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.

Potential Impact:
The Impact strategy is organised in two parts:
1. Engagement with research stakeholders
We will engage with the international research community via the World Climate Research Programme (WCRP), which via their Working Group on Coupled Modelling oversees the Coupled Model Intercomparison Project Phase 6 (CMIP6); letter of support on the response after carbon emissions cease from Dr Chris Jones (Hadley Centre). There is a World Climate Research Programme Grand Challenge concerned with 'Carbon feedbacks in the Climate System', which focuses research initiatives aimed at the need for better process understanding of the Southern Ocean carbon sink: Williams gave an invited keynote lecture on Ocean feedbacks at their workshop at University of Bern in April 2018, which led to a carbon feedback review paper by Williams for Current Climate Change Reports and he is also part of a CMIP6 AR6 carbon-cycle feedback paper. In addition, we are working with researchers actively involved in modelling the Southern Ocean carbon cycle (letter of support from Prof Mick Follows, MIT).

Similarly we will engage with the wider Southern Ocean observing community via the SCAR body Southern Ocean Observing System (SOOS). SOOS acts as a body to coordinate international observations, and this information will be of great value to guiding their recommendations, which then feed through to set wider observing initiatives. Meijers sits on SOOS as a member of the scientific steering committee.

Finally, our strong links to existing and upcoming large research programmes (e.g. ORCHESTRA, RoSES, UKESM, SOCCOM and SO-CHIC) will result in a two-way flow of information with these groups. We will also be able to provide wider context to the 8 million Euro ERC SO-CHIC programme which will be looking at the role of the Weddell Sea in the net Southern Ocean heat and carbon uptake, and explicitly compare our results with their WP6 (lead N. Gruber) budget assessments. Meijers is a WP lead in SO-CHIC.

2. Engagement with non experts, school children and the general public
In order to engage with school pupils, students, teachers and the general public, we are creating a series of short accessible videos- combinations of film and animations with commentaries - to explain the key processes involved over both short and long term timescales.
We are have recently produced 3 animations on the carbon cycle (each 3 minutes long): "Where is the emitted carbon going?", "Paris or bust" and "How is surface warming related to carbon emissions?" on our OceanClimateAtUoL YouTube channel.

This series will be extended by producing a new additional short animation on the carbon cycle in the Southern Ocean: "How is carbon cycled in the Southern Ocean?". We will include cartoons in the animations, which will be actively drawn so the sketch unfolds, making the complexity of the science appear fun and accessible.

The video outputs will be distributed via our OceanClimateAtUoL YouTube channel and hosted at BAS. Our latest carbon videos are being shown on plasma screens throughout the University of Liverpool on 20 September 2019 (a climate day) and our 3 previous videos on Sea Level: A Liverpool View have been used 'off-line' at Open Days, school outreach and events like the Mersey Tall Ships Festival, the James Cook ship visit to Liverpool and the Big Bang Exhibition of Science for school children (organised by STEM in June in Liverpool 2019).

We will also want to make teachers aware of this resource via our MerseySTEM network. Milestones Success of this impact plan will be measured by (i) engagement with research stakeholders (especially WRCP Grand Challenges); (ii) engaging with civic leaders to our science briefings and YouTube analytics to provide measurement of our various video output.

The outcomes of the project will also be summarised in a final report produced and distributed at BAS.