Shallow-water Frictional Effects in Island Wakes

Only one eddy is present in the island wake in the lee of Rattray Island, a 1600 m wide island in shallow coastal waters off Australia. This observation is contrary to the predictions of a number of two- and three-dimensional, fixed grid, numerical models which all predict that two eddies are present, one eddy being shed from each of the two tips of the island. The discrete vortex model suggests this observation results from the frictional effects in sub-grid scale details of the topography close to the tips of the island. This model also successfully reproduces the observations of a Karman vortex street in the lee of islands in Rupert Bay, Canada. By contrast, fixed-grid models predict no such eddy shedding. Island wakes are thus sensitive to details of the bathymetry near the separation points and these must be either parameterized in fixed grid models, whether regular or boundary fitted, or explicitly calculated in non-fixed grid models such as the discrete vortex model.