Despite global warming, Antarctic sea ice expanded during most of the first four decades of satellite observations. However, in 2016 the Antarctic sea ice area plummeted, in a change far outside the range of previously observed variability. Neither the increasing trend nor the rapid decline are authentically simulated by climate models, casting doubt on their ability to represent associated processes including Southern Ocean heat and carbon uptake, melting of the Antarctic Ice Sheet, and many other aspects of the Southern Hemisphere climate.

We propose to generate a new mechanistic understanding of the drivers and impacts of Antarctic sea ice variability, including the dramatic decline in 2016. To achieve this overarching goal, we will launch an ambitious, internationally-leveraged field programme to provide the first comprehensive, systematic, year-round measurements of atmospheric and oceanic conditions at the Antarctic air-sea-ice interface. We will use these observations to constrain a hierarchy of numerical models and apply them to understand the processes controlling the historical decadal sea ice expansion and 2016 decline. Finally, we will assess the short-term and decadal consequences of Antarctic sea ice variability. Through this interlinked programme of observations, model development and model evaluation, DEFIANT will deliver a step-change in our understanding of the Antarctic sea ice system.