On mechanisms that determine synoptic time scale convection over East Africa

摘要

This study explores potential mechanisms to explain the marked synoptic scale variability in convection seen over East Africa during the July-September season. Regression composites based on analysis of 22 years of brightness temperature and reanalysis datasets highlights a complicated evolution for this synoptic convection. Active convection in the region is typically preceded by a coherent eastward moving convective signal that is followed by a period with a prominent westward moving convective signal away from the region. Results suggest that this sequence is associated with eastward propagation of convectively coupled Kelvin waves (CCKWs) that subsequently trigger westward-moving African easterly waves (EWs). Results also indicate that synoptic convection in the East African region is impacted by westward moving upper tropospheric EWs originating on the tropical easterly jet in the South Asian region. These upper EWs propagate through East Africa with about a 4-day period, consistent with the synoptic convection variability observed over East Africa. Consistent with local forecaster experience, analysis of the regional spatial patterns of convection and associated moisture flux convergence show that a coherent signal propagates northeastwards from the equatorial Congo region to East Africa on synoptic timescales. The evolution resembles the CCKW-associated signal suggesting that CCKWs contribute to this synoptic variability.