Aerosol loading over the Indian Ocean and its possible impact on regional climate

40-55This paper provides a review of aerosol forcing results from the Indian Ocean Experiment (INDOEX) and also summarizes the follow-on modeling studies that examine the impact of the haze on regional climate. Every dry season from November to May, anthropogenic haze spreads over most of the northern Indian Ocean, and South and Southeast Asia. The INDOEX documented this Indo-Asian haze at various scales during 1995-2001. The haze particles consisted of several inorganic and carbonaceous species, including absorbing black carbon clusters, fly ash and mineral dust. Because of black carbon contributing as much as about 14% to the fine particle mass, the single-scattering albedo estimated by several independent methods was consistently around 0.9 both inland and over the open ocean. Anthropogenic sources contributed as much as 75% (10%) to the aerosol loading and the optical depth. The regional aerosol forcing resulting from the direct and indirect effects was derived by integrating the multi-platform observations of satellites, aircraft, ships, surface stations and balloons with 1- and 4-D models. The haze layer reduces the net solar flux at the surface by as much as 20 to 40 Wm⁻² on a monthly mean basis and heats the lowest 3 km atmosphere by as much as 0.4 to 0.8 K/day, which enhances the solar heating of this layer by 50 to 100%. The INDOEX also documented year-to-year fluctuations of the haze forcing. For instance, the southernmost extent of the haze varied from about 10ºS to about 5ºN. In assessing the haze impacts on the cold dry-season regional climate, we conducted two CCM3 experiments with two extreme locations of the forcing: 1) extended haze forcing (EHF) and 2) shrunk haze forcing (SHF). Over India where the forcing is centered, the simulated climate changes are very similar between EHF and SHF. The most important effect of the haze is a surface cooling, and a strengthening of the inversion in the lower troposphere. The surface cooling has been confirmed by observations. The stabilization of the boundary layer results in a reduction of evaporation and sensible heat flux from the land. Rainfall patterns get substantially disrupted in local and remote regions, with the disruption being very sensitive to the southern extent of the imposed haze forcing. Both forcings lead to global circulation/precipitation perturbations; and the EHF produces about an order of magnitude larger responses. One key remote response to the haze is the suppression of convection in the western equatorial Pacific, which has implications for ENSO variability. Since the western Pacific convection suppression would weaken the trade winds over the Pacific and induce warm anomalies in the eastern basin, we speculate that the Great Indo-Asian haze might have an important role in the amplitude and frequency of El Niño events during the recent decades. The haze-ENSO connection is further demonstrated by the Cane-Zebiak Pacific Ocean/atmosphere model. The focus of the studies thus far has been on the dry season (November to May) aerosols. The role of anthropogenic aerosols during the wet season from June to September is to be explored