Overshooting convection drives winter mixed layer under Antarctic sea ice

Antarctic sea ice regulates atmosphere-ocean heat and gas exchanges by insulating the upper-ocean mixed layer. Simultaneously, winter sea ice formation intensifies ocean mixing and deepens the mixed layer, influencing further ice growth. Understanding this interaction is crucial for accurate Antarctic sea ice and climate predictions, yet winter observations remain scarce due to harsh conditions. Here, we use high-resolution state-of-the-art large eddy simulations to unravel the upper-ocean dynamics linking vertical heat flux to sea ice growth. Our results show that sea ice formation drives energetic meter-scale saline convective plumes that penetrate the pycnocline, overshoot their neutral depth, and rebound upward, entraining warm subsurface water, thereby moderating ice formation. A scaling theory is developed to estimate plume-induced turbulent heat flux and applied to an ice-ocean reanalysis dataset, revealing its large-scale impacts. Overshooting convection significantly limits Antarctic sea ice expansion and explains regional thickness variations, underscoring its importance for future climate projections.