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The Sensitivity of African Wave Disturbances to Remote Forcing

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  • a Center for climate Systems Research, Columbia University and NASA/Gaddard Institute for Space Studies, New York New York
  • | b Laboratoire de Modelisation du Climat et de l'Environnement, Gif-sur-Yvette, France
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Abstract

Ensembles of three simulations each, forced by June–September 1987 and 1988 sea surface temperatures, respectively, were made with a new version of the general circulation model of the National Aeronautics and Space Administration/Goddard Institute for Space Studies. Time series of 6-h meridional winds at about 780 mb over West Africa were spectrally analyzed to detect African wave disturbances, whose properties for the two ensembles are compared and contrasted. The realistically simulated, stronger 1988 tropical easterly jet and the associated stronger upper-tropospheric divergence are components of interannual differences in the SST-forced planetary circulation, which correspond to higher amplitudes of African wave activity and concomitant excesses in 1988 Sahel rainfall rates. Results do not show, however, that most of the heavier precipitation was spatially organized by African wave structures. The excess rainfall is associated with stronger mean southerly circulation in the lower troposphere, which carried more moisture into the Sahel. Nevertheless, because waves modulate winds, convergence, humidity, and precipitation, the study suggests that they serve as a teleconnection mechanism, whereby extreme Pacific Ocean SST anomalies are able to influence climate variability in Africa's Sahel.

Abstract

Ensembles of three simulations each, forced by June–September 1987 and 1988 sea surface temperatures, respectively, were made with a new version of the general circulation model of the National Aeronautics and Space Administration/Goddard Institute for Space Studies. Time series of 6-h meridional winds at about 780 mb over West Africa were spectrally analyzed to detect African wave disturbances, whose properties for the two ensembles are compared and contrasted. The realistically simulated, stronger 1988 tropical easterly jet and the associated stronger upper-tropospheric divergence are components of interannual differences in the SST-forced planetary circulation, which correspond to higher amplitudes of African wave activity and concomitant excesses in 1988 Sahel rainfall rates. Results do not show, however, that most of the heavier precipitation was spatially organized by African wave structures. The excess rainfall is associated with stronger mean southerly circulation in the lower troposphere, which carried more moisture into the Sahel. Nevertheless, because waves modulate winds, convergence, humidity, and precipitation, the study suggests that they serve as a teleconnection mechanism, whereby extreme Pacific Ocean SST anomalies are able to influence climate variability in Africa's Sahel.

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