Climate response to soil dust aerosols

The effect of radiative forcing by soil dust aerosols upon climate is calcu lated. Two atmospheric GCM (AGCM) simulations are compared, one containing a prescribed seasonally varying concentration of dust aerosols, and the oth er omitting dust. Each simulation includes a mixed layer ocean model, which allows SST to change in response to the reduction in surface net radiation by dust. Dust aerosols reduce the surface net radiation both by absorbing and reflecting sunlight. For the optical properties of the dust particles a ssumed here, the reflection of sunlight is largely offset by the trapping o f upwelling longwave radiation, so that the perturbation by dust to the net radiation gain at the top of the atmosphere is small in comparison to the surface reduction. Consequently, the radiative effect of soil dust aerosols is to redistribute healing from the surface to within the dust layer. Beneath the dust layer, surface temperature is reduced on the order of 1 K, typically in regions where deep convection is absent. In contrast, surface temperature remains unperturbed over the Arabian Sea during Northern Hemis phere (NH) summer. even though the dust concentration is highest in this re gion. It is suggested that the absence of cooling results from the negligib le radiative forcing by dust at the top of the atmosphere, along with the f requent occurrence of deep convection, which ties the surface temperature t o the unperturbed value at the emitting level. Where convection is absent, cooling at the surface occurs because radiative heating by dust reduces the rate of subsidence land the corresponding mass exchange with the convecting region). Thus, the temperature contrast betwe en these two regions must increase to maintain the original transport of en ergy, which is unperturbed by dust. It is suggested that cooling over the A rabian Sea during NH winter, despite the much smaller dust loading, is perm itted by the absence of convection during this season. Thus, the change in surface temperature forced by dust depends upon the extent of overlap betwe en the dust layer and regions of deep convection, in addition to the magnit ude of the radiative forcing. Surface temperature is also reduced outside o f the dust cloud, which is unlikely to result solely from natural variabili ty of the AGCM. It is suggested that the perturbation by dust to Indian and African monsoon rainfall may depend upon the extent to which ocean dynamical heat transpor ts are altered by dust.