West African convection regimes and their predictability from submonthly forecasts

Abstract

Recurrent convection regimes are identified during the extended West African Monsoon (WAM) season (May–Nov) using a $k-means$ clustering of 1980–2013 NOAA daily Outgoing Longwave Radiation (OLR), and are well reproduced in 1996–2015 ECMWF week-1 reforecasts despite systematic biases. One regime of broad drying across the Sahel in the early (May–Jun) and late (Oct) WAM is of particular interest regarding the prediction of onset date. This regime is associated with an anticyclonic cell along the Atlantic coast of West Africa leading to a weakened monsoon flow and subsiding anomalies across the Sahel. Teleconnections of this regime with the Indian monsoon sector are identified through modulations of the Walker circulation alongside relationships to MJO phase 3 more than 10 days in advance, when convection is enhanced over the Indian Ocean. Other regimes are associated with westward propagating anomalous convective cells along two distinct wave trains at $15^\circ \hbox {N}$ and $24^\circ \hbox {N}$ during the core (Jul–Sep) and late (Oct–Nov) WAM, respectively, and translate into wet anomalies transiting across the Sahel. A regime of broad Sahel wetting in the core WAM, more frequent since the 1990s, is related to global SST warming, agreeing with the observed recovery of Sahel rainfall. ECMWF skill in forecasting regime sequences decreases from week-1 to -4 leads, except in the case of the above-mentioned regime associated with early season dry spells, translating into the potential for skillful WAM onset date predictions. Our analysis suggests that sources of predictability include relationships to the MJO and the Indian monsoon sector, which need to be further examined to benefit subseasonal forecasting efforts in West Africa, and ultimately agricultural planning and food security across the Sahel.