Continental Shelf and Coastal Processes

Monthly anomaly of SST from a composite SST product (1993-2003) (Ridgway, 2007b)
Monthly anomaly of SST from a composite SST product (1993-2003) (Ridgway, 2007b)

The coastal shelf in the South East Australia region is relatively narrow (10-25km wide) except for Bass Strait, which is has an average depth of 50-70 metres and connects Tasmania with mainland Australia and the Great Australian Bight to the Tasman Sea. This region supports a diversity of marine ecosystems that includes submerged temperate rocky reefs and canyons on each side of the strait that supports high biological diversity and endemism.

In addition, a diversity of marine industry activities such as fishing, shipping and oil and gas extraction take place in this particular region. Tides and wind action mix the waters in Bass Strait, however, the central region becomes stratified during the summer months.

Elsewhere in the South East Australia region there are no well-defined shelf currents due to the narrow shelves. However, the interaction between the ZC, EAC, and Subtropical Front makes for a dynamic shelf environment at the confluence of 3 different systems. Along the southern shelf the circulation is predominantly the result of wind forcing, with an eastward current over the continental shelf flowing from Cape Leeuwin to the southern tip of Tasmania.

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

Boundary Current/Shelf Interactions: 

  • What are the influences of the boundary currents and the associated meso-scale and sub-meso-scale eddies off south east Australian waters over the continental shelf and how do they impact cross shelf exchange and water properties?
  • How does the EAC and ZC interact with the continental shelf/slope break and topographic features (such as canyons) generating upwelling, internal waves, and mixing?
  • What effect does the dissipation of internal tides from the Tasman Sea, which break on the eastern shelf, have on the productivity in this region- and is this likely to change with a changing climate?

Upwelling and Downwelling: 

  • What are the frequency, magnitude, and drivers of upwelling/downwelling processes and slope water intrusions in the coastal waters off South East Australia and how do they influence cross shelf exchange of properties?
  • Does tide or wind driven upwelling increase the productivity of Bass Strait?
  • What are the mechanisms for upwelling and the magnitude and spatial extent in the Bonney Coast.
  • What role do canyons play in variations in upwelling/downwelling cycles in southern Australian waters?
  • What is the spatial and temporal extent of the downwelling in eastern Bass Strait?
  • Is the southern extension of the EAC providing any likelihood for summer upwelling fluxes in Bass Canyon?
  • Are there changes noted in the fisheries that can be mapped to changing dynamics in the Bass Strait upwelling/downwelling system? 

Shelf Currents:

  • What is the magnitude and variability (interannual, seasonal, and higher frequency) of currents off south east Australia?
  • What are they dynamical drivers of currents off south east Australia and how do the EAC, ZC and ACC interact? 

Wave Processes:

  • What are the roles of internal and coastally trapped waves, including tides in continental shelf processes?
  • What is the tidal regime and how does it influence shelf processes?
  • What is the influence of coastally trapped waves on shelf circulations?
  • What is the contribution of the various wave processes to mixing over the shelf and at the shelf edge?
  • What is the contribution of internal tides and waves to on-shelf transport of nutrients and off-shelf transport of pollutants?
  • Are internal waves important? Internal waves may also be important in vertical mixing as found on the northwest shelf slope. 
  • Is there likely to be a change in the CTW propagation through Bass Strait given the likely shifts in geostrophic currents and seasonal wave patterns?

Offshore pipeline stability design is of vital importance in the offshore industry. Stability of an offshore or subsea pipeline is greatly influenced by the external environmental conditions. This is especially true around near shore crossings where current and wave interactions can induce sediment transfer and scour around pipelines. As the need for natural resources, such as oil and gas grow within Australian waters, so too does the infrastructure needed to facilitate this industry.