Seasonal Evolution and Variability Associated with the West African Monsoon System

Abstract

In this study, the seasonal variations in surface rainfall and associated large-scale processes in the tropical eastern Atlantic and West African region are investigated. The 6-yr (1998?2003) high-quality Tropical Rainfall Measuring Mission (TRMM) rainfall, sea surface temperature (SST), water vapor, and cloud liquid water observations are applied along with the NCEP?NCAR reanalysis wind components and a 4-yr (2000?2003) Quick Scatterometer (Quik SCAT) satellite-observed surface wind product. Major mean rainfall over West Africa tends to be concentrated in two regions and is observed in two different seasons, manifesting an abrupt shift of the mean rainfall zone during June?July: (i) near the Gulf of Guinea (about 5°N), intense convection and rainfall are seen during April?June and roughly follow the seasonality of SST in the tropical eastern Atlantic, and (ii) along the latitudes of about 10°N over the interior of the West African continent, a second intense rain belt begins to develop in July and remains there during the later summer season. This belt coexists with a northward-moving African easterly jet (AEJ) and its accompanying horizontal and vertical shear zones, the appearance and intensification of an upper-tropospheric tropical easterly jet (TEJ), and a strong low-level westerly flow. Westward-propagating wave signals [i.e., African easterly waves (AEWs)] dominate the synoptic-scale variability during July?September, in contrast to the evident eastward-propagating wave signals during May?June. The abrupt shift of the mean rainfall zone thus turns out to be a combination of two different physical processes: (i) evident seasonal cycles in the tropical eastern Atlantic Ocean, which modulate convection and rainfall near the Gulf of Guinea by means of SST thermal forcing and SST-related meridional gradient; and (ii) the interaction among the AEJ, TEJ, low-level westerly flow, moist convection, and AEWs during July?September, which modulates rainfall variability in the interior of West Africa, primarily within the ITCZ rain band. Evident seasonality in synoptic-scale wave signals is shown to be a good indication of this seasonal evolution.