Influence of bottom topography on cross-shelf circulation forced by time dependent wind

Abstract

A series of numerical experiments is conducted in order to examine the role of topographic irregularities in generation of subinertial cross-channel barotropic currents and to obtain quantitative estimates of the offshore flow amplitude, its ratio to the magnitude of the alongshore currents and the alongshore correlation scale. A periodic (along the coast) channel with geometry representative of continental margins is considered. Topographic disturbances have multiple alongshore wavenumber contributions, kT, and the amplitude proportional to kT-2 (except one experiment). The motion is forced by spatially uniform and temporally varying alongshore wind stress, which drives the background current along the channel. The background current adjusts to topographic disturbances and, in particular, Barotropic Shelf Waves (BSW) result. That gives rise to the offshore currents. The amplitude of the cross-channel flow is shown to increase with kT in the long wave limit (order of 100 km in the model) and, in contrast, to decrease with kT in the short wave limit (less than 20 km in the model). As a rule, the strongest response is attained on the intermediate scales where lee waves form most efficiently. Hart's {1990} quasi-geostrophic solution provided quantitative parameters to explain the scale dependence and helped to interpret the results of simulations meant to examine sensitivity of the cross-channel flow characteristics to variation of the governing parameters. It is shown that the structure of the resonant wave is established by the combination of spatial properties of the bottom bumps, and by the period and amplitude of the fluctuating background current. On the basis of this analysis, it is demonstrated how the BSW dispersion diagram can be used for diagnostic purposes. The averaged (in time and along the channel) amplitude of subinertial cross-channel currents ranged from negligible values to about 5.5 cm/sec, indicating that, indeed, rather substantial offshore flow can be generated due to the effect of the topographic irregularities. The alongshore correlation scale was no smaller than about 5 km and in a few simulations (for instance, with lee-wave-favourable mean wind included) reached 30 - 50 km.