TRANSPORT AND RECIRCULATION OF AEROSOLS OFF SOUTHERN AFRICA - MACROSCALE PLUME STRUCTURE

A pall of aerosols and trace gases frequently occurs over southern Afr ica to a depth of similar to 500 hPa, blanketing vast areas, particula rly in the austral winter and spring. Large-scale offshore transport o f these aerosols and trace gases in extremely large plumes from interi or continental areas of the subcontinent to the Indian and Atlantic Oc eans is a common occurrence. The nature of the transport plumes, their climatology, chemical composition and morphology are discussed. In th e vertically integrated, surface-to-500 hPa layer, poleward of about 1 5 degrees S, transport into the Indian Ocean is shown to be about 60% greater into the Indian Ocean than into the Atlantic Ocean. Recirculat ion of atmospheric constituents is considered and estimates of aerosol mass fluxes over central southern Africa are presented. Of the total of about 50 Mt yr(-1) of aerosols being transported at the central mer idian, 44% is shown to be recirculated material. The rest exits the su bcontinent directly without recirculation. Preferred plume corridors o f exit and entry are postulated for different localities on the east a nd west coasts. Two case studies of east-and west-coast plumes apparen tly flowing uniformly out of southern Africa are examined. The illusio n of uniformity in plume structure is shown to be misleading. Both plu mes are shown to be above and separated from the marine boundary layer . Each is over 1500 km in width and 3-5 km deep. Likewise, both are ca pped by absolutely stable layers at similar to 500 hPa and exhibit a c omplex structure of both outflowing aerosols and trace gases and inflo wing, recycled and recirculated material. Indications of the compositi on of the recirculated material are given and implications of the plum e transports are considered. (C) 1998 Elsevier Science Ltd. All rights reserved.