AEROSOL DEPOSITION OFF THE SOUTHERN AFRICAN WEST COAST BY BERG WINDS

Abstract

ABSTRACT Berg winds off the west coast of southern Africa are associated with wintertime subcontinental and ridging anticyclones. Most of the transport patterns associated with Berg winds involve air recirculated over the subcontinent with anticyclonic recirculation occurring mainly over central southern Africa and northeastern South Africa. Although not frequent, Berg winds are considered the most important atmospheric agents of aerosol transport and deposition off the west coast because of their high velocities and subsidence offshore. Berg wind transport off the west coast has north northeasterly to northeasterly maximum frequency transport plumes, extending more than 100km offshore from Lüderitz and Keetmanshoop. The Keetmanshoop offshore plume transports a total of 72 752 tonnes of mainly aeolian dust annually towards the coast with 982 tonnes deposited offshore throughout the plume. Off Lüderitz, Berg wind transport has two plumes: the southeasterly plume which transports 35 894 tonnes of mainly aeolian dust annually with 437 tonnes deposited off the coast, and the easterly plume, which transports 36 537 tonnes annually, depositing 3 018 tonnes within 100km offshore. A total of 3 455 tonnes of aerosols are transferred annually into the Benguela upwelling system from the two Lüderitz plumes. The Lüderitz and Keetmanshoop plumes deposit aerosols off the west coast at an estimated rate of 74 μ m−2. Although not significant by global standards, an estimated annual dust input of4 437 tonnes into the South Atlantic Ocean during Berg wind days by both the Lüderitz and Keetmanshoop plumes is likely to be responsible for contributing some aerosols to supplement nutrients in the highly fertile Benguela upwelling system.