A paper led by Julian Mak has just appeared in Ocean Modelling demonstrating that eddy saturation – insensitivity of the volume transport of a circumpolar current to surface wind stress – is an emergent property of our new geometric eddy parameterisation. In effect, we modify the widely-used Gent-McWilliams eddy parameterisation by solving a parameterised eddy energy budget and rescaling the eddy transfer coefficient to match the formderived in Marshall et al. (2012). This is for the same physical reasons as described in our recent paper in Geophysical Research Letters. We are currently working towards implementing this geometric eddy parameterisation in an Ocean General Circulation Model.
Our paper presenting a simple model of eddy saturation – the surprising insensitivity of the strength of the Antarctic Circumpolar Current to surface wind forcing – has been published in Geophysical Research Letters and highlighted as a Research Spotlight in EOS.
The model invokes three ingredients: a momentum budget, an eddy energy budget, and a relation between the “eddy form stress” and eddy energy that we have identified in previous work. The model also predicts that circumpolar transport increases with increased bottom friction, a counterintuitive result that is confirmed in eddy-permitting calculations. These results suggest an unexpected and important impact of eddy energy dissipation, through bottom drag or lee wave generation, on ocean stratification, ocean heat content, and potentially atmospheric CO2.
Much of our recent work has been building on a geometric framework for parameterising eddy fluxes that we first published in 2012 and 2013. A consequence of this framework is that if the eddy energy and stratification are known, then the eddy diffusivity is determined uniquely aside from a non-dimensional scaling factor that is no greater than 1.
A paper led by Scott Bachman has just appeared in Ocean Modelling in which we use a high-resolution numerical model to diagnose the eddy diffusivity in an unstable front and compare it to several theoretical predictions. Not only does the geometric eddy diffusivity fare better than the competition, but it out-performs our wildest expectations. This is a particularly exciting result as it may allow us to improve the parameterisation of unresolved eddies in the ocean models used for climate prediction.
Subtropical gyres are the ocean’s deserts. Classically this is believed to result from surface wind forcing driving convergent surface currents and downwelling, preventing the supply of essential nutrients from the cold, nutrient-rich beneath. Nevertheless modest biological activity is found, at odds with this simple paradigm.
We have just published a paper in the Journal of Physical Oceanography which shows that turbulent eddies oppose the wind-driven downwelling. The following is a movie produced by Ed Doddridge, my former graduate student and lead author on the paper.
Particles tracked for ten model years. The small lines represent individual particles and the large circles show the centroid of the particles. The green particles move with the Eulerian-mean velocities, while the purple ones move with the full, eddying velocities. The background colours show the potential temperature on a meridional slice through the model domain. These tracked particles provide a method for estimating the “Generalised Lagrangian-mean velocity” of the subtropical mode water. The results demonstrate that the eddy transport opposes the Eulerian-mean downwelling.
We are currently writing a follow-up paper to explore the implications for biological activity in the subtropical oceans.
(thanks to Ed Doddridge for his post on which this is based)