The splitting of eddies along boundaries
Abstract:This paper addresses the question of what happens to an eddy that is forced “violently” against a boundary by, say, an advective current or another vortex. The detailed temporal evolution of such a collision on an f-plane is examined using a barotropic, as well as a one-and-a-half-layer contour dynamics model with surgery procedures. Both the barotropic eddy and the one-and-a-half-layer eddy initially have two circular potential vorticity fronts: an inner front at which the velocity increases to a maximum from zero at its center, and an outer front (e.g., the edge of the eddy) at which the velocity reduces to zero.
At t = 0, the circular eddy is conceptually cut off by the wall. It is demonstrated that such a cut corresponds to the violent forcing of the eddy against the wall. One intuitively expects that, as a result of the collision, the eddy would simply leak fluid along the wall (forming a thin jet) until it shrinks to such a size that it is merely “kissing” the wall (Nof, 1988a). In contrast to this intuition, however, it is found that, after the eddy is cut by the wall (i.e., t > 0), the annulus fluid (i.e., the fluid between two fronts) is gradually advected along the wall forming a new eddy next to the interior (i.e., the region inside the inner front). After formation, both the off-spring eddy and the parent eddy migrate along the wall away from each other. This migration is mainly due to the image effect that is created by the wall and separates the eddies even farther. As time goes on, the migration intensifies because the mutual advection of the eddies forces them farther into the wall so that the image effect increases.
These results of our contour dynamics study are in good agreement with additional experiments of an isopycnic, primitive equation model. Namely, both of these studies illustrate that an eddy-wall collision causes the parent eddy to split into two migrating eddies, one that contains the core of the parent and the other that contains fluid from the rim.
Possible applications of these models to eddies pushed against the shelf in the Gulf of Mexico are discussed.
Document Type: Research Article
Publication date: November 1, 1993
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