Summer surface circulation along the Gascoyne continental shelf, Western Australia

The Gascoyne continental shelf is located along the north-central coastline of Western Australia between latitudes 21 degrees and 28 degrees S. This paper presents CTD and ADCP data collected in November 2000 together with concurrent wind and satellite imagery, to provide a description of the summer surface circulation pattern along the Gascoyne continental shelf and slope. It is shown that the region comprises a complex system of currents that are influenced by offshore eddies, wind stress, varying shelf-widths, coastal topography and outflow from the hypersaline Shark Bay. Four different water types and current systems were identified from the field measurements. The Leeuwin Current (LC) is the major current flowing through the region. It transports lower salinity, warmer water along the 200 m isobath, poleward. The signature of the LC gradually transformed from a warm (24.7 degrees C), lower salinity (34.6) water in the north to a cooler (21.9 degrees C), more saline (35.2) water in the south resulting from geostrophic inflow of offshore waters. The width and depth of the current also changed continuously responding to the changing bottom topography and the orientation of the coastline: in the northern section under the influence of the narrow shelf and steep slope, the current was strong (similar to 0.75 ms(-1)) and extended deeper into the water column. In contrast, the current decelerated (to similar to 0.2-0.4 ms(-1)) when flowing past the wider continental shelf offshore of Shark Bay and then accelerated along the southern section along the steep continental slope. Downwelling events were persistently associated with the current. The Ningaloo Current (NC) was confined to the northern Gascoyne shelf within 35 km of the coast. Although upwelling was detected along the northern section of the study region, adjacent to the Ningaloo coral reef, water properties suggest a recirculation of LC water from the south. Changes in the shelf width at Point Cloates have a significant influence on the NC resulting in bifurcation of the northward current. The higher salinity outflow from Shark Bay influences the continental shelf region immediately offshore of the main entrances to the Bay through the mixing of the higher salinity outflow water with the shelf waters. The Capes Current, a wind-driven current originating to the south of the study region was identified as a cooler, more saline water mass flowing northward. (c) 2005 Elsevier Ltd. All rights reserved.