West African monsoon demise: Climatology, interannual variations, and relationship to seasonal rainfall

The climatology and interannual variations of the West African monsoon demise are investigated using the Tropical Rainfall Measuring Mission and the Global Precipitation Climatology Project precipitation data sets along with the ERA-Interim reanalysis. During monsoon demise, the rainfall maximum progresses southward from the Sahel to the Gulf of Guinea. The West African monsoon demise is smooth, with no evidence of a meridional jump of the rainfall maximum as occurs during monsoon onset. The climatological monsoon demise date, defined by using a 2 mm d(-1) rainfall threshold for the Sahel, is 20 October. The demise date varies up to 25 days during the period 1979-2012, between 5 and 30 October. An early (late) demise is associated with an anomalously strong (weak) North Atlantic subtropical high, which extends over the Mediterranean and the Sahara throughout the demise period. The monsoon season total rainfall is found to be significantly correlated with the demise date. In a composite of early (late) demise cases, northerly (southerly) moisture flux anomalies, which enhance (reduce) moisture divergence, are associated with an anomalously strong (weak) North Atlantic subtropical high. In addition, the early (late) demise of the West African monsoon is accompanied by cold (warm) sea surface temperature anomalies in the subtropical North Atlantic.