Dynamic and thermodynamic transformations in the atmosphere associated with precipitation over West Africa

Abstract

The precipitation associated with atmospheric circulations in summer, over West Africa shows large intraseasonal and interannual variability. The changes in the circulation characteristics were of importance, as they contributed significantly to the interseasonal variability of the rainfall. The NCEP-NCAR reanalysis dataset were used to investigate the wind field, vorticity term, stability index, precipitable water, moisture term, depth of convectively geopotential, and precipitation for August and September for the period 1984-2004. In the present study, dynamic and thermodynamic characteristics of the atmosphere associated with summer rainfall during wet and dry periods were investigated. Rainfall was examined using a 500-station archive. The vorticity, moisture budget, and moist static energy were analyzed between 850 and 200hPa in Auqust and September for 1984 (dry year) and 2004 (wet year). Significant changes in the wind and vorticity fields at 850hpa and 200hpa took place over the sub-region of West Africa. A large magnitude stability indices, strong monsoon circulation, and wet condition over the Sahel region were observed in August of 2004; and on the other hand, small magnitude stability indices, very weak circulation; and relatively dry condition were observed in the same month for 1984. However, in' September the value of stability index was smaller and hence a reduced activity of monsoon circulation and low rainfall was observed during the period