Major Boundary Currents and Inter-basin Flows

Boundary Currents in the Australian region (Image: CSIRO)
Boundary Currents in the Australian region (image provided by CSIRO)

The waters around Australia form a complex intersection of the Pacific and Indian Oceans. The three major boundary currents are the East Australia Current (EAC), which forms the western boundary current of the South Pacific gyre, and the Leeuwin Current (LC), a unique eastern boundary current of the Indian Ocean, and the third ‘boundary current’ flowing along Australia’s longest coastline is the westward drift component of the Antarctic Circumpolar Current (ACC) found at the Subtropical Front (STF). The south east Australia region is influenced by all 3 major currents.

The influence of the different currents in the region varies seasonally, with summer and winter patterns of the surface circulation influenced by the EAC and the Zeehan Current (ZC) respectively. The Zeehan current is a current that runs southeastward along the continental shelf edge of western Bass Strait and western Tasmania. On the east coast in summer (Jan-Mar) there is a polewards flow of warm, saline water forced by an episodic coastal boundary flow. The cross-shelf pressure gradient driving this flow is formed from the difference between the seasonal changes in coastal sea level and the offshore eddies. There is a seasonal reversal of this flow in winter with cool, fresh, modified subantarctic surface waters drawn up from the south.

Off the west coast the ZC operates 180o out of phase with the EAC. The strongest pole-ward flow is in winter when it projects warm, relatively saline waters down the western Tasmania coast and around the southern tip of Tasmania. There is a sharp division between the EAC and ZC influence adjacent to the Tasman Peninsula off southeast Tasmania.

The following high-level science questions will guide the Southeast Australia IMOS observing strategy in this area:

Fluxes:

  • How do the mass, heat, and salt transports of the EAC, LC and ACC and the coastal currents in South Eastern Australia vary on seasonal, interannual, and multi-decadal timescale?
  • How do boundary current transports respond to and influence regional climate?
  • What are the relative contributions of air-sea fluxes, the mean flow, and eddies to the regional heat and freshwater budget?

Drivers:

  • What is the cause of variations in current strength?
  • How much of the change observed can be attributed to human drivers?
  • What is the relationship between boundary currents and modes of climate variability?

Dynamics:

  • What is the dynamical connection between boundary currents, interbasin flows and gyres in the South East Australian region?
  • What controls variations in the strength of the eddy field associated with the major Australian current systems that affect the South East Australian region?