THE WEST AFRICAN MONSOON: DYNAMICS, DECADAL VARIABILITY AND INTERACTION WITH THE EASTERN TROPICAL ATLANTIC SSTS
This dissertation documents regional modeling studies of various aspects of West African Monsoon dynamics and variability. The physical processes responsible for observed characteristics seasonal evolution of the monsoon, its decadal variability and interaction with the Eastern Atlantic Ocean are presented. The primary mechanism for abrupt latitudinal shift of precipitation maximum from the Guinea coast to the Sahel region, "the monsoon jump", in June is found to be the deepening of the monsoon flow into the continent by inertial instability. This instability is forced by the pressure gradient associated with low level condensational heating due to the pre-existing boundary layer moisture supply onto the warm land surface. The decadal variability of the West African precipitation, in particular, the causes of the 1980s droughts over the Sahel and the partial recovery of the 1990s, are also studied using the regional atmospheric model. The primary source of variability of precipitation over the Sahel is found to be variability in the moisture transport across its western coast. This moisture transport is influenced by circulation patterns associated with the regional SST anomalies. The 1980s warming over the Indian Ocean, for example, introduces divergence and anticyclonic easterly wind anomalies which drive moisture away from the region and cause widespread drought. In a similar manner, warming over tropical Atlantic Ocean also contributes to the drought. The modeled recovery of the 1990s is found to be due to changes in the scale and distribution of the anomalous SSTs both over the tropical Atlantic and Indian Oceans. These changes introduce cyclonic circulation patterns that favor westerly transport of moisture towards the Sahel. The response of the region to an SST forcing is related to the background moisture distribution and is enhanced by circulation patterns associated with the changes in the local condensational heating. Finally, a coupled regional model is used to investigate the two way interaction between theWest African Monsoon and seasonal SST variations over the Eastern Tropical Atlantic Ocean. The model allows thermodynamic and dynamic interactions between the atmospheric planetary boundary layer and the ocean mixed layer. The model results show that the seasonal cooling over the Gulf of Guinea between spring and summer is related to increased entrainment and evaporation due to acceleration of the southerly winds driven by the African heat low. Over northeastern tropical Atlantic, the warming due to solar heating is augmented by the suppression of evaporation. The northward migration of the ITCZ leads to weakened northeasterly winds and reduced evaporation. The warming of the northeastern Atlantic, on the other hand, weakens the anticyclonic easterly winds associated with the Atlantic High and strengthen the cyclonic westerly monsoon winds.
Journal of ClimateVolume 20 5264-5284
This is a PhD dissertation of Samson Hagos
American Meteorological Society
West African Monsoon; Atlantic SST