Katherine Hill

Dynamics and impacts of East Australia Current variability off the southeast coast of Australia

Supervisors: Dr Steve Rintoul (CMAR), Dr Ken Ridgway (CMAR), and Prof. Richard Coleman (UTAS)

Scientific Background

A long-term record from Maria Island shows that the waters off the east coast of Tasmania have warmed significantly over the last 50 years. The Maria Island record also shows significant decadal variability. Salinity has also increased during summer months over this time period. The temperature and salinity changes are consistent with a strengthening of the poleward extension of the East Australia Current (EAC), but the dynamics driving such a change have not been explained.

Changes have also been observed in species distributions over this time period. Sea urchins previously restricted to the Australian mainland have expanded southward off the east coast of Tasmania, which may in turn have contributed to an observed decline in giant kelp on the east coast. Introduced pests have also spread south as the waters have warmed. The salmon aquaculture industry is sensitive to changes in water temperature, with recent warm summers leading to a significant decline in production and profit. The relationship between recent ecosystem changes and variability in the EAC extension has not yet been explored in any detail.

Harris et al. (1988) documented correlations between large-scale climate variability and a number of fisheries and ecosystem variables in Tasmanian waters. They identified the number of days with strong westerly winds as the primary forcing factor, which in turn was linked to large-scale climate factors such as El Nino. Thresher et al (2004) suggested that cooling at 1000 m depth south of Tasmania over the last three centuries reflected an increase in the poleward flow of the EAC along the Tasmanian coast, which was in turn driven by a poleward shift of the westerly winds. While the correlations in these studies are suggestive, the dynamical explanation for the observed changes is incomplete.

Project outline and objectives

The project will use a variety of data sets and circulation model output to characterize and understand the variability of the waters off the southeast coast of Australia and relate variability in the physical environment to observed ecosystem changes. By identifying the physical mechanisms driving variability in the region, the study will also provide insights into the likely impacts of climate change and assist in the development of management strategies.

The specific objectives of the project are:

1. To quantify the variability of the EAC extension off southeast Australia.
2. To identify the dynamical mechanisms driving variability in the EAC extension on time-scales from seasons to centuries.
3. To relate variability in the EAC extension to changes in ecosystems and fisheries in Tasmania and southeast Australia waters.

The project will use a variety of data sets to document variability of the physical environment including the unique long-term Maria Island coastal station record; the CSIRO Climatology of Australia's Regional Seas (CARS) atlas; and satellite measurements of sea surface height, sea surface temperature and ocean colour. A range of models will be used to diagnose the mechanisms driving the observed variability, from simple wind-driven circulation models (eg to test hypotheses regarding sensitivity of the EAC extension to changes in winds), to ocean general circulation models driven with observed forcing over the last 50 years. Changes in the physical environment will be related to changes in ecosystems and marine resources using fisheries statistics and observations of ecosystem and species distribution changes.