Loop Current Ring Shedding: The Formation of Cyclones and the Effect of Topography

Abstract

Abstract The formation of cyclones in the vicinity of the Loop Current ring during the separation stage is analyzed in the frame of a high-resolution ECMWF daily wind-forced Miami Isopycnic Coordinate Ocean Model (MICOM) simulation. Mesoscale cyclones, observed in sea surface height maps in the vicinity of the Loop Current in a necking-down position, are found to contribute to the separation of the ring from the Loop Current as they grow between the Loop Current and the ring in the MICOM simulation. To understand the origin of the cyclones, the instability of the Loop Current idealized as an isolated vortex is studied. After noticing the cyclonic vorticity belt around the Loop Current, and based on the vertical distribution of potential vorticity in a Loop Current ring in the MICOM simulation, the linear stability of a shielded vortex is studied in the quasigeostrophic formalism. To simulate the effects of the planetary vorticity gradient and topography on the Loop Current, the nonlinear states of the idealized Loop Current are analyzed using the adiabatic MICOM code. Results from the analysis of the MICOM simulation show that cyclones are the products of a vortex rim instability. The modal analysis of the Loop Current instability reveals that mode 4 is the fastest-growing mode and that baroclinic (barotropic) instability is intensified in the deep (surface) layers. These results are confirmed by the analytical study of the idealized Loop Current. The nonlinear state shows that a Loop Current–like vortex is indeed a pentapole on an f plane. On the β plane, the northern cyclone is separated from the anticyclone by the β 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, and 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.