A modeling study of the Climatological current field and the trajectories of upwelled particles in the East Australian current

Abstract

The climatological current field off the coast of New South Wales, Australia, is investigated using results from a field experiment and a diagnostic, numerical modeling study. In particular, the flow dynamics are examined near Smoky Cape (30°55′S) in the vicinity of the East Australian Current separation point. Modeled velocity fields compare favorably with observed velocity measurements that were obtained in this region during November 1998. An investigation of the role of advection in the alongshore momentum equation reveals that UVx and VVy change sign about the separation point, representing a shift in the flow regime from onshore advection and southward acceleration (north of the separation) to seaward advection and weaker southward currents (south of the separation). Bottom stress increases on the continental shelf downstream of the separation point, coincident with high horizontal divergence throughout the region. South of Smoky Cape, the near-bottom flow is characterized by a low Burger number, coincident with a subcritical Richardson number. These characteristics imply that the timescale in which bottom boundary layer shutdown occurs will be lengthened, enabling a persistent flow of potentially nutrient rich water into the coastal region. A series of Lagrangian particle tracking experiments designed to investigate the fate of the upwelled water are presented. Motivated by an observed chlorophyll-a maximum along the 25.25σθ isopycnal, water of this density is used as a proxy for nutrient-enriched water. It is found that upwelled water originates at depth to the north of Smoky Cape and at times well offshore. South of Smoky Cape, coastal recirculations are evident, suggesting that upwelled water could be trapped in eddy fields adjacent to the coast and thus explaining why phytoplankton blooms are prevalent in the region south of Smoky Cape.