A GCM Investigation of Dust Aerosol Impact on the Regional Climate of North Africa and South/East Asia

Abstract:

The interactions between dust and other physical processes have been found to play an important role in the dust-induced climate change. However, there are large uncertainties regarding whether, where, and how the dust enhances or suppresses precipitation. The climatic effects of dust aerosols in North Africa and South/East Asia have been investigated using an atmospheric general circulation model, NCEP/GCM/SSiB (Simplified Simple Biosphere Model) and the three-dimensional aerosol data simulated by the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model. GCM simulations show that due to the scattering and absorption of solar radiation by dust particles, surface temperature decreases over both regions, accompanied by a reduced sensible heat flux. However, precipitation responses are different in these two regions. Due to differences in dust location and the associated heating with respect to the rainfall band and circulation, the effect of dust could either enhance or suppress precipitation. Over the North Africa region where dust particles are mainly located to the north of rainfall band, heating of the air column by dust particles forces a stronger ascent motion over dust layers, which induces an anomalous subsidence (or a weakened upward motion) and suppressed cyclonic circulation to its south where precipitation reduces. In South/East Asia, dust particles are located in the upper troposphere over the major rainfall band during the monsoon season, especially Southwest India and the coastal area of Bay of Bengal. Heating of the air column increases upward motion and strengthens cyclonic circulation. Therefore, cloud and precipitation increase over South/East Asia associated with dust effect. During the pre-monsoon season, when dust particles are located to the north of the monsoon rainfall band, the heating effect results in shifting precipitation northward. The anomalous upward motion over dust regions will induce a subsidence to its south and subsequently reduce precipitation over that region. Therefore, the responses of circulation and precipitation to aerosol forcing depend on the relative location of dust aerosols with respect to rainfall band, which may explain the fact that contradictory results exist regarding whether the aerosol effect would enhance or suppress precipitation.