Interaction of a monopolar vortex with a topographic ridge

J.H.G.M. van Geffen and P.A. Davies
Geophys. & Astrophys. Fluid Dynam. 90, 1--41 (1999)

1. Introduction

Vortices are common features under many geophysical circumstances. Observations (see e.g. Richardson 1993a,b; Bower et al., 1995; Kamenkovich et al., 1996; Bograd et al., 1997) have, for instance, shown the existence of several kinds of vortices in the Earth's oceans, such as Meddies, Gulf Stream eddies and anticyclones, Agulhas eddies, etc. These eddies are quite abundant; Richardson (1993b) estimates that there are roughly 1000 discrete eddies in the North Atlantic. The vortices move due to a combination of the latitudinal variation of the Coriolis force (the so-called beta-effect) and the general background oceanic flow. As such vortices move, they inevitably encounter submarine topographic features at the the bottom of the ocean or the edges of coastal shelves.

Interactions with topographic features are known to influence the trajectories of the vortices and it is possible that the vortex is destroyed by the topography. For instance, Richardson (1993a,b) gives the example of a Meddy off the westcoast of Portugal that had a catastrophic end in the vicinity of Hyères Seamount after an estimated life time of 2.5 years. As a result, the Meddy's contents -- Mediterranean water with its high salinity and pollution -- was released into the Atlantic Ocean. Shapiro et al. (1995) report observations of a Meddy that was damaged by a collision with a group of seamounts but kept its coherent structure (though it lost some of its contents).

Eddies are thus thought to play an important role in transporting water properties within the oceans and it is therefore of interest to understand the basic mechanisms behind the interaction of a vortex with a bottom topography. In this regard, a two-layer model has been used by Kamenkovich et al. (1996) for studying Agulhas eddies that encounter a ridge, which showed that Agulhas eddies that cross the ridge can carry their contents far into the South Atlantic subtropical gyre. Smith and O'Brien (1983) used a two-layer model for an eddy on a coastal shelf. Such eddies move along the shore with a tendency to go onshore (offshore) for a cyclonic (anticyclonic) vortex (see also Grimshaw et al., 1994). The latter tendency is caused by the beta-effect (or its dynamical equivalent of a sloping bottom, with "north" towards decreasing fluid depth; see e.g. Van Heijst, 1994): a cyclonic (anticyclonic) monopole moves on the northern hemisphere to the northwest (southwest). It is also this effect that causes a cylonic vortex to climb out of a conical valley and to the top of a hill (Carnevale et al., 1991). Verron and Le Provost (1985) showed that in a flow over an isolated seamount (anti)cyclonic vortices may be formed, depending on the flow characteristics and the presence or absence of the beta-effect. Ezer (1994) showed that eddies are formed in the region where the Gulf Stream passes over a chain of seamounts.

The present paper employs a simple one-layer two-dimensional (shallow water) model to study the basic features of vortex-topography interactions. As a first step in this investigation, a cyclonic monopole that encounters a smooth ridge located at the equator is simulated, where the height and width of the ridge and its orientation are varied. The numerical model used for the simulations is outlined in Section 2. In Section 3 the effects of a topographic disturbance on an approaching vortex are discussed and a simple example is given to show that these effects are well represented by the model. The monopole used for the interaction-study and the mechanism for its motion (the beta-effect) are introduced in Section 4. Sections 5 to 7 present the results of the simulations and some concluding remarks are formulated in Section 8.

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