Impact of Arabian Sea pollution on the Bay of Bengal winter monsoon rains

Accumulation of pollution over the southern Arabian Sea has been documented in numerous studies that followed the INDOEX field project of 1999. In this study several examples of Arabian Sea pollution are shown using MODIS/CALIPSO data sets during the winters of 2002 through 2007. We identify the Bombay Plume that makes its way into the Arabian Sea from the west coast of India. A second part of this work is on modeling the regional impacts of Arabian Sea pollution. We use the NASA Goddard Earth Observing System (GEOS) model to carry out many comparative forecast simulation experiments that include the pollution based on MODIS and control runs that utilize climatological estimates of pollution aerosols. The model invokes both the direct and indirect effects of aerosols. Our observations are as follows: (1) The Arabian Sea experienced above normal rain during these periods for the MODIS experiments as compared to the control. (2) The most interesting feature is the divergent outflow center, in the upper troposphere, over polluted regions of the atmosphere over the Arabian Sea as a consequence of tropospheric aerosol heating. (3) An important related feature is a compensating downward lobe with a divergent inflow in the upper troposphere centered over the Bay of Bengal. (4) The presence of this downward lobe over the Bay of Bengal relates to a reduction of winter monsoon rains over the southeast coast of India. (5) We also show evidence of similarly reduced winter monsoon rains using raingauge data over the southeast coast of India during high-pollution events identified by MODIS data.