Prediction of dry-season precipitation in tropical West Africa and its relation to forcing from the extratropics

Precipitation during the boreal winter dry season in tropical West Africa is rare but occasionally connected to high-impacts for the local population. The dynamics and predictability of this phenomenon have been studied very little. Here a statistical evaluation of the climatology, dynamics, and predictions of dry-season wet events is presented for the region 7.5–15°N, 10°W–10°E. The analysis is based upon GPCP merged satellite-gauge pentad rainfall estimates and five-day ERA-40 precipitation forecast, and covers the 23 dry seasons (November–February) 1979/80–2001/02. Wet events are defined as pentads with an area-averaged precipitation anomaly of more than +200% with respect to the mean seasonal cycle. Composites of the 43 identified events indicate an association with a trough over northwestern Africa, a tropical plume on its eastern side, unusual precipitation at the northern and western fringes of the Sahara, and reduced surface pressure over the Sahara, which allows an inflow of moist southerlies from the Gulf of Guinea to feed the unusual dry-season rainfalls. The results give evidence for a pre-conditioning by another disturbance about one week prior to the precipitation event. The ERA-40 forecasts show a high temporal correlation with observations, a general wet bias, but a somewhat too low number of wet events. With 53% of all identified events correctly forecasted and only 32% of forecasted events not verified the model shows a moderate skill in contrast to the prediction of many other tropical precipitation systems. A separate consideration of hits, misses, and false alarms corroborates the previously proposed hypothesis that a strong extratropical influence enhances the quality of predictions in this region. New results using ERA-Interim forecasts and GPCP and TRMM daily rainfall products for the 10 winters 1998/99 through 2007/2008 will be presented. Overall, findings of the present study should encourage weather services in West Africa to take advantage of available dry-season precipitation forecasts in terms of the dissemination of early warnings.