Large-Scale Recirculation of Air over Southern Africa

Abstract

Kinematic air parcel trajectory analysis is used to determine patterns of horizontal air transport in 2000 km ? 2000 km areas over southern Africa. From these, composite zonal and meridional transport fields are derived for the subcontinent to estimate the extent to which recirculation of air and aerosols may take place in the lower troposphere between the surface and 500 hPa. The nature and degree of recirculation beneath the persistent 500-hPa absolutely stable layer is demonstrated, and transport by recirculation in discrete streams is shown to constitute 44% of the total transport over the region. From a determination of air volume fluxes and estimates of aerosol concentrations, the total mass flux of aerosols by direct transport and by recirculation in conditions during which semipermanent, subtropical, continental anticyclones prevail is estimated to be about 51 Mton yr?1 in the surface-to-hPa layer. Recirculation comprises approximately 22 Mton yr?1 of this amount. Of the recirculated transport, about 5 Mton yr?1 is recirculated to the west in easterly transport and 17 Mton yr?1 to the east in westerly transport.