Tidal mixing near the Kimberley coast of NW Australia

Abstract

A complex of reefs and islands atop raised topography that is 140 km long and 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 images of sea surface temperature in spring, summer and autumn suggest that tides commonly mix sub-thermocline waters to the surface along this ridge, particularly along its western half, which includes Adele Island and Churchill, Albert, Beagle and Mavis reefs. The cool surface waters (~3˚C cooler than surroundings) are seen where the depths are less than ~50 m, such that they closely reflect the bottom topography. The semi-diurnal tides have a maximum daily range of ~10 m and it is when the range exceeds ~3 m that mixing to the surface occurs. Thus, the mixing can cease during some neap periods. Historical data from the shelf show the sub-thermocline water to be nutrient-rich (nitrate concentrations of 6.0 µM), so the mixing is likely to be important for the marine ecosystem. As winter approaches, the distinctive cool mixed region around the Adele–Mavis banks is overwhelmed by surface cooling, and historical ship data reveal that the stratification on the surrounding shelf disappears.