Overshooting convective plumes and their role in winter Antarctic sea-ice growth

Antarctic sea ice plays a critical role in regulating heat and gas exchanges between the atmosphere and the Southern Ocean by insulating the upper-ocean mixed layer. During winter, sea ice formation drives vertical mixing, deepening the mixed layer and entraining subsurface heat that feeds back on subsequent ice growth. Recent years have seen an alarming decline and increased variability in Antarctic sea ice. This trend highlights the urgent need to better understand and represent the processes controlling ice growth in predictive models. Despite its importance for climate projections, this coupled ice–ocean feedback remains poorly constrained because wintertime observations are sparse. Here, we use high-resolution, state-of-the-art large-eddy simulations to identify a previously unrecognized process in the Southern Ocean: overshoot convection. We show that sea ice formation generates energetic, meter-scale saline plumes that penetrate beyond the mixed layer and overshoot into the pycnocline. These plumes rebound upward, entraining warmer subsurface waters associated with Circumpolar Deep Water back into the mixed layer and enhancing upward heat fluxes that moderate further ice growth. Recent years have seen an alarming decline and increased variability in Antarctic sea ice, underscoring the urgency of improving the representation of such processes in predictive models. We develop a theoretical framework to quantify plume-driven heat fluxes and apply it to an ice–ocean multi-year reanalysis dataset over the recent decade, demonstrating the large-scale relevance of overshoot convection. Our results indicate that this process limits Antarctic sea ice growth and spatial expansion, providing a physical explanation for observed regional variations in ice thickness. Crucially, overshoot convection is absent from current climate-scale models, highlighting a key missing process in projections of future Antarctic sea ice evolution and climate change.