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 datasets 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 southward progression of the rainfall maximum lags behind that of the subsolar point.

The climatological monsoon demise date, defined by using a 2 mm/day rainfall threshold for the Sahel, is October 20th. The demise date varies up to 25 days during the 1979-2012 period, between October 5th and 30th. An early (late) demise is associated with a strengthening (weakening) of the North Atlantic subtropical high, which extends over the Mediterranean and Sahara throughout the demise period.

The monsoon season total rainfall is found to be significantly correlated with the demise date, but not with the onset date. In a composite of early (late) demise cases, northerly (southerly) moisture flux anomalies, which enhance (reduce) moisture divergence, are associated with the associated strengthening (weakening) of the 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. These anomalies are in place from September, suggesting a potential for seasonal prediction of the monsoon demise and, therefore, seasonal rainfall totals.