Tidal mixing near the kimberley coast of NW Australia

A complex of reefs and islands atop raised topography that is 140 km long a nd up to 35 km wide extends roughly westward from the Kimberley coast of NW Australia. The surrounding seafloor has depths of 60-80 m. Satellite image s of sea surface temperature in spring, summer and autumn suggest that tide s commonly mix subthermocline waters to the surface along this ridge, parti cularly along its western half, which includes Adele Island and Churchill, Albert, Beagle and Mavis reefs. The cool surface waters (similar to 3 degre es C cooler than surroundings) are seen where the depths are less than simi lar to 50 m, such that they closely reflect the bottom topography. The semi -diurnal tides have a maximum daily range of similar to 10 m and it is when the range exceeds similar to 3 m that mixing to the surface occurs. Thus, the mixing can cease during some neap periods. Historical data from the she lf show the sub-thermocline water to be nutrient-rich (nitrate concentratio ns of 6.0 mu M), so the mixing is likely to be important for the marine eco system. As winter approaches, the distinctive cool mixed region around the Adele-Mavis banks is overwhelmed by surface cooling, and historical ship da ta reveal that the stratification on the surrounding shelf disappears.