Revisiting the statistical relationship between remote climate indices and West African monsoon variability

This paper revisits the strength and stationarity of linear statistical relationships between indices of monsoon rainfall derived from rain gauge data for three regions in tropical West Africa (West Sahel, Central Sahel and Guinea Coast) and various indices describing remote variations in the ocean-atmosphere climate systems from 1921 to 2009. The results reveal that both the Atlantic Multi-decadal Oscillation (AMO) and the Atlantic Meridional Mode (AMM) show a positive linear correlation to Sahel rainfall. The percent variance explained (PVE) ranges from 10-25%. The correlation stems from periods longer than eight years for AMO, but AMM does show a correlation on interannual time scales for West Sahel (WS) that was absent in the 1970s and 1980s. The PVE by El Niño Southern Oscillation (ENSO) indices is, though statistically significant, on the order of 10% and found mainly on the interannual time scale. A strong and stable correlation with PVEs larger than 50% is found between the sea surface temperatures in the Atlantic 3 region (ATL3, 0-20°W, 3°S-3°N) and the Guinea Coast rainfall. On the other hand, the correlation between ATL3 and WS rainfall changed from significantly negative to significantly positive after the 1970s. The eastern Mediterranean SSTs are found to be significantly related to Central Sahel (CS) rainfall, especially in recent years with PVEs between 36 and 47%. Multi-linear regression analyses reveal that the relative importance of the Indian Ocean is 42% in the optimal regression model. For the CS this value is 37% for the Eastern Mediterranean SSTs, and 70% in case of the Guinea Coast using the ATL3 index. However, except for the Guinea Coast, non-stationarities in the correlation between the climate state indexes and WAM rainfall suggest the need of the application of different regression models depending on the active “teleconnection regime”.