Deep oceanic convection: the outsized role of small-scale processes

Dr Louis Clement, Dr Eleanor Frajka-Williams, Prof Alberto Naveira-Garabato, Dr Marilena Oltmanns
Rationale: 

Deep-water formation (convection) at high-latitudes, and more specifically in the Labrador Sea in the subpolar North Atlantic, forms a key part of the large-scale ocean circulation. This enables the exchange of gases between the atmosphere and deep ocean, and thus contributes to the global oceanic sequestration of carbon. In a warming climate, added volumes of freshwater into the Labrador Sea, associated with the increased melting of Greenland ice sheet and of Arctic sea-ice, are expected to reduce or shutdown convection, which would reduce the ocean’s potential to mitigate climate change. In the western Labrador Sea, freshwater is exchanged between the basin boundary and interior due in part to small-scale (submesoscale <10 km) processes. Recent evidence revealed that eddies, sea-ice, and atmospheric (wind and buoyancy) forcing can all influence these small-scale processes. However, high-resolution observations in the Labrador Sea are scarce due to extreme winter conditions. Thus, the mechanisms responsible for the cessation of convection remain poorly understood and are largely neglected by climate models.

 

This project will use multi-year measurements from autonomous platforms (gliders) to investigate the role of small-scale processes on the cessation of convection. The student will determine the competing effects of eddies, atmospheric forcing, and sea-ice on the lifecycle of convection.

 

Methodology: 

The candidate will primarily focus on two glider missions completed in winter 2020 and 2022. These missions provide multiple high-resolution transects of physical properties (temperature, salinity, velocity, and turbulent mixing) across the Labrador Sea boundary. The student will characterise the presence of submesoscale instabilities and their dependence on atmospheric forcing and on sea-ice. In addition, the candidate will determine the overall contribution of these submesoscale processes to the intensification or suppression of deep convection. Using multi-year observations will reveal whether the large sea-ice extent conditions of winter 2022 contributed to a year of anomalous convection.

 

The candidate may further use existing Argo floats, mooring deployments, and ship-based hydrographic data to characterise the interannual variability of polar and Atlantic water masses but also to extend the small-scale analysis from gliders in space and time. Satellite observations of sea-ice and atmospheric reanalysis product will reveal the role of the large-scale atmospheric variability onto small-scale oceanic processes. Finally, the use of high-resolution simulations may complement the analysis of submescoscale processes by estimating their overall contribution onto the freshwater budget of the Labrador Sea. This evaluation will deliver invaluable insights into the necessity to include these key small-scale processes in climate models.

 

Location: 
NOC/UoS
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 hosted in the Ocean Circulation and Processes Team at the National Oceanography Centre.

Specific training will include:

  • Opportunities to take part in fieldwork, which may include a large oceanographic research vessel or a glider deployment from small boats offshore Greenland.
  • Training in numerical and statistical techniques in Python and version-controlled software (Git).
  • Processing and analysis of glider data and various observational datasets.
  • Potential training in Big Data analysis (Pangeo) and open-source software.
  • Opportunities to take part in summer schools and glider training to learn about glider deployment and data processing.
  • Opportunity to attend national/international conferences to disseminate the candidate’s results and to broaden their scientific network.

 

Eligibility & Funding Details: 

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

 

Background Reading: 
  • Naveira Garabato, A. C., R. Ferrari, and K. L. Polzin, 2011: Eddy stirring in the Southern Ocean. J. Geophys. Res., 116, C09019.
  • Oltmanns, M., J. Karstensen, and J. Fischer, 2018: Increased risk of a shutdown of ocean convection posed by warm North Atlantic summers. Nat. Clim. Change 8, 300–304.
  • Thompson, A. F., Lazar, A., Buckingham, C., Naveira Garabato, A. C., Damerell, G. M., and Heywood, K. J., 2016: Open-ocean submesoscale motions: a full seasonal cycle of mixed layer instabilities from gliders. J. Phys. Oceanogr., 46, 1285—1307.