an offset at the surface was observed over the AS (~23%) and that at TOA was ~19%, obviously at high wind conditions. This implies that over the AS sea-salt aerosols (in coarse mode) contribute significantly to LWF compared to other adjacent oceanic regions.