Abstract:

The Leeuwin Current is an anomalous poleward-flowing eastern boundary current that transports warm tropical waters southward and imposes significant ecological influences on the coastal marine ecosystems off Western Australia. In this study, we attempt to map the spatial structure of the sea surface temperature (SST) signature of the Leeuwin Current using MODIS satellite data, which is used to investigate the spatial and temporal variability of the Leeuwin Current for the period July 2002 to December 2012. A topographic Position Index (TPI) method is used to map the Leeuwin Current's SST signature from the MODIS SST images. The semi-automatic classification process involves automatically identifying areas occupied by warm SST waters of the Leeuwin Current and manually discarding detached Leeuwin Current features. The TPI mapping results enabled us to quantitatively examine the spatial and temporal variability of the Leeuwin Current's SST signature, including its spatial structure, extent, and cross-shelf migration, as well as its associated surface chlorophyll a characteristics. This study showed that the TPI mapping is able to capture the complex spatial structures of the Leeuwin Current, including its meanders, offshoots and eddies along its path, as reflected in the SST data. Using an eddy-resolving, data-assimilating numerical ocean model, we were able to relate the Leeuwin Current's SST signature with its surface current speed signature, though generally the speed signature is on the offshore side of the SST signature by about 25 to 30 km. Temporal wavelet analyses indicated that the variability of the Leeuwin Current's SST structure is dominated by seasonal cycle. During austral winter, the warm SST signature of the current is close to the shelf break, having greater spatial extent and higher chlorophyll a concentrations. In contrast, during austral summer, the SST signature of the current moves offshore, with reduced spatial extent and chlorophyll a concentrations. The Leeuwin Current also has notable inter-annual variation, to a large extent in response to ENSO variability in the Pacific. In El Niño years the current's SST signature tends to have reduced spatial extent, move further inshore, and increase its chlorophyll a concentrations. The opposite occurs during the La Niña years, especially after the unprecedented 2010–11 La Niña/Ningaloo Niño event when there was a significant reduction of the surface chlorophyll a concentration within the Leeuwin Current's SST signature. The mapping method developed in this study will be also useful in detecting climate driven variability of other ocean boundary current systems.

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