Topographic effects on barotropic vortex motion: No mean flow

The impact of the island topographic beta effect on hurricane-like vortex t racks is studied. Both f plane and spherical geometry without a mean flow a re 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 beta effect on the vortex track. Experiments on the f plane indicate that the d rift speed along the clockwise path is proportional to the square root of b eta (e)v(max). The applicability of the square root law on the f plane is d ependent on the degree to which the local beta (e) effect is felt by the vo rtex. The experiments on the sphere also demonstrate that the speed along t he clockwise path is larger for a vortex with a larger maximum wind v(max). The occurrence of hurricane-like vortex trapping, however, is not sensitiv e to the value of v(max). When there is no background flow, the vortex will drift to the northwest in the presence of the planetary vorticity gradient . The beta drift speed acts to keep the vortex from being trapped. The inse nsitivity of the vortex trapping to v(max) on the sphere appears to be due to the possible cancellation of stronger planetary band topographic beta ef fects. The experiments suggest that the topographic scale must be comparabl e to (if not larger than) the vortex radius of maximum wind for the trappin g to occur. Nonlinear effects are important in that they hold the vortex to gether and keep it moving without strong dispersion in the island-circulati ng path. This vortex coherency can be explained with the beta Rossby number dynamics. The global shallow-water model calculations used in this study i ndicate that the vortex trapping increases with peak height, topographic le ngth scale, and latitude (larger topographic beta effect). In general, the trapping and clockwise circulating path in the presence of a planetary vort icity gradient will occur if the scale of the topography is greater than th e vortex radius of maximum wind and if the planetary beta parameter is less than the topographic beta parameter.