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Abstract

Sea surface temperature (SST) anomalies in the tropical Atlantic have been associated with precipitation anomalies in West Africa that form a dipole pattern with centers over the Sahel and the Gulf of Guinea. Whilst this was clear before the 1970's, the dipole pattern almost disappeared after that date, as the anti-correlation between rainfall anomalies in the Sahel and Guinea dropped abruptly. Simultaneously, the anti-correlations between Sahel rainfall and tropical Pacific SSTs strengthened. It has been posited that these changes after the 1970's developed as rainfall over West Africa started to co-vary with SSTs in the global tropics. In this covariability, enhanced summer rainfall over West Africa with a monopole pattern corresponds to warmer SSTs in the tropical Atlantic and Maritime Continent, and colder SSTs in the tropical Pacific and western Indian Oceans. The present paper describes the hitherto unexplored seasonal evolution of this co-variability and the physical mechanisms at work. Sensitivity experiments with two atmospheric general circulation models demonstrate that, after the 1970's, the impacts of SST anomalies in the Indo-Pacific counteract those in the Atlantic in terms of generating rainfall anomalies over the Sahel, and that this superposition of effects is primarily linear. Therefore, at interannual timescales, the change in the patterns of co-variability between West African rainfall and tropical SSTs can explain the non-stationary relationship between the anomalies in these two fields.