Mathieu Rouault: Top

THE INFLUENCE OF THE AGULHAS CURRENT ON SOUTH AFRICAN EXTREME WEATHER

Mathieu Rouault, Chris Reason, Isabelle Jobard, Johann Lutjeharms

Department of Oceanography, University of Cape Town. 7701, Rondebosch, South Africa

Laboratoire de Meteorologie Dynamique,Ecole Polytechnique, Paris, France

Mathieu Rouault's Web Site | Email rouault@physci.uct.ac.za

Abstract
Surface station, METEOSAT, TRMM, NCEP re-analysis and ECMWF backward trajectories data are used to examine the evolution of a severe storm and flood event that occurred over the southern coastal regions of South Africa on 14-15 December 1998 . Heavy rainfall occurred in two widely separated locations and tornadoes were reported. The storm 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. Moisture flux transects through the storm region and back trajectories of air parcels suggest that a major source of low level moisture for the storm was the Agulhas Current region. With about 4 orbits a day above South Africa, TRMM data was extremely useful to document the spatial extension and intensity of the storm. Heavy rainfall occurred mainly over the Agulhas Current and coastal region releasing large amounts of latent heat. The maximum surface rain rates occurred during all three TRMM orbit passes above the Agulhas Current between 25-26º E and 34-36º S where the SST ranged from 22º to 25º C. Observational evidence for the significant latent heat fluxes in the area was obtained during the Agulhas Current Air Sea Exchange Experiment (ACASEX), the first dedicated air-sea interaction experimental cruise in the Agulhas Current. It took place during autumn 1995 to study the influence of the Agulhas Current on the atmospheric marine boundary layer near the coast. ACASEX resulted in a thorough investigation of the exchange between ocean and atmosphere and the structure of the marine atmospheric boundary layer . Most of the measurements showed that the core of the Agulhas current, about 80-km wide, transfers about 5 times as much water vapor to the atmosphere as the surrounding water. A maximum latent heat flux of 600 W.M-2 was measured during the cruise. Atmospheric stability in the marine boundary layer is usually unstable above the current, stable above the inshore cold water and near neutral offshore with an unstable convective marine boundary layer over the current. Mean specific humidity and potential temperature of the boundary layer increased significantly with a concomitant boundary layer deepening over the current. In the presence of alongshore winds an atmospheric moisture and thermal front developed over the inshore SST front

Abstract

Surface station, METEOSAT, TRMM, NCEP re-analysis and ECMWF backward trajectories data are used to examine the evolution of a severe storm and flood event that occurred over the southern coastal regions of South Africa on 14-15 December 1998 . Heavy rainfall occurred in two widely separated locations and tornadoes were reported. The storm 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. Moisture flux transects through the storm region and back trajectories of air parcels suggest that a major source of low level moisture for the storm was the Agulhas Current region.

With about 4 orbits a day above South Africa, TRMM data was extremely useful to document the spatial extension and intensity of the storm. Heavy rainfall occurred mainly over the Agulhas Current and coastal region releasing large amounts of latent heat. The maximum surface rain rates occurred during all three TRMM orbit passes above the Agulhas Current between 25-26º E and 34-36º S where the SST ranged from 22º to 25º C.

Observational evidence for the significant latent heat fluxes in the area was obtained during the Agulhas Current Air Sea Exchange Experiment (ACASEX), the first dedicated air-sea interaction experimental cruise in the Agulhas Current. It took place during autumn 1995 to study the influence of the Agulhas Current on the atmospheric marine boundary layer near the coast. ACASEX resulted in a thorough investigation of the exchange between ocean and atmosphere and the structure of the marine atmospheric boundary layer . Most of the measurements showed that the core of the Agulhas current, about 80-km wide, transfers about 5 times as much water vapor to the atmosphere as the surrounding water. A maximum latent heat flux of 600 W.M-2 was measured during the cruise. Atmospheric stability in the marine boundary layer is usually unstable above the current, stable above the inshore cold water and near neutral offshore with an unstable convective marine boundary layer over the current. Mean specific humidity and potential temperature of the boundary layer increased significantly with a concomitant boundary layer deepening over the current. In the presence of alongshore winds an atmospheric moisture and thermal front developed over the inshore SST front

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