Wilton Aguiar: The Southern Ocean’s thermodynamic response to sustained low sea ice
Following several decades of modest expansion, Antarctic sea ice extent contracted abruptly in 2015 and has since remained anomalously low. In winter 2023 alone, 2.33 million km² of ocean became ice-free—a loss rarely observed over the last 40 years. Antarctic sea ice has not recovered to its previous higher-extent regime, indicating that sea ice loss is reshaping Southern Ocean heat storage and transport patterns. However, isolating the impact of persistently low sea ice extent on Southern Ocean heat storage and transport is challenging because of the interconnectedness of the ocean, sea ice, and atmosphere in the Earth system.
In this study, we use a high-resolution, eddy-rich regional model of the Southern Ocean, together with an idealised ghost flux approach, to isolate the oceanic response to a sustained reduction in Antarctic sea ice extent. Reduced sea ice extent produces a three-layer thermal response. In the deep ocean (z>3000m), increased eddy activity strengthens southward heat transport, enhancing heat convergence and warming at high southern latitudes. At the surface (z<100m), enhanced summer shortwave penetration leads to surface warming. In winter, deeper mixed layers and enhanced heat loss within the retreating sea ice edge drive subsurface cooling (between 100 m and 300 m). As climate models project further reductions in Antarctic sea ice over the coming decades, these processes are likely to become increasingly important in shaping the future distribution and storage of heat in the Southern Ocean.