Work Package 3 (Impacts and Predictability of changes in Antarctic sea ice) will extend DEFIANT’s scope to cover longer timescales and broader geographical regions. We will achieve this aim using an interdisciplinary approach, combining CMIP6 analysis, new simulations with the NEMO-SI3 and HadGEM3-GC5 models, and analysis of observations and state estimates.

WP3 will investigate the impacts of sudden sea ice loss on globally-relevant ocean circulation patterns, which join the Southern Ocean to the global climate system: the ACC, subpolar gyres, and dense bottom water production. We will identify sudden sea ice decline events in the CMIP6 archive, and analyse the typical evolution of the ice-ocean system in the years preceding and following such events. Observations and state estimates will reveal how Southern Ocean water mass transformations and circulation patterns were affected by sea ice loss in 2016. Extended simulations with NEMO-SI3 will assess the impact of 2016-like events on the Weddell Gyre and the ACC, as well as dense water formation and deep ocean heat storage.

WP3 will also address whether major sea ice loss events are a new normal or internal variability, and whether they will occur more frequently as climate change continues. Analysis of different forcing scenarios in CMIP6 will reveal the relative roles of anthropogenic forcing and internal variability in triggering such events. NEMO-SI3 simulations will determine how the system responds to idealised changes in forcing, such as extreme temperature or wind, that may initiate sea ice loss. Forcing perturbation experiments with HadGEM3-GC5 will evaluate how changes in anthropogenic forcing, such as greenhouse gases, stratospheric ozone depletion, and ice sheet melt water, impact the coupled ice-ocean-climate system.

Finally, we will identify the implications of Antarctic sea ice loss for the heat and carbon budgets. State estimates, which are observationally constrained, will allow analysis of the short-term heat and carbon budgets. We will then analyse the long-term heat budget in HadGEM3-GC5 simulations and the CMIP6 archive, to reveal how sea ice loss affects ocean heat storage on decadal and multi-decadal timescales.

WP3 is led by Dr Kaitlin Naughten and Prof Alberto Naveira Garabato, and the work will be performed across several institutions: the British Antarctic Survey, University of Reading, University of Southampton, National Oceanography Centre, University of Leeds, and the UK Met Office.