Simulation of a low-level westerly jet and its role in West Africa precipitation variability

A mode of regional climate variability on interannual time scales over West Africa is investigated, and its connection to past climate is examined. Variability in precipitation is related to the existence and strength of a low-level westerly jet, recently identified in high-resolution satellite observations and well simulated in a regional climate model (RCM). In today's climate, relatively wet conditions are associated with an intensification of this jet, accompanied by increased moisture transport into Sahelian Africa from the Atlantic. RCM simulations of the African Humid Period, 6-8 ka, show that the westerly jet was also stronger during that time of enhanced summertime insolation which was marked by a large increase in precipitation rate over the Sahel. The "greening" of the Sahara and Sahel of the African Humid Period was related to enhanced moisture advection and convergence by the westerly jet and not with, for example, enhanced southerly monsoon flow across the Guinean coast. Also associated with the strengthened westerly jet during the African Humid Period was an intensified thermal low which was vertically expanded through 675 hPa. The Sahelian High and African Easterly Jet, features of the present day 675 hPa level, were not characteristic of the African Humid Period.