Continental Shelf and Coastal Processes

Temperature depth profiles from shelf waters off SA (for February 2010) indicating cold (blue ) upwelled water that is largely sub-surface underlying warmer (red) surface water. Data from CTD tags deployed on Australian sea lions.
Temperature depth profiles from shelf waters off SA (for February 2010)

The shelf waters of southern Australia host a large summertime upwelling system spanning a distance of ~800 km with upwelling hotspots centred in the Bonney Coast and eastern Great Australian Bight (GAB) regions. Both historical and SA-IMOS data indicate that upwelling in the eastern GAB originates to the south and south-east of Kangaroo Island, is largely sub-surface, and directed along the 100 m isobath to the north and north-west.  In addition, the results of the numerical simulations indicate that the low-frequency weather-band flow (associated with upwelling) is essentially directed along isobaths.

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

a) There are several mechanisms for upwelling that include: 
(i) Ekman wind forcing and modification due to alongshore divergence and CTWs,
(ii) upwelling by the bottom boundary layer of the FC,
(iii) canyons, and
(iv) ENSO variability.

The latter (iv) will be discussed in the next section.  The mechanisms (ii) and (iii) will need data and studies that better define the magnitude and spatial extents of the FC. Studies of mechanisms (i) will need the current data streams (CTD, ADCP) to be continued and replicated for the Bonney Coast and complemented by hydrodynamic modelling that allows the data streams to be extrapolated in space and time.

Long (>10 year) time series are needed so that regression analyses can be made. We have in mind here that proxies for variables such as alongshore velocity and bottom temperatures may exist (e.g. coastal sea level and Sea Surface Temperature (SST)). In this case, expensive moorings might be sensibly replaced using sea level and SST data to provide useful estimates of upwelling.

b) What is the spatial and temporal extent of cross-shelf exchange associated with winter downwelling? Such downwelling may represent a major component of cross-shelf exchange. In addition, where does the water come from to replace the downwelled water and does the downwelling enhance the associated eastward shelf currents?

c) Are internal waves important? Internal waves may also be important in vertical mixing as found on the northwest shelf slope. Preliminary SAIMOS temperature logger data from the shelf slope also indicates such waves to exist and the observational data streams need to be re-continued.

d) What is the role of the intense surface waves in (i) modifying the ocean circulation and mixing, and (ii) determining the distribution of the benthos?  A waverider buoy is needed in the mid-GAB as well as Gulf St Vincent to monitor changes in wave climatologies. These wave data could be used to validate coupled wave-hydrodynamic models. What is the wave climate in the Gulf St Vincent and how does it affect patterns of sediment transport, coastal erosion and pollutant dispersion along the Adelaide Metropolitan Coast?

e) What is the role of turbulent mixing processes on vertical diffusion and rates of diapycnal mixing? Ocean microstructure measurements are needed in the estuarine and shelf waters to quantify vertical budgets of heat, salt, nutrients and momentum. Observations of vertical diffusivities of heat, salt and nutrients can be used to validate wave-hydrodynamic models and biogeochemical fluxes which underpin new production in the four SA-IMOS oceanographic sub-regions.