Aerosol Radiative Forcing Derived from SeaWiFS-Retrieved Aerosol Optical Properties

Abstract

To understand climatic implications of aerosols over global oceans, the aerosol optical properties retrieved from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) are analyzed, and the effects of the aerosols on the earth's radiation budgets [aerosol radiative forcing (ARF)] are computed using a radiative transfer model. It is found that the distribution of the SeaWiFS-retrieved aerosol optical thickness is distinctively zonal. The maximum in the equatorial region coincides with the intertropical convergence zone, and the maximum in the Southern Hemispheric high latitudes coincides with the region of prevailing westerlies. The minimum aerosol optical thickness is found in the subtropical high pressure regions, especially in the Southern Hemisphere. These zonal patterns clearly demonstrate the influence of atmospheric circulation on the oceanic aerosol distribution. Over global oceans, aerosols reduce the annual-mean net downward solar flux by 5.4 W m⁻² at the top of the atmosphere, and by 5.9 W m⁻² at the surface. The largest ARF is found in the tropical Atlantic, Arabian Sea, Bay of Bengal, the coastal regions of Southeast and East Asia, and the Southern Hemispheric high latitudes. During the period of the big Indonesian fires (September–December 1997), the cooling due to aerosols is more than 10 W m⁻² at the top of the atmosphere, and more than 25 W m⁻² at the surface in the vicinity of Indonesia. The atmosphere receives extra solar radiation by more than 15 W m⁻² over a large area. These large changes in radiative fluxes are expected to have enhanced the atmospheric stability, weakened the atmospheric circulation, and augmented the drought condition during that period. It would be very instructive to simulate the regional climatic impact of the big Indonesian fires during the 1987–88 El Niño event using a general circulation model.