East Australian Current and New Zealand Regional Ocean Circulation Model

The East Australian Current (EAC) sheds some of the largest and most energetic eddies in the world's oceans. The EAC is a southern hemisphere equivalent of the Gulf Stream, but has a much weaker analog of the Labrador Current in spite of the large Antarctic Circumpolar Current (ACC) which lies to its south. This may be related to blocking by the Campbell Plateau south of New Zealand and the associated more southward course of the ACC.

The DieCAST Ocean Model has been applied to the South West Pacific Ocean spanning 146 E (Tasmania), 18 S (Fiji Islands), 160 W and 65 S with resolution. This area has New Zealand located roughly near its center.

Model Results for sea surface temperature, sea surface height and surface velocity from a DieCAST ocean model-simulated anticyclonic eddy off Sydney, Australia, in the East Australian Current.
DieCAST is a z-level model using a semi-allocated horizontal grid and fourth-order-accurate horizontal advection and baroclinic pressure gradient formulation. It uses unfiltered real bathymetry and is robust with very low dissipation. It has been applied successfully to the Gulf of Mexico, South China Sea, the Labrador Current, the Strait of Sicily, the Great Lakes, the North Atlantic Ocean, the Arctic Ocean, the California Current, the Adriatic and the Mediterranean Seas.

We have investigated the effects of parasitic, cool-core (cyclonic) eddies on the dynamics of the EAC itself and its shed anticyclonic eddies. We have focused on the retroflection region east of the city of Sydney, where the EAC flows eastward across the Tasman Sea towards the northern tip of New Zealand. Simulations using the DieCAST Ocean Model show that cool-core parasitic eddies often originate along the shelfbreak and may interact strongly with the much larger, offshore warm core eddies after being swept offshore by strong warm core frontal currents.

The results are consistent with many detailed observations, including the dynamics of the East Australian Current (EAC) (Bennet(1983), Gresswell(1981)). Although EAC mesoscale cyclonic eddies are found only in deep water, intense, small scale cyclonic parasitic eddies often originate along the shelfbreak and near the headlands, and get advected offshore in spurts, around the south side of major warm core anticyclonic EAC eddy features. These parasitic eddies interact strongly with the EAC large scale warm core eddies and affect separation and reconnection with the main coastal current as well as participating in collisions, mergers and subsequent fission of EAC eddies.

The model is in apparently good agreement with significant observed flow features, including the major eddy field of the ACC south of New Zealand, the Tasman front, and detailed features near New Zealand, including the East Cape Current, the Hikurangi Eddy, the Southland Current and the Mernoo Gap jet.

These model results show that once offshore, the cyclonic parasitic eddies may grow in amplitude and scale. The scale increase reflects geostrophic adjustments as the eddies grow to a scale corresponding to the deepwater Rossby radius-of-deformation.

Related Papers
Bowman M.J., D.E. Dietrich and B.G. Sanderson. Non linear interactions of East Australian Current eddies. In: Oceanic Fronts and Related Phenomena: Konstantin Fedorov International Memorial Symposium. UNESCO Intergovernmental Oceanographic Commission Workshop Report No. 159, pp. 48-53 (2000).

The work of Bennett, A.F. and Greswell, G.

For More Information on DieCAST
The DieCAST Model Website