The physics and biology of the ocean carbon sink: how air-sea interactions affect organic carbon uptake and sequestration in the Southern Ocean.

The water masses leaving the Southern Ocean form a major conduit of atmospheric carbon into the interior of the global ocean. Part of this transport is via the ‘solubility carbon pump’ whereby CO2 dissolved in seawater is transported into the interior ocean. Another component is via the ‘biological carbon pump’ (BCP), a range of processes turning inorganic carbon in surface waters into organic material which then propagates to depth via gravitational sinking, mixing, and subduction. A key aspect of the BCP is how deep sinking organic material penetrates across layers of different density before being converted into soluble form; the potentially large consequences of variability of the water masses in which the carbon ultimately finds itself are still uncertain. The Southern Ocean is the formation region for globally significant mode waters which may transport the majority of erstwhile organic carbon into the Atlantic, Pacific, and Indian Oceans. Inter-annual variability in atmospheric forcing (e.g. winds, heating, freshwater input) can exert a strong influence on mixed layer depth and hence on the volume of mode waters formed in this region. The consequences for the BCP of the Southern Ocean remain unquantified.

Exploring the sensitivity of carbon uptake and transport to changes in atmospheric forcing is challenging. The significant amount of time needed to complete even a single model run limits the number of scenarios that can be explored. However, adjoint models allow the sensitivity of such properties to be explored in a much more computationally efficient manner. The student will make use of an adjoint model constructed as part of the NERC ORCHESTRA project to explore inorganic carbon uptake in the Southern Ocean. The adjoint modelling framework will be used to determine sensitivities of transport across the northern boundaries of the Southern Ocean to changes in atmospheric forcing. These results will be combined with data on remineralisation depth from a variety of projects (including the ongoing NERC Carbon Uptake and Seasonal Traits in Antarctic Remineralisation Depth [CUSTARD] project) to determine variability in the partitioning of organic carbon across the Southern Ocean water masses.

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 NOC. Specific training will include: instruction in the use of the adjoint model developed by the British Antarctic Survey; a possible opportunity to participate in a research cruise; training in the use of biogeochemical models, including the accelerated Transport Matrix framework.