Ocean–Atmosphere Interaction in the Agulhas Current Region and a South African Extreme Weather Event

M. Rouault Ocean Climate Research Group, Department of Oceanography, University of Cape Town, Rondebosch, South Africa

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S. A. White Ocean Climate Research Group, Department of Oceanography, University of Cape Town, Rondebosch, South Africa

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C. J. C. Reason Department of Environmental and Geographical Science and Department of Oceanography, University of Cape Town, Rondebosch, South Africa

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J. R. E. Lutjeharms Ocean Climate Research Group, Department of Oceanography, University of Cape Town, Rondebosch, South Africa

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I. Jobard Laboratoire de Météorologie Dynamique de l'Ecole Polytechnique, CNRS, Paris, France

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Abstract

Ocean–atmosphere interaction above warm western boundary currents such as the Gulf Stream, the Kuroshio Current, and the Agulhas Current often leads to very high evaporation rates. In the case of the Agulhas Current, which flows just off the southeast coast of Africa, such high latent heat fluxes may lead to increased low-level advection of moisture onshore and local intensification of storm systems. Observational evidence for the significant latent heat fluxes in the Agulhas Current area was obtained during the Agulhas Current Air Sea Exchange Experiment, which showed that about 5 times as much water vapor is transferred to the atmosphere above the 80–100-km-wide core of the current than from the neighboring waters. Using NCEP reanalyses, Meteosat, and Tropical Rainfall Measuring Mission (TRMM) satellite data and local station and radiosonde observations, this study investigates 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. Moisture flux transects through the storm region and backward trajectories of air parcels suggest that low-level onshore flow of moisture from the Agulhas Current region played a significant role in the storm evolution. However, because the NCEP data on which these moisture fluxes are based are known to significantly underestimate the surface latent heat flux when compared with ship observations, it is suggested that the actual contribution of the Agulhas Current moisture source to the storm may have been even greater than is documented in this paper.

Corresponding author address: Dr. Mathieu Rouault, Department of Oceanography, University of Cape Town, R. W. James Bldg., Rm. 125, Rondebosch, Cape Town, South Africa. Email: rouault@physci.uct.ac.za

Abstract

Ocean–atmosphere interaction above warm western boundary currents such as the Gulf Stream, the Kuroshio Current, and the Agulhas Current often leads to very high evaporation rates. In the case of the Agulhas Current, which flows just off the southeast coast of Africa, such high latent heat fluxes may lead to increased low-level advection of moisture onshore and local intensification of storm systems. Observational evidence for the significant latent heat fluxes in the Agulhas Current area was obtained during the Agulhas Current Air Sea Exchange Experiment, which showed that about 5 times as much water vapor is transferred to the atmosphere above the 80–100-km-wide core of the current than from the neighboring waters. Using NCEP reanalyses, Meteosat, and Tropical Rainfall Measuring Mission (TRMM) satellite data and local station and radiosonde observations, this study investigates 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. Moisture flux transects through the storm region and backward trajectories of air parcels suggest that low-level onshore flow of moisture from the Agulhas Current region played a significant role in the storm evolution. However, because the NCEP data on which these moisture fluxes are based are known to significantly underestimate the surface latent heat flux when compared with ship observations, it is suggested that the actual contribution of the Agulhas Current moisture source to the storm may have been even greater than is documented in this paper.

Corresponding author address: Dr. Mathieu Rouault, Department of Oceanography, University of Cape Town, R. W. James Bldg., Rm. 125, Rondebosch, Cape Town, South Africa. Email: rouault@physci.uct.ac.za

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