Instability Dynamics of a Subtropical Jet and Applications to the Azores Front Current System: Eddy-Driven Mean Flow

Abstract

The Azores front is associated with an eastward flowing subtropical jet, which coincides with the northern edge of the North Atlantic subtropical gyre and forms a southern branch of the Gulf Stream. An intensive study of the eddy-driven mean flow induced by the nonlinear instability of the jet has been conducted. Using a primitive equation numerical model initialized with the summer hydrological fields typical of the Azores front, the authors show that the mesoscale patterns that appear after a few months are largely determined by baroclinic instability processes. the jet and its associated mesoscale turbulence have a strong meridional asymmetry due to the strongly sloping thermocline. In general, an anticyclonic meander, which grows and deforms northwestward, generates a pinched-off anticyclone while a cyclonic meander, which grows and deforms southwestward, degenerates in a thin filamental structure that will dissipate before any cyclone can be pinched off. The authors propose a mechanism that relates this asymmetry of the turbulent fields to the mean meridional gradient of Ertel potential vorticity. the rectification of the meridionally asymmetric mesoscale turbulent eddies is able to create a westward flowing countercurrent on the northern flank of the main jet. The Eliassen?Palm flux vector divergence or, equivalently, the turbulent transport of Ertel potential vorticity perturbation drives the countercurrent in the upper layers, while the Coriolis force acting over the ageostrophic meridional circulation is responsible at lower levels. This ageostrophic meridional circulation is dipolar, with an upwelling below the jet axis and downwellings on either sides. The dipole is forced by the particular structure of the eddy transport of zonal momentum associated with meridional asymmetry. It is an important communication link between the surface and deeper ocean layers in a frontal area. These results are in good agreement with observational data from the Azores Front system and are probably relevant to other zonal jets that lie on the poleward sides of subtropical gyres.