Seasonal circulation around Tasmania: An interface between eastern and western boundary dynamics

Abstract

The seasonal circulation connecting the Pacific and Indian oceans around Tasmania is documented using seasonal maps from satellite altimetry and sea surface temperature (SST) and in situ mean fields from a high‐resolution climatology of the region (Commonwealth Scientific and Industrial Research Organisation (CSIRO) Atlas of Regional Seas (CARS)). In general, the surface variability displays a very different character off the east and west coasts. Off the east coast the variability is generated externally, being dominated by the western boundary dynamics of the East Australian Current (EAC). In the west it is weaker and arises from the seasonal rise and fall of the coastal sea level, which is due to the seasonal reversing wind patterns. A high‐variability tongue associated with the EAC stretches poleward off the east coast, tracing out a waveguide between the major topographic structures. It has a broad peak in power between 180 and 365 days (>0.08 m), which is a surface height expression of the energetic mesoscale eddy activity associated with the EAC. Distinctive summer and winter patterns of the surface circulation are described which are influenced by the EAC and the Zeehan Current (ZC), respectively. On the east coast in summer (January–March), there is a poleward advection 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 Zeehan Current operates 180° out of phase with the EAC. It is strongest 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.