, Volume 25, Issue 5, pp 517-535
Date: 13 Aug 2005

Dynamics of the boreal summer African monsoon in the NSIPP1 atmospheric model

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Abstract

A nine-member ensemble of simulations with a state-of-the-art atmospheric model forced only by the observed record of sea surface temperature (SST) over 1930–2000 is shown to capture the dominant patterns of variability of boreal summer African rainfall. One pattern represents variability along the Gulf of Guinea, between the equator and 10°N. It connects rainfall over Africa to the Atlantic marine Intertropical Convergence Zone, is controlled by local, i.e., eastern equatorial Atlantic, SSTs, and is interannual in time scale. The other represents variability in the semi-arid Sahel, between 10°N and 20°N. It is a continental pattern, capturing the essence of the African summer monsoon, while at the same time displaying high sensitivity to SSTs in the global tropics. A land–atmosphere feedback associated with this pattern translates precipitation anomalies into coherent surface temperature and evaporation anomalies, as highlighted by a simulation where soil moisture is held fixed to climatology. As a consequence of such feedback, it is shown that the recent positive trend in surface temperature is consistent with the ocean-forced negative trend in precipitation, without the need to invoke the direct effect of the observed increase in anthropogenic greenhouse gases. We advance plausible mechanisms by which the balance between land–ocean temperature contrast and moisture availability that defines the monsoon could have been altered in recent decades, resulting in persistent drought. This discussion also serves to illustrate ways in which the monsoon may be perturbed, or may already have been perturbed, by anthropogenic climate change.