Water Vapour Transport Over Africa and the Atlantic Ocean During Summer 1979

Abstract

AbstractThe water vapour transport over Africa and the eastern Atlantic Ocean during the 1979 summer is studied. the analysis is based on wind fields from the European Centre for Medium Range Weather Forecasts and humidity fields derived from a three‐layer precipitable water data set.The evolution of the bi‐weekly mean fields of water vapour flux in the surface to 850 mb layer shows the progressive penetration of moisture over West Africa during the summer. At low levels during the maximum activity of the African monsoon, the southerly flow penetrates up to 20°N and there is a large belt of westerlies around 10°N. the fluxes between 700 and 500 mb suggest the importance of the African easterly jet (AEJ) as a major artery for moisture for West Africa.Vertically integrated fluxes across different sections have been analysed. Across the equator at the longitudes of the Gulf of Guinea the flux shows a decreasing trend due to the northward propagation of the intertropical discontinuity over west Africa. Near 22°N, the flux is mainly from the north at the beginning of the summer and becomes southerly during surges of the monsoon. the flux from West Africa toward the Atlantic Ocean is large, indicating that moisture from the Gulf is transported upward and advected westward by the AEJ. the fluxes oscillate with two main periods, around 3‐5 days and 5‐7 days.Water vapour budgets have been estimated for boxes located over the Gulf of Guinea and West Africa. the Gulf of Guinea is a source of water vapour transported northward and westward by the trade winds. Over West Africa at low levels, moisture comes from the Gulf of Guinea. In the mid‐troposphere there is a strong easterly moisture flux associated with the AEJ and originating from central Africa. This moisture seems to come initially from the Gulf of Guinea. Because it is mainly divergent, the mid‐tropospheric flux may have little influence on rainfall over West Africa. the budget study also emphasizes the importance of evapotranspiration over the belt north of the Gulf of Guinea where water vapour is recycled and transported northward.