Slicks (Part 2)
In my blog on moods of Sydney Harbour, I touched on the topic of slicks. I frequently see them on relatively calm mornings from my verandah and they never cease to amaze me with their clarity and spatial variability. Peter Cowell has been a fan of slicks since the early 70s and offered the following observations and comments. It is a privilege to communicate his thoughts in this ACS blog and look forward to any further discussion. It should be noted that Peter’s comments do not specifically relate to Sydney Harbour.
Slicks may form because oils accumulate where water is descending. The general proposition is that the oils are natural biological products (although hydrocarbon contaminants can also make a contribution). Oil accumulation occurs along the crest lines of internal waves, and along velocity-shear fronts due to the tendency to form helical flows, or simply satisfy conservation of mass at the front due to the tendency for water within adjacent water masses to mix. Being buoyant, the oils can’t descend with the sea water. The viscosity of the oil suppresses capillary wind waves, hence the slicks. When stronger winds generate ultragravity waves (wind chop), the quantity of oil is generally insufficient to suppress these waves. The slicks are no longer evident, although scum lines can take their place if white capping produces enough froth. All of this is best known in relation to Langmuir circulation, but the principles should apply to all lines of descending flow, whatever their origins.
When swarms of slicks are evident offshore, they are more likely associated with internal waves related to thermal stratification, although slicks on velocity fronts are probably present as well, because fronts are fairly ubiquitous. The fronts are mostly associated with a coastal irregularity (headlands, including those between bays/tributaries). Exceptions occur where sharp changes in water depth occur. This has been observed above and along the line of the flood tide delta at Lilly Pilly in Port Hacking and at the crest of the flood tide delta in Port Stephens. Years ago Mike Kingsford noted the presence of velocity fronts in Botany Bay and documented entirely different fish communities on either side of the front… the implication being the fronts form a curtain that the fish do not cross.
Peter Cowell observed velocity fronts offshore when running shore-normal echo sounder transects off Diamond Bay (during work on the inner shelf sand bodies). These surveys were run under quiescent conditions, times that were conducive to the presence of slicks. Running these transects along a compass bearing would require pointing the bow 5 or 10 degrees (or may be more) into the alongshelf current, to maintain the shore-normal line. Then when a slick was crossed, there was a need to point the bow in the opposite direction off line against the alongshelf counter current: a compelling experience.
We know that others who have worked offshore from Sydney and within the Harbour have observed fronts and eddies that may be linked to slick formation. We welcome further comments.
Peter Cowell and Bruce Thom
Words by Prof Bruce Thom and Dr Peter Cowell. Please respect the authors’ thoughts and reference appropriately: (c) ACS, 2018, posted 3 November 2018, for correspondence about this blog post please email firstname.lastname@example.org