A Simple Question on the Sydney Shelf?

Ask a coastal engineer and an oceanographer the same question, and you’ll get two different answers. They're both right, and yet they're both wrong.

I was recently asked to look at near-bed currents on the Sydney Shelf. I'd done my reading (as a coastal scientist) and it was a clear slam-dunk that the shelf is wave-dominated with tiny tidal currents. Following a similar method I'd applied on the Gold Coast in ~12-m water depth, I started to evaluate the considerable record of instrumentation, thankfully supported by the enormous volumes of work previously done by WRL!

At a depth of ~30m, waves produce much slower currents than closer to shore. So, I decided that the assumption of wave-dominance might be on shaky ground and pivoted to undertake an evaluation of other mechanisms for current development.

Doing a new round of reading (as an oceanographer) it was a clear slam-dunk that shelf currents are eddy-dominated, developed through shedding from the East Australian Current, with tiny tidal currents. Sounds familiar, but different? This time there's a considerable body of work done by UNSW, BOM (see the amazing plot below of surface currents) and a bunch of others... However, just like nearshore waves, relative influence of eddies has a cross-shelf transition, as well as variation through the water column.

Once again, long-term instrumentation came to the rescue.

Analysis of ADCP records from 65-m and 100-m depths provided an indication of the cross-shelf transition, demonstrating some of the complexities of surface and benthic shear. Stronger currents were developed under a wide array of synoptic conditions, each with a different oceanographic signature. The highest surface currents were associated with TC Zelia (Jan-2011) and the strongest near-bed currents were developed by a rather curious underflow event. This included a 12-hour perturbation and was remarkably coincident with a tsunami on the other side of the world (a proper coincidence of timing!). So, while eddies are the prevailing mechanism for oceanographic currents on the outer shelf, they are not necessarily the only mechanism which needs to be considered, closer to shore.

In the end, at a depth of ~30m, the influence of wave-orbitals and shelf currents was almost equal, demonstrating the need to include both processes in the assessment. Also, despite their relative neglect in both coastal and oceanographic reporting, tidal currents remained a persistent secondary mechanism for currents within the inner shelf observations.

The interesting learning point for me here...? The perspectives of wave-dominated and eddy-dominated conditions are true for the locations where they have been identified. However, the inner shelf is the transition zone between these two perspectives... where coastal science and oceanography need to shake hands and get on with the business of working together!