Revisiting atmospheric and oceanic teleconnection impacts on West African Monsoon rainfall variability

The contribution revisits linear statistical relationships between indices of monsoon rainfall for three regions in tropical West African and various indices describing remote variations in the ocean-atmosphere climate systems. The West African regions are the West Sahel, Central Sahel and the Guinea Coast, and the monthly station rainfall data used to compile the standardized indices have been enlarged and updated to cover the period June-September 1921-2009. Remote indices used comprise, but are not restricted to, the Atlantic Multidecadal Oscillation (AMO), Atlantic Meridional Mode, indices of the state of El Niño-Southern Oscillation (ENSO), sea surface temperature in oceans adjacent to Africa (Atlantic and Indian Oceans, Mediterranean Sea), and the All Indian Monsoon Rainfall Index. The study focuses on the evolution of statistical relationship over various time window lengths for fixed starting years and starting years for fixed window length to reveal the non-stationary behavior of relationships. It also distinguishes between contributions from interannual and decadal time scales to a given statistically significant relationship. Finally, a multiple linear regression analysis is used to explore maximum variance that can be explained by the remote indices. Some pertinent findings are: (a) Both AMO and AMM show a positive linear correlation to Sahel rainfall that dropped in the 1960s and 1970s and explained at most 10-25% of variance. The correlation stems from periods longer than 8 years for AMO, but AMM does show a correlation on interannual time scales for West Sahel suggesting a closer physical relation between AMM and rainfall in the WS. (b) The percent variance explained by ENSO indices is, though statistically significant, on the order of 10% and found on the interannual time scale. (c) A stable correlation between the sea surface temperatures in the ATL3 region (0-20°W,3°S-3°N) and the Guinea Coast rainfall is found for the 89-year investigation period and related to year-to-year co-variations. On the other hand, the correlation changed from significantly negative to positive for the West Sahel after the 1970s. (d) The multi-linear correlation analyses yielded six (four) predictors for the Sahel (Guinea Coast) and the explained variance is on the order of 30-60% (25-60%).