83rd Annual

Monday, 10 February 2003: 2:00 PM
The South-Asian Brown Cloud: A Coupled Ocean-Atmosphere Model study of its impact on the Monsoon, El-Nino and the Hyrdological Cycle
Veerabhadran Ramanathan, SIO/Univ. of California, La Jolla, CA; and J. T. Kiehl, T. Bettge, W. M. Washington, and . C. Chung
The south-Asian Brown Cloud is an absorbing aerosol layer of about 3 km thick whose origin is fossil fuel combustion and biomass burning by a population of over 3 billion. Long-range transport transforms this absorbing haze into a widespread regional scale aerosol layer covering most of asia and the northern indian ocean which lead to large reductions (10 to 15%) in the surface solar irradiance, a corresponding increase in atmospheric solar heating (30 to 100%), stronger low level inversions, suppression of rainfall, and less efficient removal of pollutants. The starting point of this study is the Indian Ocean Experiment which provided estimates of the radiative focing at the surface and for the atmosphere. We employ this data in the NCAR community climate system model (CSM) to examine the regional and global impact of the Asian Brown cloud. The reduction in the surface radation cools south Asia by as much as 0.6K during the winter time. The fundamental impact of the atmospheric heating by the haze is to push the inter-tropical convergence northwards, suppress precipitation over the western Tropical pacific warm pool, southern Africa and western Asia. The suppression of precipitation over the warm pool weakens the trades and triggers El-Nino like response with global impacts. The haze induced surface cooling of the Arabian sea leads to weakening of the summer monsoon rainfall. The study provides new insgihts into the impact of absorbing aerosols on the tropical hydrological cycle.

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