Summer circulation dynamics within the Perth coastal waters of southwestern Australia

The dynamics of the summer circulation in the coastal waters off Perth in Western Australia were investigated during a two-month field experiment. The study included the deployment of an array of moorings spanning the outer shelf, the inner shelf, within the inshore Perth coastal lagoon, and in the large coastal embayment of Cockburn Sound. The results revealed highly transient coastal circulation patterns that responded to variability in both the locally- and remotely-generated forcing. Local wind forcing played a primary role in driving much of the alongshore current variability at the shallower (<20 m depth) inshore sites, with a well-defined peak wind forcing time scale of ~1 week that fell within the synoptic weather band in the region. Due to the mean northward wind stress that persisted during this summer period, a mean northward current of 0.05–0.1 m s−1 was observed at these inshore sites. Large-scale variations in alongshore water level (pressure) gradients also episodically generated strong along- and cross-shore current oscillations throughout the region. Major events were associated with the propagation of coastally-trapped waves generated by a tropical low pressure system far (~1000 km) to the north of Perth, which propagated down the Western Australia coast. On the outer shelf, local wind forcing played a minor (but still not a negligible) role in driving alongshore current variability, with this momentum balance instead dominated by the alongshore pressure gradient variability. Due to the unusually large alongshore pressure gradient that persists year round along the Western Australia coast, currents on the shelf were on average southward. However, large-scale northward reversals of the shelf flow were also observed when northward wind stresses were sufficiently large and/or the local alongshore pressure gradient became episodically weak.