Remarkable near-surface eastward flows in the South Indian Ocean: Understanding the dynamical links between the Indian Ocean subtropical gyre, Indonesian Throughflow, Leeuwin Current and subduction south of Australia

Supervision Team

Background

A remarkable aspect of the upper ocean circulation in the south Indian Ocean (SIO) is the presence of near-surface eastward flowsacross this basin. Parts of the eastward flow are concentrated into discrete bands and have been identified as the south Indian Countercurrent (SICC) and the Eastern Gyral Current (EGC).  These features have been detected in observations, although details about their precise location, intensity and variability are still to be described.  These currents have been simulated successfully by some numerical models, although their basic dynamics remain unclear.  The most remarkable aspect of the eastward flows is that they are in the opposite direction to that predicted by both Ekman and Sverdrup theory. Recent advances have been made linking Australian and regional climate variability to variability in the Indian Ocean. Before we can further our understanding of the Indian Ocean’s influence on Australian climate, we need to understand the first-order dynamics controlling its circulation and watermass variability.

Modelling studies have proposed three mechanisms for the cause of the eastward flow, which is compensated by subsurface westward flows.  The first is the damping of Rossby waves, forced by the Indonesian Throughflow (ITF), away from the eastern boundary.  The second is localised regions of downwelling, such as caused by convection south and southwest of Australia and at the West Australian coast in the Leeuwin Current. The third is the large decrease in steric height in the SIO due to the meridional density gradient from light ITF and tropical waters to denser southern waters, giving rise to eastward flow. In support of the latter idea, when the ITF is shut off in numerical ocean models there is a reduced meridional density gradient and weaker eastward flows. 

Ideas for why the eastward flow concentrates in bands (EGC and SICC) include 1) winds and/or buoyancy fluxes causing a convergence of the surface flow; and 2) Rossby waves emanating from regions of subduction south of Australia propagating west and being deflected north by the subtropical gyre.  In support of the latter process, revealed in model simulations, the southern edges of the observed eastward flow all connect to the SW tip of Australia.

Project outline and objectives

The student will be part of a team that includes the supervisors, a postdoctoral research fellow, Dr Stuart Godfrey, and Drs Jay McCreary and Ryo Furue of the University of Hawai’i International Pacific Research Center. 

The PhD project will focus on the role of subduction south of Australia in driving the near-surface eastward flows, and will link into the work of the team in answering the broader questions.  The specific objectives for the project are

1. analyse observations from the Indian Ocean Thermal Archive and the CSIRO Atlas of Regional Seas to build a detailed picture of the location, intensity and variability of circulation pathways in the southeastern Indian Ocean, and potential Rossby wave paths emanating from south of Australia, in particular.
2. Conduct a similar analysis using the high-resolution output from the BLUElink Ocean Forecast Australia Model (OFAM).  The model’s high spatial resolution and multi-year time series will allow a quantification of the year-round variability in SIO currents, and detailed mapping of the subduction regions in the SIO.  It will also allow an exploration of the cross-correlations between the eastward flows, ITF, Leeuwin Current, subducted water flow, Rossby waves and atmospheric forcing.
3. Investigate ocean processes that drive the near-surface eastward flows using a hierarchy of models less complex than OFAM including analytical, linear-continuously-stratified box models and general circulation models. This part of the work relies on close interaction with Drs McCreary and Furue and the Postdoctoral Research Fellow.