2005 LAYERED OCEAN MODEL WORKSHOP

The formation of cyclones in the vicinity of the Loop Current during the shedding process is analysed in terms of vortex instability. First, using a high resolution ECMWF-daily wind forced MICOM simulation, the potential vorticity anomaly and the energy conversion terms are calculated to evidence the timing of barotropic and baroclinic instability during a ring formation and separation cycle. The scenario obtained shows that cyclones are the products of the mixed barotropic-baroclinic vortex instability, which grows around the rim of the Loop Current ring while it is attached to the Loop Current. Cyclones contribute to the separation of the ring from the Loop Current as they grow between them. This scenario, based on observations, was proposed much earlier by Vukovich et al. (1979) and Vukovich and Maul (1985). To understand the role of instability in the Loop Current ring shedding process and the interaction of the unstable Loop Current ring with the topography, we build upon the MICOM simulation a simplified quasi-geostrophic model of the Loop Current ring. The Loop Current ring-like vortex is a type 'R' shielded vortex, whose cyclonic rim becomes unstable as the width of the rim decreases and high instability modes grow. The analytical model confirms that a mode 4 baroclinic instability is intensified in the deep layers of the ring while barotropic instability is surface intensified. The nonlinear state shows that a Loop Current ring-like vortex is indeed a pentapole on an f-plane: the anticyclone is surrounded by four cyclones. On the beta-plane, the northern cyclone is separated from the anticyclone by the beta-effect and both drift westward. When the topography of the Gulf of Mexico is taken into account, namely the Campeche Bank, the southward slope north of the Loop Current, and the Florida shelf east of the Loop Current, several effects are observed: (1) the northern corner of the Campeche Bank erodes the Loop Current ring and its cyclones, and interacts with the vortex's most unstable mode; (2) the northern southward slope scatters the northern cyclone while the anticyclone remains coherent and propagates to the west; (3) realistic westward propagation speeds are obtained in the presence of the northern Campeche shelf, which acts as a mirror effect on the Loop Current ring, as opposed to the Florida shelf, which tends to block the ring.