Environmental conditions for tropical cyclogenesis associated with African easterly waves

Abstract

The factors of tropical cyclone (TC) genesis that are associated with African easterly waves (AEWs) were analyzed. We detected AEWs that passed over the West African coast (WAC) using ERA-interim data from July to September 2000-2010 and examine differences between the characteristics of AEWs that either develop or do not develop TCs. We first examined the environmental conditions of the AEWs that develop TCs and their dependencies on genesis location. We found that the mid-level relative humidity near the WAC is most strongly related to the genesis location among the factors contributing to TC genesis. Composite maps of the AEWs show that wave trains at 600 hPa are west-north-westward for AEWs that develop TCs and westward for non-developing AEWs. The location of maximum of water vapor at 400 hPa coincides with the stream function center for cases of TC development, while it is shifted to the southeast for cases of non-development. We focused on a case of a nondeveloping AEW with high relative humidity near the WAC, and found that, among other possible suppression mechanisms, a dry shallow vortex originating from the Sahara Desert had an additional effect of suppressing TC genesis.