Indian Ocean Experiment: An integrated analysis of the climate forcing andeffects of the great Indo-Asian haze

Every year, from December to April, anthropogenic haze spreads over most of the North Indian Ocean, and South and Southeast Asia. The Indian Ocean Exp eriment (INDOEX) documented this Indo-Asian haze at scales ranging from ind ividual particles to its contribution to the regional climate forcing. This study integrates the multiplatform. observations (satellites, aircraft, sh ips, surface stations, and balloons) with one- and four-dimensional models to derive the regional aerosol forcing resulting from the direct, the semid irect and the two indirect effects. The haze particles consisted of several inorganic and carbonaceous species, including absorbing black carbon clust ers, fly ash, and mineral dust. The most striking result was the large load ing of aerosols over most of the South Asian region and the North Indian Oc ean. The January to March 1999 visible optical depths were about 0.5 over m ost of the continent and reached values as large as 0.2 over the equatorial Indian ocean due to long-range transport. The aerosol layer extended as hi gh as 3 km. Black carbon contributed about 14% to the fine particle mass an d 11% to the visible optical depth. 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 80% (+/- 10%) to the aerosol loading and the optical depth. The in situ data, which clearly support the existence of the first indirect effect (increased aeros ol concentration producing more cloud drops with smaller effective radii), are used to develop a composite indirect effect scheme. The Indo-Asian aero sols impact the radiative forcing through a complex set of heating (positiv e forcing) and cooling (negative forcing) processes. Clouds and black carbo n emerge as the ma or players. The dominant factor, however, is the large n egative forcing (-20 +/- 4 W m(-2)) at the surface and the comparably large atmospheric heating. Regionally, the absorbing haze decreased the surface solar radiation by an amount comparable to 50% of the total ocean heat flux and nearly doubled the lower tropospheric solar heating. We demonstrate wi th a general circulation model how this additional heating significantly pe rturbs the tropical rainfall patterns and the hydrological cycle with impli cations to global climate.