Vorticity dynamics near sharp topographic features

In ocean models, the interaction with boundaries is often parameterized as it involves small-scale processes that are usually hard to capture in a large-scale model. However, such interactions can play important roles in the model dynamics. For example, the choice of boundary conditions (free-slip vs. no-slip) has a direct impact on the vorticity (enstrophy) budget: with no-slip boundary conditions, vorticity is injected into the system, whereas with free-slip boundary conditions, there should be no vorticity injection as long as the coastline is smooth. However, we show here that at boundary singularities (e.g., corners), vorticity is injected into the domain even for free-slip boundary conditions. In this article, we use North Brazil Current rings to better understand the dynamics of eddy-topography interaction. This complex interaction is first analyzed in terms of a point vortex interacting with a wall. Within this simplified framework, we can describe the vorticity generation mechanism as a pseudoinviscid process. To quantify this vorticity injection, we first consider the inviscid limit for which we can derive an analytical formula. This theoretical prediction is then evaluated in conventional gridded ocean models. In such models, the representation of such a “viscous” boundary interaction may be affected by the grid representation and the discretization of the advection and viscous operators.