Is ocean mixing upside down? How mixing processes drive upwelling in a deep-ocean basin

Prof Alberto Naveira-Garabato, Mr Carl Spingys, Dr Eleanor Frajka-Williams, Prof Matthew Alford, Dr Kurt Polzin (Woods Hole Oceanographic Institution)
Rationale: 

Since the seminal work of Walter Munk in the 1960s, oceanographers have believed that the upwelling of cold, abyssal waters that regulates the deep ocean's ability to sequester heat and carbon for decades to millennia is driven by centimetre-scale turbulent mixing associated with breaking waves in the ocean interior. Measurements of deep-ocean turbulence over the last two decades, however, starkly contest this scenario, and instead suggest that mixing by breaking waves drives *downwelling* of abyssal waters. Inspired by this conundrum, recent theoretical investigations have developed a tantalising alternative view of the role of mixing in sustaining deep-ocean upwelling. In this new view (known as the ‘upside-down ocean mixing hypothesis’), upwelling is driven by highly localised turbulence within thin (typically tens of metres thick) layers near the seafloor, known collectively as the bottom boundary layer.

This project will assess the validity of this new paradigm, and figure out how it works, by analysing the first set of concurrent, systematic measurements of (1) large-scale mixing and upwelling, (2) their interior and bottom boundary layer contributions, and (3) the processes underpinning these contributions, in a representative deep-ocean basin: the Rockall Trough, in the Northeast Atlantic.

 

Methodology: 

The candidate will investigate the range of mixing processes occurring in the Rockall Trough through the analysis of turbulence observations acquired with ship-based fine- and microstructure profilers, autonomous gliders and specialised moored instrumentation. The investigation will entail: (a) characterising the processes at play; (b) quantifying the rates at which they induce mixing; (c) assessing their role in driving up- or down-welling across the basin; and (d) determining the relative contributions to such up- and down-welling motions of bottom boundary layer vs. off-boundary processes. The measurements to be analysed are in the course of being obtained via the three-cruise, year-long BLT-Recipes experiment (https://www.southampton.ac.uk/oes/research/projects/bottom-boundary-layer-turbulence-and-abyssal-recipes-blt-recipes.page) over 2021/2022.

Further, the candidate may perform and analyse simulations with an ultra-high-resolution numerical model of the Rockall Trough region in which measurements were gathered, in order to enable a thorough dynamical interpretation of the observations.

 

Location: 
University of Southampton
Training: 

The INSPIRE DTP programme provides comprehensive personal and professional 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 based at the National Oceanography Centre.

Specific training will include:

  • Opportunities to take part in fieldwork, possibly including the last BLT-Recipes cruise in 2022.
  • Processing and analysis of diverse observational datasets.
  • Theoretical understanding of a range of ocean mixing processes.
  • Possible opportunities to take part in glider training, e.g., at a summer school to learn about the deployment and processing of gliders and other autonomous robots.
  • Opportunities to attend national/international conferences to disseminate the candidate’s results and to broaden their scientific network.
  • Opportunities to visit co-supervisors at the Scripps Institution of Oceanography and Woods Hole Oceanographic Institution.

 

Eligibility & Funding Details: 

Please see https://inspire-dtp.ac.uk/how-apply for details.

Background Reading: 
  • de Lavergne, C., G. Madec, J. Le Sommer, G. Nurser & A. Naveira Garabato, 2016: On the consumption of Antarctic Bottom Water in the abyssal ocean. J. Phys. Oceanogr. 46, 635-661.
  • Ferrari, R., A. Mashayek, T. McDougall, M. Nikurashin & J.-M. Campin, 2016: Turning ocean mixing upside down. J. Phys. Oceanogr. 46, 2239-2261.
  • Naveira Garabato, A., E. Frajka-Williams, C. Spingys & ten others, 2019: Rapid mixing and exchange of deep-ocean waters in an abyssal boundary current. PNAS 116, 13233-13238.