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要旨:
In this study, a 25-year regional climate model run overWest Africa is evaluated and examined with respect to
causes of interannual rainfall variability related to the West African Monsoon. West African rainfall has been
subject to strong interannual and decadal variability throughout the past 50 years. Known driving forces for
this variability are large-scale changes in Atlantic sea surface temperatures (SSTs), variability due to global
atmospheric circulation changes, like for instance variability related to El Ni˜no-Southern Oscillation, but also
regional and local-scale changes in land use and vegetation cover. The interaction of these impact factors
with West African synoptic and subsynoptic processes is still not completely understood. One reason for this
lack of knowledge is that basic features of West African climate, including the African Easterly Jet (AEJ),
African Easterly Waves (AEWs) as well as monsoon dynamics, are very complex multiscale phenomena.
Climate modeling in West Africa requires the ability to simulate these effects, which can only be achieved by
mesoscale atmospheric models. Using the regional climate model REMO from the Max-Planck Institute for
Meteorology in Hamburg, a 25-year dynamical downscaling study was undertaken in order to evaluate a tool,
which will then be used for the examination of causes of rainfall variability in West Africa. The model was
used on a 0.5◦ grid over North Africa northward of 15◦S. The model evaluation leads to some confidence in
the reliability of the modeled climate. A detailed examination of composites of selected wet and dry years in
the Guinean coast region elucidates the role of SST forcing and external atmospheric forcing for interannual
rainfall variability. In general, abundant monsoonal rainfall comes along with warm tropical Atlantic SSTs,
enhanced latent heat fluxes from the ocean to the atmosphere and stronger surface wind convergence near the
Guinean Coast. This is accompanied by large-scale dynamical changes in strength and direction of both the
Tropical Easterly Jet (TEJ) over the Indian Ocean and the Subtropical Jet (STJ) over the Near East and the
Caucasian region.
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