The Southern Ocean (SO) is the most exciting and extreme region of the world ocean, with the strongest winds, coldest temperatures, and most intense storms. It is believed also to be among the largest 'sink' for atmospheric CO2, accounting for about one third of the uptake of CO2 by the global ocean and nearly one tenth of the global emissions of CO2 on average each year. Thus the evolution of the SO carbon sink has the potential to alter the rate and extent of climate change.

In spite of its importance, we don't know the state, variability, or climatic drivers of the contemporary SO carbon sink and there is much controversy over its recent evolution. The climate of the SO has been changing over recent decades: in particular, winds have intensified, (attributed in part to the depletion of stratospheric ozone and in part to increasing temperature gradients arising from climate change), ocean acidification is occurring, and there is a long term decline in krill stocks. These effects take place on top of large natural variability and poorly quantified climatic trends.

SONATA will achieve a step change in our understanding of the contemporary SO carbon sink by delivering new data and new insights, integrating observations from the ocean, from the atmosphere, and model results. We will develop three complementary streams of research, an 'Oceanic', an 'Atmospheric', and a 'Processes and drivers' view, and will bring them together using advanced mathematical frameworks to provide a single assessment with multiple constraints and reduction of uncertainties.

The Oceanic view will use existing and new observations of ocean carbon. We will undertake a new calibration experiment to better assess the large number of pH measurements now being made by about 200 sophisticated profiling floats introduced by the US SOCCOM programme. These have the potential to greatly increase the number of observations that can be used to calculate air-sea CO2 fluxes, but only if adequately calibrated. In addition we will develop and use a new technique to construct estimates of the seasonal and temporal evolution of the air-sea flux, using a model of the upper water column constrained with available hydrographic and carbon-system observations.

The Atmospheric view will collect new atmospheric CO2 data in remote SO locations comprising Halley Station (75S), the Falkland Islands (51S), and aboard the BAS research ship James Clark Ross; new atmospheric O2 data will come from a ship track that repeats a SO transect every 8 weeks, as well as from Halley Station in coastal Antarctica. Using these data and an inverse framework approach, SONATA will provide an independent assessment of the SO carbon sink, which will deliver particularly on the geographic distribution of the changes, with O2 data helping to inform the drivers.

The Processes and drivers view will use two climate-scale carbon models and a series of hindcast simulations to identify the relative contributions of (a) atmospheric CO2 concentration, (b) natural climate variability, (c) climate change, and (d) stratospheric ozone depletion to recent SO carbon trends and variability. Ocean and atmosphere observations, including new data from SONATA and SOCCOM, will be used to optimise the model and validate the results. Idealised forcing with climate models will provide the 'fingerprints' of climatic drivers that are needed to understand the observed patterns of change.

Finally the three streams of research will be integrated using a Bayesian fusion mathematical approach that considers the strengths and weaknesses of each stream of information and minimises the joint uncertainty. The SO ocean carbon sink will be assessed annually in this way. We will then test the added value of including new streams of observations in the future, including from floats, gliders, drifters, Autonomous Surface Vehicles, additional ground-based observations and satellite CO2 data.

Grant Ref: NE/P021417/1