(a) Determine the mean state and seasonality of the Southern Ocean carbon sink. This will be done by making a step change in the quantity of wintertime surface ocean fCO2 estimates, both by providing a calibration to pH-based estimates from float data, and by using historical and new hydrographic and biogeochemical observations, especially upper water column dissolved inorganic carbon and total alkalinity, to reconstruct wintertime surface fCO2 concentrations.
(b) Determine the geographical distribution of the Southern Ocean carbon sink. This will be done by putting together a new, dense, 3-dimensional database of atmospheric CO2 concentrations incorporating new data collected in SONATA with existing data. This database will then be used in an atmospheric inversion framework to quantify the geographical distribution, particularly regarding the differences in SO carbon fluxes associated with the upper and lower limbs of the SO overturning circulation.
(c) Identify the trends and climatic drivers of the Southern Ocean carbon sink. This will be done by combining our analysis of observed patterns of changes using existing and new oceanic and atmospheric data-based estimates, with expected fingerprints of climatic drivers from a series of new hindcast ocean model simulations developed specifically to reproduce the effects of observed variability in underlying processes and drivers of the SO carbon sink.
(d) Design and implement a strategy to monitor the Southern Ocean carbon sink. This will be done by integrating existing and new data-products and model simulations using a Bayesian optimisation, and will be delivered annually in conjunction with the global carbon budget updates.
(e) Design an optimal sampling strategy to reduce remaining uncertainties and constrain the evolution of the Southern Ocean carbon sink for decades to come. This will be based on the development and integration of Observing System Sampling Experiments (OSSEs) for the SO carbon sink, testing potential new data streams, for example profiling gliders and Autonomous Surface Vehicles in the ocean, and new remote island sites and satellite data in the atmosphere.
