This work package focuses on: Exchange between the upper-ocean mixed layer and the interior.

What are the leading-order processes that control the rate at which heat and carbon enter the Southern Ocean interior in its different layers, and how will these rates change in future?

Work Package two aims to determine the key ocean processes controlling the transport of heat and carbon from the surface mixed layer of the Southern Ocean into its deep interior. This transport from the surface, where the ocean exchanges properties with the atmosphere, to the deep interior, where heat and carbon may be sequestered for decades to centuries, is known as subduction.  We know that the Southern Ocean is the dominant region globally for such long term sequestration of heat/carbon and has significant impact on global climate, but the key contributing processes, regionality and timescales of this subduction are poorly understood.  In particular we wish to understand exactly how, where and on what timescales subduction strength responds to changes in forcing from the atmosphere (winds, heating and precipitation) and remote boundary conditions such as sea ice melt.  By taking up more or less heat and carbon on decadal to centennial timescales the Southern Ocean has significant potential for large scale feedbacks on the global climate system, so we also seek to understand how the subduction processes may change under projected anthropogenic forcing.

To achieve these goals WP2 will:

1. Develop new observational budgets of Southern Ocean subduction, optimised to resolve the regional water masses and processes driving subduction and quantify the relative importance of processes and their response to surface forcing changes on seasonal and interannual timescales.  These budgets will incorporate existing ship and Argo hydrographic data, as well as new ORCHESTRA autonomous glider, seal and ship based observations.
2.  Assess modelled mixed layer upwelling, transformation and subduction budgets across a wide suite of coupled climate models of varying setup and resolution, including existing CMIP5 models, the upcoming CMIP6 assessment and the Met Office’s Earth System Model.  The diverse ensemble of models analysed will allow the robustness of modelled subduction/forcing relationships to model biases and setup differences to be tested and contrasted with the observations in 1).
3. Investigate and quantify the robustness and sensitivity of modelled subduction to changes in atmospheric forcing and model parameterisations via targeted perturbation experiments of high resolution Southern Ocean models. Candidate processes and forcings for these experiments will be informed by the above observed 1) and modelled 2) budgets and the relationships and associated uncertainties identified within.
4. Examine CMIP5 and CMIP6 coupled climate projections of ocean heat and carbon uptake in the Southern Ocean to 2100 under a variety of climate forcing scenarios.  We will utilise the findings of 1-3) above to reduce the uncertainty in the model ensemble predictions, as well as identify key forcings and processes contributing to subduction response on multi-decadal to centennial timescales.