MerMEED – Eddy hunting: Dafydd Gwyn Evans

Dr Dafydd Gwyn Evans is a Post-Doc at the University of Southampton and National Oceanography Centre.

At the western boundary of all ocean basins, the swirling vortices that oceanographers call mesoscale eddies vanish. Some call these regions eddy graveyards, as some idealised model studies suggest that very little of the energy linked to these eddies escapes. We can track eddies with satellites, and watch them slowly march westward to their demise along the coasts of the United States, Argentina, Australia and Japan.

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Yet, it’s not clear what happens to the energy that sustains a mesoscale eddy when it finally reaches the eddy graveyard. Theoretical studies using ocean models suggest that the energy dissipates through processes that generate turbulence and mixing. If this is the case, these processes may provide a sorely missed piece to the complex jigsaw puzzle that is the energy budget of the global ocean.

Using a specially designed field campaign the MerMEED project (which in case you’re interested stands for Mechanisms Responsible for Mesoscale Eddy Energy Dissipation) seeks to understand why eddies dissipate in these regions. The goal is to use satellites to spot eddies approaching our study region off the Bahamas, and when the time is right, to embark on short expeditions to directly measure the levels of turbulence and dissipation that occur when the enhanced currents of an eddy hit the steeply sloping sea-bed near the Bahamas. Autonomous underwater gliders and moored current meters (on the existing RAPID-WATCH moorings) will also be deployed to map and monitor the multi-month changes in these eddies. My role as the MerMEED post-doc is to bring all this data together to unravel the importance of local dissipation in the energy balance of eddies.

gwyn-2In September 2016, we spotted an anticyclonic eddy destined for our study region. We predicted it would reach the Bahamas by October and slowly dissipate over the course of 2-3 months. So, we set about preparing for our first expedition. Hoping to charter a vessel in the Bahamas we packed our equipment into a container and shipped it to the Caribbean. It was around this time that Hurricane Matthew tore a path through the Bahamas and that our intended vessel reported damage to the shipboard crane. Realising that the Bahamians would likely have more important problems to deal with than those of eddy energy dissipation, we went in search of a replacement vessel. It was the R/V Walton Smith of the University of Miami that came to the rescue.

With an eddy and a boat, by the 2nd of December we found ourselves floating a few miles east of Great Abaco. Using the acoustic Doppler current profiler mounted to the hull of the Walton Smith, we set about mapping the strength and direction of the current. As expected, the western half of the clockwise rotating eddy was driving a strong northward flow that ran along the sloping sea-bed. North of Great Abaco, this flow separated from the sloping sea-bed, forming an intense northward jet.

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To measure turbulence and dissipation we used very sensitive shear and temperature profilers. This showed two regions of particular interest. We found the first at a location where the northward flow, still tied to the sea-bed at this point, is forced over a particularly rough part of the sea-floor. Here the flow is abruptly slowed near to the sea-bed, generating enhanced turbulence and dissipation. We also saw evidence internal wave generation using the Doppler current profiler. We found the second region of interest to the northeast of Great Abaco, where the flow formed an intense northward jet. Here we saw exciting evidence of enhanced turbulence and dissipation where this jet meets the surrounding ambient fluid. With this new data and the promise of more from future expeditions, we begin the process of understanding exactly what the data shows, and whether the processes we uncover contribute to the notoriety of the eddy graveyard. Stay tuned to @mermeeduk for updates!

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