MODELING THE DYNAMICS OF THE TRANSIENT, BAROTROPIC RESPONSE OF CONTINENTAL-SHELF WATERS TO TROPICAL CYCLONES

The dominant dynamic balances which occur during the transient, barotropic response of continental shelf waters to tropical cyclones are investigated, using an idealised two-dimensional numerical model. Three main stages of the response are identified: a build-up stage as the cyclone approaches, a directly forced stage when the cyclone is within a distance roughly corresponding to its diameter (twice radius to maximum wind), and a relaxation stage, after the cyclone has propagated away or dissipated. During the directly forced stage when the maximum response occurs, the dominant momentum balances are used to derive approximate expressions for the maximum currents and sea levels. Geometrical factors such as whether the storm track is alongshore or cross-shore, and storm characteristics such as its forward speed, radius to maximum winds and ambient to central pressure difference are all important in determining the magnitude of the response. During the relaxation stage, freely propagating edge waves and topographic gyres are clearly evident. The topographic gyres result from the vortex-like circulation relaxing over a sloping bottom following the passage of a cyclone and are the manifestation of locally induced continental shelf waves. Characteristics of the transient response of the idealised model are compared with those obtained using a model of the northwest shelf of Australia. It is shown that, although the complex bathymetry of the northwest shelf introduces local effects, the principal results derived using the idealised model are still valid. Thus in addition to clarifying the key processes that occur in the various stages of the response, a diagnosis of the momentum balances occurring in the idealised model produces useful formulae for estimating maximum currents and sea-levels generated by a tropical cyclone.