Monday, 7 July 2014
The effects of multi-decadal variations of sea surface temperature (SST) on the West African monsoon rainfall and circulation, and consequent change in dust emission and transport are examined based on model experiments with the NASA GEOS-5 GCM with prescribed SST. Based on Singular Value Decomposition (SVD) between Sahel rainfall and global SST distribution, SST patterns representing the 1950s and 1980s are selected and prescribed for 7-year simulations for each period. Results show that the subtropical high is strengthened and expanded southwestward in 1980s when the northern Atlantic Ocean SST is colder compared to the 1950s. As a result, the rain band in the tropical Atlantic Ocean and Sahel moves southward and rainfall is reduced over the Sahel. Stronger surface winds associated with strong surface temperature gradients increase dust emission in the southern Saharan desert and northern Sahel. Combined with circulation changes induced by strengthening the subtropical high and reduced wet deposition, dust transport is increased in the deep tropics (10-20N), but reduced in the northern path (20-30N)
To examine the effect of aerosol radiative forcing, additional experiments are conducted with reduced dust absorption. Preliminary results indicate that higher dust radiative forcing lifts dust particles to higher altitudes. Stronger dust radiative forcing also increases meridional temperature gradients and strengthens the African Easterly Jet, resulting in an increased residence time of dust and farther westward transport.
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