U.S. Minerals Management Services > Publication Abstracts

1. Baroclinic Eddy Interactions with Continental Slopes and Shelves

2. Loop Current Eddy Interaction with the Western Boundary in the Gulf of Mexico

Baroclinic Eddy Interactions with
Continental Slopes and Shelves

G.G. Sutyrin, G.D. Rowe, L.M. Rothstein, I. Ginis

Journal of Physical Oceanography
January 2003
Volume 33, pp. 283-291

Abstract

The evolution and propagation of large anticyclonic eddies similar to the Loop Current eddies found in the Gulf of Mexico are studied in a two-layer intermediate equation numerical model. It is found that the propagation of these eddies is governed primarily by the ß-effect, lower-layer flow driven by the surface eddies, and the image effect. As the eddies move westward under the influence of ß, they drive deep circulation that typically includes a cyclone near the eddies’ trailing edge. This cyclone accelerates the eddy southward, leading to a path substantially different from that found in reduced-gravity models of similar surface eddies. When the eddy encounters a continental slope, the lower-layer flow is dispersed by topographic Rossby waves, and the eddy loses its southward propagation component. If the continental slope is backed by a shelf that is wider then the eddy’s radius, a deep anticyclone will drive the surface eddy rapidly southward until it crosses the shelf break and encounters the boundary, where the eddy will turn northward under the influence of the image effect. Eddies initialized over a zonal slope, which cannot develop the deep circulation that accelerates the eddy southward because of topographic Rossby wave dispersion, propagate nearly zonally until they encounter the western boundary. Over a northern slope such as that found in the Gulf of Mexico, these eddies will retroflect and propagate westward under the influence of the image effect if the northwest corner shelf is narrow, but they will turn southward along the western boundary if the northwest corner shelf is wide.

Loop Current Eddy Interaction with the Western Boundary
in the Gulf of Mexico

S.A. Frolov, G.G. Sutyrin, G.D. Rowe and L.M. Rothstein
Submitted to the Journal of Physical Oceanography August 2003

Abstract

A two-layer, intermediate equations (IE) model that, uniquely, allows for the intersection of the bathymetry with the layer interface is used to study the interaction of Loop Current Eddy(LCE)-type anticyclones with the realistic boundary topography found in the western Gulf of Mexico. Three types of shelf configurations are studied. In the first, the shelf is relatively wide while the continental slope is relatively narrow (designed to realistically represent the Gulf of Mexico coastal topography profile at 25^{o}N). In the second, the shelf is relatively narrow while the continental slope is relatively wide (designed after the Gulf of Mexico topographic profile at 23^{o}N). Finally, a ‘hybrid’ topography is configured for the purpose of isolating the dynamics for analysis. The anticyclone was initialized in the upper layer over the abyssal plane away from the topography, with the lower layer initially at rest so that the eddy can naturally develop a lower-layer flow and a westward propagation, driven by the \beta –effect, until it encounters the coastal topography.

The physical mechanism that has the most significant effect on the evolution of the LCE in all topographic configurations studied here is the interaction of the LCE with cyclones formed directly to its north via the process of off-slope advection of potential vorticity (PV) in the upper layer. The LCE interaction with those cyclones that are generated via this mechanism results in the LCE becoming elliptic and rotating clockwise with its center following a cyclic trajectory.

The amplitude of the cyclic motion produced by LCE interactions with cyclones is controlled by a different physical mechanism. The mechanism consists of the LCE interacting with bottom-intensified eddies that can be generated beneath the LCE over regions of flat topography adjacent to the continental slope. The bottom-intensified eddies are generated due to stretching and compression of the lower layer by a rotating elliptic LCE. The net effect of these eddies is to significantly amplify the cyclic motion of the LCE. The width of the continental slope is the critical parameter controlling the strength of the LCE interaction with deep eddies and, therefore, the amplitude of the cyclic motion. The amplitude of the cyclic motion is significantly larger for the western topography with a narrow continental slope, which allows the deep eddies to form beneath a large portion of the LCE.

A combination of the above physical mechanisms produces a characteristic pattern of LCE evolution during its interaction with the western boundary around 25^{o}N and 23^{o}N in the Gulf of Mexico. The pattern consists of a cyclone formation north of the LCE and subsequent cyclic LCE motion, especially pronounced around 25^{o}N. This characteristic pattern can be identified in some observed cases of LCE interaction with the western boundary in the GoM. In particular, the interaction of ‘Fast Eddy’ with the western boundary around 25^{o}N, which occurred during the fall of 1986, was observed to produce a cyclone north of the eddy. The eddy was then observed to move away from the shelf and south, following the characteristic pattern identified in our experiments.