Understanding rainfall variability over West Africa and climate sensitivity in the tropics : observational analysis and regional climate model simulations



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West African rainfall plays an important role in tropical climate and hydrological cycle. This study aims to advance our understanding of the West African rainfall across different timescales, from the diurnal cycle to interannual variations. Then, the study is extended to understand the climate sensitivity in the tropics with an increase of greenhouse gases.

First, on a short timescale, the diurnal cycle of warm season rainfall over West Africa is investigated. In both the climatology and the 2006 case study, most of West Africa shows a single diurnal peak of rainfall in either the afternoon or at night. Afternoon rainfall peaks are associated with an unstable lower troposphere. Nocturnal rainfall peaks are associated with rainfall systems propagating westward. They occur most frequently about 3°-10° of longitude downstream of regions with afternoon rainfall peaks. Rainfall systems in the convection-permitting simulation show similar westward propagation with afternoon peaks associated with elevated topography and nocturnal peaks downstream of the topographic afternoon rainfall regions. However, the model fails to reproduce the observed afternoon rainfall peaks in several regions far away from the downstream of elevated topography.

Second, on a longer timescale, 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 smoothly from the Sahel to the Gulf of Guinea. The climatological monsoon demise date is October 20th. The demise date varies between October 5th and 30th. An early (late) demise is associated with an anomalously strong (weak) North Atlantic subtropical high. The monsoon season total rainfall is found to be significantly correlated with the demise date.

Finally, idealized regional model simulations are used to understand the climate sensitivity over the broad tropical region. The model is configured with idealized continents and oceans. With a doubling of CO2, the land and the ocean warm at different rates. The changes of the continental heat lows, subtropical highs and land-based rainfall in the tropics are explored and linked to the surface warming.