Wave activity and convection in equatorial Africa

Easterly waves are a common feature in southern equatorial Africa during the austral spring and autumn seasons, October-November and March-to-May, respectively. Maximum activity is in the zone between the equator and 10 o S and the rate of westward propagation is relatively constant. There is tremendous year-to-year variability in the number of waves, with most frequent development during the 1980s and weakest development in the 1950s. The waves in the two seasons contrast in terms of number and longitude of genesis. In March-to-May there are generally 2 to 4 waves per month, but 4 to 6 in October-November. In the latter season the genesis shows two longitudinal peaks, one associated with the East African highlands around 25 to 30 o E and the other at the Atlantic coast around 10 to 12 o E. In March-to-May genesis generally occurs in the sector from 10 E to 22 o E. There is a distinct diurnal cycle in development, with a maximum around 0 Z and a minimum around 12 Z. This suggests a link to mesoscale activity and convection, however no relationship has been found so far. The convection appears to be triggered by orographic effects, which result in nocturnal convergence flow into the Zaire Basin from the surrounding highlands.