资源环境科技发展态势分析平台

Precipitation patterns over West Africa display a broad range of timescales, from the intraseasonal to decadal timescales, as well as multidecadal shifts. Here we investigate the dominant patterns of the July to September precipitation over the region and the related atmosphere-ocean anomalies during the satellite-era from 1983 to 2017. Using extended empirical orthogonal function analysis of an ensemble of nine precipitation datasets, we identify two dominant modes that together account for about 33% of the variance. The Sahel mode displays spatially coherent increases in precipitation over much of West Africa and a decrease at the Guinea Coast, and is closely reproduced by linear trend analysis. Linear trends explain 25–53% of the Sahel variance from the drier mid-1980s to the wetter mid-1990s. The Guinea Coast mode displays robust precipitation anomalies south of the Sahel, with strong interannual variability and a statistically non-significant trend. The Sahel mode is associated with a northerly displacement of the Inter-Tropical Convergence Zone (ITCZ), warm North Atlantic with cold blob in the subpolar gyre region, warm Mediterranean Sea, warm tropical southwest Indian Ocean and negative Pacific decadal variability pattern. The Atlantic Niño leads the Guinea Coast mode by two months. Both Sahel and Guinea Coast modes are substantially influenced by the interactions between meridional (displacements of the ITCZ) and zonal (variations of the Walker Circulation) atmospheric circulations. The southerly displacement of the ITCZ, convection, upper-level divergence and surface convergence in the equatorial Atlantic associated with the Guinea Coast mode is horizontally compensated by strong surface divergence and upper-level convergence over the equatorial Pacific where anomalous cooling prevails, implying a strong role for the Walker Circulation during this period.