Topographic Effects on Barotropic Vortex Motion: No Mean Flow

Abstract

The impact of the island topographic ? effect on hurricane-like vortex tracks is studied. Both f plane and spherical geometry without a mean flow are considered. The simulations used in this study indicate the existence of a track mode in which vortices are trapped by the topography and follow a clockwise island-circulating path. The trapping of a hurricane-like vortex can be interpreted in terms of the influence of the island topographic ? effect on the vortex track. Experiments on the f plane indicate that the drift speed along the clockwise path is proportional to the square root of ?evmax. The applicability of the square root law on the f plane is dependent on the degree to which the local ?e effect is felt by the vortex. The experiments on the sphere also demonstrate that the speed along the clockwise path is larger for a vortex with a larger maximum wind vmax. The occurrence of hurricane-like vortex trapping, however, is not sensitive to the value of vmax. When there is no background flow, the vortex will drift to the northwest in the presence of the planetary vorticity gradient. The ? drift speed acts to keep the vortex from being trapped. The insensitivity of the vortex trapping to vmax on the sphere appears to be due to the possible cancellation of stronger planetary ? and topographic ? effects. The experiments suggest that the topographic scale must be comparable to (if not larger than) the vortex radius of maximum wind for the trapping to occur. Nonlinear effects are important in that they hold the vortex together and keep it moving without strong dispersion in the island-circulating path. This vortex coherency can be explained with the ? Rossby number dynamics. The global shallow-water model calculations used in this study indicate that the vortex trapping increases with peak height, topographic length scale, and latitude (larger topographic ? effect). In general, the trapping and clockwise circulating path in the presence of a planetary vorticity gradient will occur if the scale of the topography is greater than the vortex radius of maximum wind and if the planetary ? parameter is less than the topographic ? parameter.