Southern Ocean carbon indices and metrics (SARDINE)

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The Southern Ocean provides the primary window by which the atmosphere affects the properties of the interior ocean and, in turn, how the deep ocean affects the atmosphere. Surface waters formed here are transferred at depth over the global ocean and deep waters are returned to the sea surface. The role of the global ocean in the carbon cycle is strongly controlled by this dual activity: surface waters leaving the Southern Ocean act to drawdown atmospheric carbon dioxide, while deep waters are carbon rich and so when deep waters return to the sea surface carbon dioxide is outgassed to the atmosphere.There are ongoing changes in the volume of Southern Ocean mode waters, which are projected to vary in a nonlinear manner with different emission scenarios, together with changes in the overturning circulation associated with atmospheric changes over Antarctica. What is unclear are the associated changes in ocean carbon uptake in the Southern Ocean together with the wider effects on global carbon and climate metrics. For example, if the stratification strengthens and ventilation weakens in the Southern Ocean, what then are the resulting effects for the carbon and climate system?We need to know how the Southern Ocean is sequestering heat and carbon, and how that uptake is likely to alter with climate change. By drawing upon ventilation and overturning diagnostics from a mainly physical programme (ORCHESTRA), we will identify how much carbon is taken up in different water masses and identify the changes in carbon pools corresponding to different physical, biological and chemical processes.We need to know how the Southern Ocean alters global carbon feedbacks in the climate system. By identifying the changes in ocean carbon inventories in Earth system model projections for 1% annual rise in CO2, we will reveal the effect of the Southern Ocean on global carbon-cycle feedbacks, as well as identify how their components alter with changes in physical, chemical and biological processes.Similarly, we need to know how the Southern Ocean affects global climate metrics, including how much surface warming is expected for a given cumulative carbon emission, how much carbon may be emitted to avoid exceeding warming targets and the extent of delayed peak warming after emissions cease. Through combined heat and carbon diagnostics of Earth system models following Representative Concentration Pathways, we will identify the effect of the Southern Ocean on the global surface warming response to carbon emissions, the maximum permitted carbon emission to avoid warming targets and how much global surface warming might occur after emissions cease.This work is crucial if we are to understand how Southern Ocean circulation and ventilation changes affect global carbon and climate metrics in a warming world. The Southern Ocean dominates global ocean heat and carbon uptake, but is poorly represented in global climate models. There is a wide spread across current Earth system models in their ocean physical and biogeochemical representation of the recent past, and often dramatic differences between models in their projections for the future under climate forcing. This study will allow us to attribute for the first time the contribution of the physical, biological and chemical processes in the Southern Ocean to various critical global climate metrics. Through a synthesis of these climate metrics with new understanding of the physical and biogeochemical properties of the Southern Ocean revealed by ongoing projects ORCHESTRA and RoSES, we will be able to produce new interpretations of the spread in projected climate futures and identify the effect of the Southern Ocean on the uncertainty.