92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Thursday, 26 January 2012
Direct and Semi-Direct Effects of Aerosols on the Tropical Climate
Hall E (New Orleans Convention Center )
Salil Mahajan, ORNL, Oak Ridge, TN; and K. J. Evans, J. J. Hack, and J. E. Truesdale

High-resolution (1x1 degree) global tropospheric aerosol datasets with monthly temporal resolution are generated using the atmospheric component of the Community Earth System Model (CESM1.0) coupled to an active bulk aerosol model for the pre-industrial era and the period 1960-2000. The model set-up is forced with the latest estimate of surface and elevated emissions of anthropogenic and natural aerosol precursors and oxidants for the two periods and the tropospheric concentration of different aerosol species are derived from the model for each period. AMIP-style experiments conducted with the new T85 spectral configuration of Community Atmosphere Model (CAM4.0), with CAM4.0 uncoupled aerosol-cloud microphysics, when forced with the new high-resolution aerosol datasets reveal that the inter-annual variability in the tropospheric aerosol radiative forcing induced by the inter-annual variation of tropospheric aerosols can enhance inter-annual variability of regional climate response, particularly that of the tropical Atlantic and the South-East Asian precipitation response. Further, AMIP experiments also reveal that the presence of tropospheric aerosols has a significant impact on the trends in regional surface temperature and precipitation response in the later part of the 20th century. Experiments are also conducted with the T85 spectral configuration of CAM4.0 coupled to a slab ocean model forced with the new aerosol datasets. We find that the current level of tropospheric aerosols can impact the land-sea contrast, and cross-equatorial SST gradient leading to a significant reduction in the Indian monsoon and shift in the ITCZ over the tropical Atlantic in the coupled atmosphere-slab ocean model simulation. These phenomena are amplified in the coupled atmosphere-slab ocean model experiment when compared to atmosphere-only runs, suggesting a dominant role of air-sea thermodynamic coupling in the climate response to direct radiative aerosol forcing. Preliminary analysis of the coupled atmosphere-slab ocean model experiments indicates that diabatic heating of the troposphere by absorbing aerosols - the semi-direct effect, has a little role to play in the southwards movement of the ITCZ over the tropical Atlantic, but has a significant impact on the suppression of South-East Asian monsoons.

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