Ocean turbulence in Energetic Eddies using Autonomous instruments

Mesoscale eddies – swirling vortices of water 100-200 km across—are ubiquitous in the world’s oceans. They are highly energetic, contributing substantially to the ocean’s kinetic energy budget, and acting to redistribute energy and properties. While eddies are present in all ocean basins, they disappear from satellite data preferentially at western boundaries (Zhai et al., 2010). Depending on the regions and processes involved in dissipation, the large-scale ocean circulation responds differently. The motivation for this project is to better improve the representation of the mesoscale eddy energy budget in ocean models, including climate models.

Measuring turbulence directly, however, is time consuming and expensive, requiring ship support for traditional methods. As a consequence, turbulence datasets rarely capture the sporadic nature of spatially- and temporally-varying ocean turbulence. More recently, turbulence sensors are being deployed on autonomous underwater vehicles (Creed et al. 2016). These platforms offer a transformative approach to making measurements of spatially- and temporally-varying ocean turbulence, enabling measurements at scales not accessible by traditional methods (longer timescales, full time-depth resolution, etc). However, in order to enable more widespread adoption of these new platforms, further development is needed.

This project will be affiliated with the NERC project MerMEED (Mechanisms responsible for mesoscale eddy energy dissipation) using field observations and numerical simulations from near the Bahamas in the Atlantic to (1) use new instrumentation and refine novel methodology for estimating turbulent dissipation in the ocean, and (2) to use these data to investigate the processes responsible for eddy dissipation at western boundaries. Preliminary work suggests a curious asymmetry between dissipation in clockwise and counterclockwise circulating eddies (Clement et al., 2016).

The project will be focused on data from the autonomous underwater vehicles Seagliders, including both standard Seagliders and a Seaglider instrumented with a MicroPOD - a small turbulence probe measuring 512 times per second. The glider data will be used to refine a new method for estimating turbulent dissipation – the large-eddy method adapted for stratified waters. This technique would then enable future scientific investigations of the space-time variability of dissipation in the ocean. Data from the Seagliders will be validated against turbulence measurements made during short, 2-week field expeditions off the Bahamas (2016, 2017 and 2018) using freefall microstructure profilers (the “gold standard” for ocean turbulence measurements). Additional time varying data are available from the RAPID mooring array in the same region, spanning full-depth profiles within 75 km of the Bahamas and covering the time period from 2016-2018.

Development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners. The student will be registered at the University of Southampton and hosted at University of Southampton.

Specific training for this project will include

· Training in analyzing observations from autonomous underwater vehicles (Seagliders) and from microstructure profilers (free fall and mounted on the Seaglider)

· Scientific programming using Matlab, python or similar

· Sea-going experience will be available either through this project or similar

The student will become familiar with key physical processes involved in generating and dissipating mesoscale eddies, and eddy-wave-topography interactions.

In addition, a wide range of career development opportunities is available through the graduate school at the National Oceanography Centre and through the Natural Environment Research Council. The student will be encouraged to present their work at national and international conferences, as well as through local symposia for PhD students.