AN AIR TRANSPORT CLIMATOLOGY FOR SUBTROPICAL SOUTHERN AFRICA

An air transport climatology is derived for subtropical southern Afric a (Africa south of 15 degrees S) by classifying daily synoptic situati ons into predominant circulation types. The annual variation of these provides the basis for determining month-by-month transport. Percentag e zonal transport in easterly and westerly directions, levels of trans port, and times of transit are derived from forward trajectory analyse s using European Centre for Medium-range Weather Forecasts (ECMWF) dat a for a 7-year period. It is shown that semi-permanent subtropical con tinental anticyclones, transient mid-latitude ridging anticyclones and midlatitude westerly disturbances produce major transport into the so uth-western Indian Ocean in the Natal plume. Only quasistationary trop ical easterly waves result in appreciable transport into the tropical South Atlantic Ocean in the Angolan plume. Total transport is a functi on of circulation type and frequency, as well as plume dimensions. Tra nsport in continental highs follows an annual cycle reaching peak valu es in excess of 70 per cent in winter. That in easterly waves also exh ibits an annual cycle, but one peaking in summer, when up to 55 per ce nt transport may occur in north-western regions. Transport in ridging highs and westerly perturbations is much less and occurs throughout th e year, with a slight tendency to peak in spring. Recirculation of air is shown to be considerable when anticyclonic conditions prevail. Mon thly, seasonal, and annual mass fluxes over and out of southern Africa are determined from transport fields, frequency of occurrence of circ ulation types and from measurements of aerosol concentrations. An annu al mass flux of aerosols some 134 Mtons is generated over the subconti nent. About 60 Mtons year(-1) are deposited, and approximately 29 Mton s year(-1) are exported westward over the Atlantic Ocean and 45 Mtons year(-1) eastward over the Indian Ocean. Twenty-six million tons of th e 74 Mtons of aerosols exported annually to the adjacent oceans on eac h coast are a product of recirculation. Deposition within 10 degrees l atitude of the coast is nearly 10 times greater on the east than on th e west coast.