Continental Shelf and Coastal Processes

Schematic of near surface circulation in January and July derived from a numerical model. Dotted arrows indicate either weak currents (Gulf of Carpentaria and Arafura Sea) or interactions between the ITF and coastal currents (Timor Sea), Schiller 2011
Schematic of near surface circulation in January and July derived from a numerical model

Partly because of the vast extent of Western Australia ranging from the tropics to the Southern Ocean as well as the varying orientation of the coastline, there are distinctly different oceanographic regimes around the continental shelf from north to south.

Northwest Shelf

The circulation on the broad continental shelf northeast of Exmouth is complex and poorly understood. The Holloway Current is a surface layer poleward flowing ocean current that brings water perhaps from as far north as the Banda and Arafura seas, southward over the continental shelf of northwest Australia at the end of the northwest monsoon. A simple view of the generating mechanism is the seasonal south-westerly wind piles up water in the Arafura Sea and Gulf of Carpentaria during the peak of the Australian monsoon, and the current flows southward as the wind relaxes during the monsoon transition.

There are also large-amplitude internal tides that propagate across the North West Shelf (NWS) particularly in summer, when the water column is strongly stratified due to intense solar insolation. The high evaporation along the Pilbara coast and the high freshwater input along the Kimberley coast result in cross-shelf density gradients which are also capable of influencing cross-shore exchange.  High evaporation together with winter cooling results in higher density water (cooler more saline water) along the coast which results in a high density gravity current along the sea bed termed ‘shelf dense water cascade’ and results in nearshore waters being advected offshore.

West coast (Exmouth to Cape Leeuwin)

Because the Leeuwin Current typically flows along the edge of the continental shelf (albeit with periodic mesoscale meanders and eddies looping the flow away from the shelf), the currents along the outer shelf have a generally southward tendency. By contrast, the shallow nearshore waters) are largely driven by the prevailing wind regime, resulting in almost exclusively northward flow between October and February in response to the dominantly southerly wind stress prevailing during the summer months, and dominantly southward currents in winter.

There are summer counter-currents that have been regionally named the Ningaloo Current (flows between Shark Bay and Northwest Cape, and probably further north) and the Capes Current (Shark Bay to Cape Leeuwin. In southwestern Australia, there are also strong cross-shelf gradients in salinity during the summer months when evaporation can raise the nearshore salinity to over 36.4 psu compared with ~35.8 psu along the outer shelf.

In winter, on the other hand, salinities are typically ~35.2 psu to 35.6 psu across the shelf. Because of the strong seasonality of the wind regime, the wave climate also has a pronounced seasonal cycle. Superimposed on the background swell from the Southern Ocean which is experienced throughout the year, moderate seas are generated by the sea breeze system in summer and heavy seas and swell accompany winter storms.

South coast

Off Cape Leeuwin and Esperance the currents are generally towards the east (with some topographic steering from nearby islands in the Recherche Archipelago), indicative of the Leeuwin Current running along the shelf and on occasion flooding across the shelf to the coast. The flow appears to be strongest in winter and weakest between November and February when some appreciable wind-related current reversals occurred.

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

  • What is the influence of ITF waters on continental shelf and coastal regions of the
  • Kimberley and Pilbara?
  • Is the Holloway Current a major feature of the regional circulation in north-west Australia and if present, is it driven by alongshore and/or cross-shore pressure gradients? What generates the pressure gradients? Does interannual variation modulate the seasonal cycle?
  • What is the northward extent of the Ningaloo Current and the response of the regional currents?
  • What are main the interactions between Capes and Leeuwin Current?
  • What  is  the hydraulic  connectivity  and  variability between the different regions in the northern, north-west and south-west of Australia?
  • What are the main driving forces (seasonal?) of shelf currents in the region?
  • What is the relative importance of Coastally Trapped Waves on driving continental shelf circulation?
  • What is the tidal regime in the region? In particular, the tides in deeper water in the northern region of Australia
  • What are the roles of alongshore density gradients generated by (1) freshwater inputs and (2) evaporation in cross shore exchange through baroclinic forcing? What is the role of wind on cross-shore exchange?
  • What are the roles of currents and upwelling in the supply of nutrients to shelf ecosystems?
  • What are the key processes in the region which needs to be included in numerical models?
  • What data streams are required for model forcing, validation, and assimilation?
  • How can we facilitate the use of Bluelink and in particular, improve accuracy and resolution?
  • How can we develop data assimilating hydrodynamic models (ultimately) to as to extrapolate observations and improve observing system design?