The Dynamics of the Loop Current and Shed Eddies in a Numerical Model of the Gulf of Mexico

Abstract

The dynamics of the circulation in the Gulf of Mexico have been investigated using simple, efficient numerical models capable of simulating consistently observed dynamical features, including the Loop Current and the shedding of large anticyclonic eddies from the Loop. Over 150 model experiments were integrated to statistical equilibrium, typically 3–5 years. One popular hypothesis holds that the Loop Current sheds anticyclonic eddies in response to annual variations in the inflow through the Yucatan Straits. However, a striking result from the models is their ability to simulate the observed quasi-annual eddy shedding period with no time variations in the inflow. The model-predicted eddy diameters, amplitudes, and westward propagation speeds are also realistic. The dominant instability mechanism in the eddy shedding is a horizontal shear instability of the first internal mode, a barotrooic rather than a baroclinic instability. Therefore, a reduced-gravity model with one vertical mode is able to simulate the basic dynamics of the Loop Current-eddy system. Rossby-wave theory and a conservation of absolute vorticity trajectory analysis were used to explain the behavior of the Loop Current, including its northward penetration into the Gulf, the latitude of westward bending, the shedding period for the eddies, as well as their diameter, and their westward propagation speed. A regime diagram for the reduced-gravity model was constructed in terms of the Reynolds number Re and the beta Rossby number R B = v c /βL p 2 , where v c is the velocity at the core of the current, L p is half the port separation distance and β is differential rotation. Eddy shedding can be prevented by reducing Re or by increasing R B . Bottom relief acts to inhibit baroclinic instability, yielding solutions more closely resembling those from the reduced-gravity model than the two-layer flat-bottom model. Topography also influences the paths of the shed eddies and, in the presence of sufficient deep water inflow through the Yucatan Straits, prevents Loop Current penetration, westward bending, and eddy shedding. In effect, the West Florida Shelf acts to reduce the port separation, increase R B , and shift the Loop Current into a stable regime. The signatures of barotropic and baroclinic instabilities in the two-layer Gulf of Mexico model were studied using upper and lower layer pressure fields and eddy-mean energetics. Both instability processes tend to drive a deep flow characterized by modon 1 generation and they exhibit similar vertical phase relationships. However, in these experiments the westward propagation speeds associated with baroclinic instability are typically two to three times faster.