Tidal stirring and mixing on the Australian North West Shelf

Abstract

Strong tides on the Australian North West Shelf lead to vigorous vertical stirring and mixing. This activity near the shelf break is directly quantified with a numerical model. The model has a spatial and temporal resolution of about 800 m and 13 s respectively. The model result shows internal tides steepening as they shoal onto the slope, which is near critical for the semidiurnal tide. The main energy balance of the internal tides is between incoming internal tidal energy flux and the dissipation. The vertical and horizontal distribution of the heat flux carried by the internal tides is complex and difficult to parameterise. When area-averaged in the model region (38 km by 36 km), the vertical heat flux is in the order of –10−5 deg m s−1 (positive upward). This translates to a vertical diffusivity of 10−4 to 10−2 m2 s−1, which is an order of magnitude larger than the subgrid vertical diffusion. Both the warming and cooling rates caused by the convergence and divergence of the heat flux are in the order of 10−6 deg s−1. The neap-spring modulation of the semidiurnal tide did not significantly modify these estimates. These values can explain the vertical heat exchange inferred from large-scale analysis of the heat budget on the Shelf.