Response of the Southern Benguela upwelling system to fine-scale modifications of the coastal wind

• Southern Benguela Upwelling dynamics related to atmospheric fine-scales.
• Use of new scatterometer data to force a hydrodynamic model at the surface.
• Short-term response of the ocean to a unique change of momentum fluxes.
• The fate of the coastal Southern Benguela waters using numerical Lagrangian experiments.
• Impact of wind drop-off on both vertical structures and cross-shore exchanges of particles.

We analyze the results of a regional model of the Southern Benguela upwelling system forced by wind stress fields derived from QuikSCAT observations. Two different horizontal resolutions are considered for the wind stress: QS25 and QS50, corresponding to native 25 and 50 km grids, respectively. The differences between both products highlight the primary importance of fine-scale momentum fluxes for both the structure and intensity of the wind- and wind-curl-driven upwelling. Using QS25, we show that the coastal Ekman transport is reduced, leading to a warmer SST and a reduced oceanic coastal jet. QS25 finer wind stress curl patterns also favor the development of a stronger and shallower poleward undercurrent. The addition of a coastal wind correction to QS25 lets us investigate the possible implications of an imbalance between Ekman transport and Ekman pumping: a wind reduction in the coastal band often reduces the SST cooling, but the two mechanisms compensate each other when the characteristic length scales of the coastal upwelling and the orography-induced wind drop-off are similar.