Numerical studies of small island wakes in the ocean

Two and three-dimensional oceanic flows around small islands patterned after Barbados, W.I. (13 degrees 10' N latitude: 59 degrees 30' W longitude) were modeled numerically to investigate island wake effects. The two-dimensional simulations closely agreed with laboratory flows, for both attached and shedding wake regimes. As expected, results for a flat bottom confirmed that the Coriolis terms strongly affect pressure but not the flow. For idealized, yet typical incident dow speeds, water column stratification, island topography and appropriate Coriolis terms, three-dimensional simulations readily produced elongated wake patterns, dominated by surface intensified von Karman-like vortices. Effects of grid resolution, viscosity, bathymetry, and Coriolis forces on wake characteristics were studied. For islands with typical bottom slopes, realistically small horizontal eddy diffusivity has a minor effect compared to bottom drag in generating vorticity. Near-shore bathymetry (viz., the absence or presence of a continental shelf surrounding the island) plays a major role in determining the scale, intensity, and shedding period of the vortices. The addition of a 15 km wide continental shelf around the island increased the shedding period by 67%, while reducing the Coriolis force by 50% reduced the period by only 14%. Although observational data is sparse, inferred flow patterns do show von Karman-like structures near Barbados, even if eddies are not located exactly as expected. The numerical computations demonstrate that shedding eddy wakes are easily generated, and lend encouragement to the further search for organized wakes downstream of the island.