Vorticity balance in the Northwestern African Upwelling

Upwelling filaments are typical features in Eastern Boundary Upwelling Systems and were the object of numerous in situ (e.g. Ramp et al., 1991, Barton et al., 2004), and numerical studies (e.g. Haidvogel et al., 1991; Johnson and Stevens, 2000).

Nowadays, physical processes that drive filaments are not perfectly understood. Recent investigations (Pelegrí et al., 2005) suggest that filaments arise from a combination of three phenomena :
- baroclinic instability of the coastal upwelling jet;
- interaction of the flow with coastline and topography shape;
- coastal convergence due to wind stress.

Our purpose is to develop a simple theory based on the potential vorticity conservation: we assume that filament generation is related to the injection of positive relative vorticity in the vicinity of Cape Ghir: as the flow gains positive vorticity, it becomes unable to continue southward and detaches from the coast.

As several processes (wind curl, bottom friction, variable topography etc}) may be responsible for positive vorticity generation, we carried out several experiments with ROMS in order to assess the role of each process.

Results show great correlation between temperature and relative vorticity, as filaments are characterized by intense positive vorticity, surrounded at the north by a region of anticyclonic vorticity. The first conclusions stand that wind-stress resolution is an issue, while topography seems to have less influence on the filament generation.

C. Troupin (1), E. Mason (2) and P. Sangrà (2)

(1) GeoHydrodynamics and Environment Research, University of Liège, Belgium
(2) Universidad de Las Palmas de Gran Canaria, Departamento de Física, Las Palmas de Gran Canaria, Spain