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The changing freshwater composition of the South Atlantic and Southern Ocean: causes and impacts


Ocean and Earth Science

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Dr P Abrahamsen , Prof A Naveira-Garabato , Prof Mike Meredith , Dr A Brearley No more applications being accepted Competition Funded PhD Project (Students Worldwide)

About the Project

Project Rationale
The Southern Ocean exerts a disproportionate influence on our planet’s climate, via the strong drawdown of anthropogenic carbon and heat from the atmosphere that occurs there. This role is intimately connected to the Southern Ocean circulation; it is the key region globally where old waters are upwelled to the surface, and new waters created that sink back into the ocean interior. Freshwater inputs to the ocean can impact this circulation, by affecting density gradients and the stability of the upper ocean. Both sea ice meltwater and the melt of Antarctic glaciers exert such an influence, and both are known to be strongly variable, both in time and space. This project will use a tracer of freshwater inputs – the stable isotopes of oxygen in seawater – from the South Atlantic sector of the Southern Ocean to distinguish sea ice melt from other sources of freshwater in historical and recent ocean datasets. By linking these measurements with conventional oceanographic data, satellite data and the outputs of computer simulations, the student will determine the impact of known freshwater inputs on oceanographic circulation, the level to which variability in these sources impacts the ocean, and how such changes might influence climatically-important processes into the future.

Methodology
When measured alongside salinity, the ratio of stable isotopes of oxygen in seawater is a key tracer for quantifying the input of sea ice melt separately from freshwater from other sources (glacial melt, precipitation). Substantial datasets of oxygen isotopes now exist, with many obtained recently from the NERC-funded ORCHESTRA programme (Ocean Regulation of Climate via Heat and Carbon Sequestration and Transport; https://www.orchestra.ac.uk). Ocean freshwater composition will be derived from these datasets using mass balance techniques, along with quantitative estimates of ocean circulation from inverse techniques applied to conventional oceanographic data, building on, and complementing, the inverse modelling that is being performed as part of ORCHESTRA. This will enable the ocean transports of different freshwater types to be determined, and their changes over time to be quantified. The availability of new datasets will enable these analyses to take place over larger spatial scales (i.e. basin scale) than has been done before. Relating these results to remotely-sensed quantifications of sea ice change and glacial ice change on Antarctica will reveal the impact on the ocean of the different freshwater inputs.

Training
The INSPIRE DTP programme provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multidisciplinary 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 British Antarctic Survey.

Specific training will include:
- Observational (seagoing) oceanography techniques and data analysis methods (BAS)
- Stable isotope mass spectrometry (NIGL, BGS)
- Operation of cavity ringdown spectrometers (BAS)
- Inverse modelling, and data analysis techniques for climate models, inverse models, and satellite data (UoS, BAS)
- Possible opportunities to participate in an Antarctic research cruise. This is not a prerequisite for completing the PhD, and is subject to ship scheduling, medical clearance, and physical fitness requirements. (BAS)
- Opportunities to present research results at national and international conferences, and at meetings of a NERC multi-centre science programme.

Funding Notes

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

References

Akhoudas, C., J.-B. Sallée, G. Reverdin, G. Aloisi, M. Benetti, L. Vignes, and M. Gelado (2020). Ice shelf basal melt and influence on dense water outflow in the Southern Weddell Sea. Journal of Geophysical Research: Oceans, 125, e2019JC015710. https://doi.org/10.1029/2019JC015710

Brown, P. J., M. P. Meredith, L. Jullion, A. Naveira Garabato, S. Torres-Valdés, P. Holland, M. J. Leng, and H. Venables (2014). Freshwater fluxes in the Weddell Gyre: results from δ18O. Philosophical Transactions of the Royal Society A, 372, 20130298. https://doi.org/10.1098/rsta.2013.0298

Haumann, F. A., N. Gruber, and M. Münnich (2020): Sea-ice induced Southern Ocean subsurface warming and surface cooling in a warming climate. AGU Advances, 1, e2019AV000132. https://doi.org/10.1029/2019AV000132
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