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What controls the flows and mixing rates around Southern Ocean seamounts?

Project Description

Throughout the global ocean, regions of rough bottom topography are important locations of water mass mixing and transformation. Such processes commonly occur through the interaction of either mean or tidal flows with the topography, generating internal waves which subsequently break and dissipate their energy (e.g. Sheen et al., 2013). These processes assume an increased importance in the Southern Ocean where the modification of water masses can impact both limbs of the global ocean overturning circulation, with consequent impacts on global climate via the sequestration of heat and anthropogenic carbon.

A particular source of uncertainty in the transformation of water masses is in the north-south transfers of water between the Antarctic Circumpolar Current (ACC) – the globe’s largest current by volume transport - and the gyre circulations that fringe Antarctica. Recent observations have suggested that transfers between the ACC and the Weddell Gyre, in the Atlantic sector, are mediated by dynamics associated with Taylor columns (Meredith et al., 2015), with large rates of water mass stirring changing the properties of the ambient water masses. However, long term data sets documenting these processes have been limited, and measurements of the isopycnal (along density surface) and diapycnal (across density surface) transformation rates are sparse.

The student will work with data acquired from a recent 2019 ORCHESTRA cruise. This includes EM-APEX float data (one float is still collecting data), alongside ship and glider-based hydrography. The student will use these data to:

1. Quantify rates of isopycnal stirring in the vicinity of a likely Taylor column located in the southern Scotia Sea.
2. Quantify the dissipation rate of turbulent kinetic energy associated with the feature and identify the key periods of variability and processes controlling the diapyncal diffusivity.
3. Identify the signature of internal waves in the float data and uncover the dynamics which control the radiation and dissipation of internal wave energy. A recent study (Cusack et al., 2016) demonstrates the applicability of such techniques.

Later in the project, the student will use the understanding gained through the above objectives to identify other locations where such processes are likely to operate and to estimate a likely transformation rate for deep waters on a regional scale.

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 at the British Antarctic Survey. Specific training may include the option to participate in ship-based fieldwork in the polar regions as part of BAS and NOC’s long-term science programmes in the Southern Ocean. The student will also be encouraged to participate in Southampton University Masters training in, e.g. physical oceanography and data analysis skills, as appropriate. There will also be the opportunity to undertake training in the piloting of underwater gliders to support the BAS autonomous vehicles programme.

Funding Notes

UK and EU students who meet the UK residency requirements will be eligible for a full NERC studentship. Students from EU countries who do not meet the residency requirements may still be eligible for a fees-only award. More information can be found in the UKRI Training Grant Terms and Conditions

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Cusack, J.M., A.C. Naveira Garabato, D.A. Smeed and J.B. Girton (2017), Observation of a large lee wave in the Drake Passage. J. Phys. Oceanogr., doi: 10.1175/JPO-D-16-0153.1.
Meredith, M. P., A. S. Meijers,
A. C. Naveira Garabato, P. J. Brown,
H. J. Venables, E. P. Abrahamsen,
L. Jullion, and M.-J. Messias (2015), Circulation, retention, and mixing of waters within the Weddell-Scotia Confluence, Southern Ocean: The role of stratified Taylor columns, J. Geophys. Res. Oceans, 120, 547–562, doi:10.1002/ 2014JC010462.
Sheen, K., J. A. Brearley, A. C. Naveira Garabato, D.A. Smeed, S. Waterman, J. R. Ledwell, M.P. Meredith, L. St. Laurent, A. M. Thurnherr (2013), Rates and mechanisms of turbulent dissipation and mixing in the Southern Ocean: Results from the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). J. Geophys. Res. Oceans, 118, 2774–2792, doi:

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