Moisture uptake in the boundary layer above the Agulhas Current: A case study

Shipboard meteorological measurements and radiosonde ascents are used to describe the modification of the marine atmospheric boundary layer as air is advected across the warm Agulhas Current toward the coast. The data were collected on April 28, 1995, during the Agulhas Current Air-Sea Exchange Experiment south of Port Alfred, South Africa, in a region characterized by a strong sea surface temperature gradient. The surface latent heat flux increased from approximately 169 W m(-2) over the seaward border to 270 W m(-2) over the current (approximately 80-90 km wide) and decreased dramatically to approximately 40 W m(-2) over the cool shelf. This spatial heat flux gradient was reflected in the overlying atmosphere by a transition in atmospheric stability from the warm current to the cool continental shelf. A convective boundary layer over the current was replaced by a stable boundary layer over the cool shelf, where a deep residual moisture layer indicated the former vertical extent of convective eddies. A progressive accumulation of moisture occurred within the boundary layer as the air mass was advected over the current. Mean specific humidity and precipitable water vapor content in the boundary layer over the shelf were 20-25% higher than over the seaward border of the current. The results give credence to the assertion by many investigators that moisture uptake above the Agulhas Current may contribute significantly to moisture convergence and rainfall over the interior of South Africa.