Climatology of gravity wave activity during the West African Monsoon

Gravity wave activity is analysed in the lower stratosphere using 6 year radiosonde data (2001-2006) above two meteorological stations in the West African tropical region such as Niamey (13.47 degrees N; 2.16 degrees E) and Ouagadougou (12.35 degrees N; 1.51 degrees W). Monthly total energy density of gravity waves is computed with temperature and horizontal wind perturbations to highlight the West African Monsoon period from June to September. Comparison with monthly total energy density calculated with temperature only supports that observed small-scale temperature and wind perturbations are mostly associated with gravity waves in the lower stratosphere especially for large values during the wet season. Above the two sites, monthly evolution of gravity wave total energy density reveals a maximum intensity of gravity wave activity in July during the West African Monsoon period. Indicators of convective activity such as mean Outgoing Longwave Radiation (OLR) and Tropical Rainfall Measuring Mission (TRMM) rain rates reveal to be adequate monsoon proxies to be compared to gravity wave energy intensity during the West African Monsoon.