Not just going with the flow: does biological production rather than deep water formation drive the Southern Ocean carbon sink?

Programme website: http://inspire-dtp.ac.uk

Project Rationale:
The Southern Ocean (SO) is thought to have absorbed ~40% of all global human-derived (anthropogenic) carbon dioxide and >75% of anthropogenic heat, thus being disproportionately influential in the ongoing mitigation of increasing atmospheric CO2 levels and related climate effects. Current explanations of SO carbon uptake and of historical climate transitions focus on the interaction between the surface and deep ocean around Antarctica, i.e. processes that impact intermediate and dense water formation as part of the global overturning circulation (buoyancy fluxes, sea ice production), and how the marine carbon system responds to them. However, new results suggest that in fact open-ocean biological production / carbon export combined with the horizontal circulation is the key process by which CO2 is removed from the atmosphere, forcing us to reframe our understanding of how the Southern Ocean carbon system functions. This project will test this changed paradigm using new observations in conjunction with novel research cruise data and an inverse model to deconvolve the carbon system in the Atlantic sector of the Southern Ocean. The focus will be on identifying the balance between physics and biology in driving SO carbon uptake, and how these are likely to change into the future.

Methodology:
The aim is to combine biogeochemical hydrographic data (from the NERC ORCHESTRA project) and new observations of the seasonal variability of carbon fluxes in the Southern Ocean (from NERC RoSES CUSTARD/SONATA projects) with historical data and numerical model outputs in an inverse model of the South Atlantic. The focus will be on partitioning the observed carbon signal into physical, biological, and preindustrial components; then, by combining with the inverse-derived circulation scheme, the project will reveal the individual ocean transports, air-sea fluxes and water column accumulation of different types of carbon across the South Atlantic, from the oligotrophic subtropics to the sea-ice covered Antarctic region. These novel estimates will then be compared to model outputs to identify the mechanisms controlling the biogeochemical fluxes.
Your major activities within the first year would include:
- Participation in Southern Ocean research cruise/s in 2019-20 (Drake Passage, Southeast Pacific), making observations of the marine carbonate system (dissolved inorganic carbon, total alkalinity).
- Analysis of hydrographic cruise data from the South Atlantic, and comparison to mooring/float observations
- Analysis of model outputs to generate relationships among biogeochemical transports, air-sea fluxes and ocean overturning metrics
During the project, it may become important to focus more on any one of the processes or to keep a more holistic view. The student will participate and contribute to the full range of activities associated with marine biogeochemical research including work at sea and in the laboratory.

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 at National Oceanography Centre. Specific training will include:
1) biogeochemical theory, laboratory and data analysis of the marine carbon system, in particular dissolved inorganic carbon by coulometry and total alkalinity by potentiometric titration
2) the theory, optimization, manipulation and data analysis of outputs from ocean inversions and CMIP5/CMIP6 climate models
3) oceanographic observations / data collection at sea