Numerical Modelling of Mesoscale Eddies in the Tasman Sea

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Copyright: Macdonald, Helen
Western boundary currents (WBC), such as the East Australian Current (EAC), are some of the world's strongest currents. Mesoscale eddies are a common feature in WBCs and can be important for the transport of heat, vertical mixing and the retention of larval species. To better understand the physical processes that drive eddy formation in the Tasman Sea, The Regional Ocean Modelling System is used to investigate the ocean state during the formation of a warm-core eddy (WCE) in the EAC during October of 2008 and a cold-core eddy (CCE) during October of 2009. The WCE formed from a retroflexion of the EAC. Lagrangian particle tracks are used and some passive tracers are introduced into the model to investigate this eddy. There are two distinct stages in the eddy's development. The first where the EAC encircles the eddy; the second where the EAC overwashes the eddy. The water entrained into the WCE comes from the EAC during overwashing. Overwashing submerges the original eddy, creating a two-layered system with interesting dynamical consequences. Potential drivers of the formation of the CCE, such as gradients in temperature and velocity, along with the impact of 3 wind forcing scenarios (upwelling, downwelling and realistic winds) are investigated. In all three cases a CCE formed, although the location, size and isotherm uplift varies. Counter-intuitively, the strongest upwelling eddy forms with downwelling winds. Analysis of energy transformation shows the prevailing source of eddy kinetic energy to the CCE was the EAC. Particle release experiments are used to investigate the cold-core eddy's source waters. Nearly all of the surface waters within the eddy originate from the continental shelf. Water entrained into the eddy came from both north, due to the southward flowing EAC, and, unexpectedly, from south of the eddy. Particles that come from south of the eddy do so due to a northward flow on the continental shelf just prior to the eddy's formation. The results show that mesoscale eddies within the western Tasman Sea, while generally following simple models for eddies, also display more complex physical interactions. This has implications in understanding how eddies can influence distribution of heat, energy and biological production in WBC regions.
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Macdonald, Helen
Roughan, Moninya
Baird, Mark
Wilkin, John
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PhD Doctorate
UNSW Faculty
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