Subduction of a Surface Vortex under an Outcropping Front.
Résumé
The possibility for a preexisting surface-intensified anticyclone to subduct beneath a surface front is investigated using an isopycnal numerical model. Subduction occurs for strong coherent vortices and is usually accompanied by strong dissipation. Two main mechanisms cause the erosion of the vortex core. The first one is induced by the velocity shear associated with the front. It results in the peeling of the vortex potential vorticity (PV) core, sometimes leading to its complete disappearance. The second mechanism occurs after the vortex has subducted. Entrainment of high positive PV fluid parcels from the front above the vortex low negative PV core modifies the stability properties of the latter. Meanders are observed to grow at the rim of the structure, which favors the formation of PV filaments. The erosion rates caused by each mechanism are discussed in relation to the jet and vortex characteristics, and to the background stratification. The trapping of high PV fluid parcels above the vortex is also shown to be partly responsible for the decrease and eventual loss of the eddy altimetric signal, after it has subducted.