No longer polar opposites: the Arctic and the Antarctic are converging towards a seasonally dominated sea-ice regime

Over the satellite period, Arctic and Antarctic sea ice extent seemed to follow opposite pathways. Arctic sea ice showed a strong and prolonged decrease until 2007 and then stalled in its rate of decline. Antarctic sea ice extent, on the other hand, displayed a small but significant increase until 2015. After 2015, Antarctic sea ice extent experienced a sharp decline, and has since been characterized by a lower mean state with enhanced variability, suggested as a new sea-ice regime. Arctic sea ice has continued transitioning towards a regime of substantially lower summertime coverage with reduced multiyear ice, increasingly resembling Antarctic sea ice in conditions and seasonality. We aim to provoke discussion by suggesting that the parallel changes in Arctic and Antarctic sea-ice behavior share a common mechanism: the breakdown of ocean stratification sustaining sea-ice decline, whilst episodic atmospheric forcing increasingly influences high-frequency sea-ice variability. In both the Antarctic and the Eurasian Basin of the Arctic, positive surface layer salinity anomalies have caused a weakening of the ocean halocline. The associated erosion of stratification makes sea ice more vulnerable to subsurface heat, and appears to underpin recent sea-ice loss and elevated variability. The convergence of the polar regions toward a seasonally dominated sea-ice regime leads us to call for continued and enhanced knowledge transfer and coordination between polar communities to accelerate understanding of recent sea-ice changes. A better understanding of the mechanisms driving changes in the coupled atmosphere–ocean–sea-ice system is needed to improve projections of future sea-ice loss and assess the cascading risks it poses to global climate and regional ecosystems.