The Atmospheric Boundary Layer above the Agulhas Current during Alongcurrent Winds

The response of the marine atmospheric boundary layer to the strong sea surface temperature (SST) gradient across the warm Agulhas Current was studied in the Agulhas Current Air–Sea Exchange Experiment, south of Port Alfred, South Africa. Shipboard meteorological and oceanographic measurements and radiosonde ascents are used to describe this response for wind regimes that are approximately parallel to the current. Surface heat fluxes increased by over 200 W m−2 from the shelf to the current while the sensible heat flux reversed sign. A characteristically stable boundary layer over the cool shelf waters was replaced by an unstable convective boundary layer over the current. The mean specific humidity and potential temperature of the boundary layer increased significantly over the current by 25% and 10% with a concomitant boundary layer deepening of 40 m (°C)−1 SST. In the presence of alongcurrent winds an atmospheric moisture and thermal front developed over the inshore SST front. This is attributed to the horizontal gradient in the surface heat fluxes. The strong SST front owes its presence here to the juxtaposition of the warm current and an inshore kinematically driven subsurface upwelling cell whose surface expression is modulated by the wind stress.