AN AIR TRANSPORT CLIMATOLOGY FOR SUBTROPICAL SOUTHERN AFRICA

Abstract

An air transport climatology is derived for subtropical southern Africa (Africa south of 15°S) by classifying daily synoptic situations into predominant circulation types. The annual variation of these provides the basis for determining month-by-month transport. Percentage zonal transport in easterly and westerly directions, levels of transport, and times of transit are derived from forward trajectory analyses using European Centre for Medium- range Weather Forecasts (ECMWF) data for a 7-year period. It is shown that semi-permanent subtropical continental anticyclones, transient mid-latitude ridging anticyclones and mid-latitude westerly disturbances produce major transport into the south-western Indian Ocean in the Natal plume. Only quasi-stationary tropical easterly waves result in appreciable transport into the tropical South Atlantic Ocean in the Angolan plume. Total transport is a function of circulation type and frequency, as well as plume dimensions. Transport in continental highs follows an annual cycle reaching peak values in excess of 70 per cent in winter. That in easterly waves also exhibits an annual cycle, but one peaking in summer, when up to 55 per cent transport may occur in north-western regions. Transport in ridging highs and westerly perturbations is much less and occurs throughout the year, with a slight tendency to peak in spring. Recirculation of air is shown to be considerable when anticyclonic conditions prevail. Monthly, seasonal, and annual mass fluxes over and out of southern Africa are determined from transport fields, frequency of occurrence of circulation types and from measurements of aerosol concentrations. An annual mass flux of aerosols some 134 Mtons is generated over the subcontinent. About 60 Mtons year−1 are deposited, and approximately 29 Mtons year−1 are exported westward over the Atlantic Ocean and 45 Mtons year−1 eastward over the Indian Ocean. Twenty-six million tons of the 74 Mtons of aerosols exported annually to the adjacent oceans on each coast are a product of recirculation. Deposition within 10° latitude of the coast is nearly 10 times greater on the east than on the west coast.