The probability of occurrence and the intensity of tropical cyclones along the Southern African East coast

[摘要] ENGLISH ABSTRACT: A tropical cyclone is a non-frontal, synoptic scale, low-pressure system over tropical or subtropicalwaters with organised convection and a definite cyclonic surface wind circulation. The system varies in size between a hundred and a few thousand kilometres in diameter with highwinds circulating around a central low pressure. The process of bringing the lower atmosphericlayers into thermodynamic equilibrium with the warm tropical waters add the energy to theatmosphere and lower the surface pressure. If favourable climatic conditions exist this leads tothe formation of a warm core vortex, which can develop into a tropical cyclone. The occurrenceof tropical cyclones follows seasonal variations, the tropical cyclone season for the SouthwestIndian Ocean being between November and March. The occurrences peak along the SouthernAfrican East Coast between Mid-January and Mid-February.The data on the location and intensity of tropical cyclones along the Southeast Africa coastlinewere obtained from the Joint Typhoon Warning Centre and span the period between 1848 and1999. The available data before 1945 consist of tropical cyclone tracks that influencedpopulated areas or were encountered by ships. It was assumed that a number of tropicalcyclones before 1945 were not recorded and therefore data collected before 1945 weredisregarded in the analysis. The development of radar in 1945 significantly improved thedetection of tropical cyclones. Some of the tropical cyclone tracks recorded between 1945 and1956 contain information about the intensity of the tropical cyclone. Since the dawn of thesatellite age in the mid 1980's, the detection of tropical cyclones and intensity measurementshave improved vastly.Monte Carlo simulation techniques were used to create long term data series based on theavailable measured data. Statistical distributions were fitted for each characteristic describingthe tropical cyclone at its nearest position to the site under investigation.Tropical cyclones frequently occur along the Southern African East Coast. The region wheremore than one tropical cyclone per 100 years can be expected is bordered by latitudes 2.5°S to32.5°S. The design parameters for structures in these regions should provide for the influencethat a tropical cyclone will have on the site. The occurrence rate and expected maximumintensity of tropical cyclones with a 1DO-year return period vary with latitude along the SouthernAfrican East Coast. The maximum number of tropical cyclones in a 1DO-year period occurs atlatitude 15°S with an expected number of tropical cyclones of 157.2 per 100 years. Themaximum expected tropical cyclone intensity in a 100-year period is 143.5 knots at latitude 17.5°S.

[发布机构] Stellenbosch University