The influence of the Agulhas Current on two South African extreme weather events

Abstract

Surface station, satellite and NCEP re-analysis data are used to examine the evolution of two severe storms that occurred over the eastern coastal regions during South Africa's summer season 1998/99. The storms in November and December were both accompanied by heavy rainfall in two widely separated locations. The storm in December proved to be more severe as it resulted in flooding while tornadoes were reported in the Umtata and Hogsback regions of the Eastern Cape. Both storms appeared to result from interaction between a continental heat low, advection of warm moist air around an anticyclone in the South-west Indian Ocean and an approaching midlevel westerly trough. NCEP derived moisture flux diagrams and back trajectories of air parcels constructed from ECMWF data suggest that the Agulhas Current region was a major source of low level moisture for both storms. TRMM satellite imagery captured heavy rainfall above the high sea surface temperatures of the Agulhas Current. TRMM measurements of rainfall and latent heat in the atmosphere show that the high sea surface temperatures of the Agulhas Current modified the mesoscale environment above the current. To what extent the mesoscale environment above the Agulhas Current modified the synoptic situations over land could be answered using regional modeling and more frequent radiosonde data.