In the tropics, deep convection and humidity interacts in a two-way process. First, convection transports moisture in the upper troposphere where it provides strong radiative effects. Second, the distribution of humidity in the immediate environment of convection regulates the convective activity. The presence of dry air can inhibit the moist convection initiation but can also favor the squall line development for convection already triggered.

In order to analyze this second mode of interaction between humidity and convection, the origin and moisture characteristics of air masses that surround the West African Moonson convective systems is investigated during the summer 1992. Using satellite observations of Upper Tropospheric Humidity derived from METEOSAT, the tropospheric humidity field and its variability is described. A spectral analysis reveals the major modes of variability in the northern subtropical band around 25°N with maximum energy in the 6-12 days band. Westward propagating modes are dominant in this band with typical speed and wave lengths of about 9 m/s and 3000 km respectively. Norther of 30°N eastward propagating modes dominate with faster speed and shorter wave lengths.

Thanks to backtrajectory computations, the extratropical origin of the dry air masses is assessed. Case study analysis for the 21st August 1992 reveals that the 500 hPa dry air masses in the convective regions originate from the Atlantic Ocean jet stream at 50N,200 hPa. This is further confirmed over the July-August season for which 4 major outbreaks of extra tropical dry air entering the moonson system are observed.

The dynamical features and associated moisture distribution of the dry intrusions will be presented and the potentialinteraction with organized convection will be discussed.