Homogenization of potential vorticity in the eastern region of the North Atlantic subtropical gyre. A modelling approach

Abstract

The eastern region of the North Atlantic subtropical gyre is an upwelling favorable region characterized by an intense quasi‐permanent southward flowing coastal jet that is fed by the eastern branch of the Canary Current and provides the eastern boundary condition for the interior flow. This boundary drainage was numerically specified for a very simple one‐layer quasigeostrophic model through modified eastern boundary conditions, specifically by allowing a meridional band of nearconstant potential vorticity (PV) near the coast. The existence of such a constant PV meridional band implies the generation of negative relative vorticity at the eastern boundary, as if induced by the horizontal shear associated to the system of coastal currents, and allows the interior water to exit the numerical domain at the eastern boundary in reasonable agreement with observations. Climatological simulations with a high‐resolution regional ocean circulation model for the Canary Basin confirm the existence of a narrow band of high anticyclonic relative vorticity along the Northwest African coast, which increases towards the south, and is responsible for the creation of a meridional band of near‐constant PV. The model is used to illustrate the PV homogenization process at several depths along this coastal band and to compute those terms that affect the vorticity balance of water parcels moving south