The impact of autumn versus winter Arctic sea-ice loss on the winter atmospheric circulation

Arctic sea-ice extent has been declining in all months of the year, with the strongest trends occurring in late summer and early autumn. This has motivated investigation into the impacts of sea-ice loss on the mid-latitude atmospheric circulation. Modelling experiments commonly find a shift towards the negative phase of the North Atlantic Oscillation (NAO) and a weakening of the westerly wind on the poleward side of the eddy-driven jet during winter in response to year-round sea-ice loss. We use coupled atmosphere-ocean climate model experiments to investigate the extent to which this winter circulation response is driven by autumn sea-ice loss, which has implications for seasonal prediction and provides insight into the physical mechanisms. We modify sea-ice albedo parameters to drive two large ensemble experiments with different seasonal cycles of sea-ice loss. The first set of experiments has year-round reduction in sea-ice extent, with a seasonal cycle similar to the projected sea-ice loss under 2 °C global warming. The second set has similar magnitude of sea-ice loss in the late summer and early autumn months, but little reduction in sea-ice extent throughout the rest of the year. Consistent with previous studies, we find that year-round sea-ice loss drives a negative NAO response, and weakened westerlies on the poleward side of the eddy-driven jet. However, the winter circulation responds very weakly to sea-ice loss occurring only in the late summer and early autumn months. This suggests that the winter atmospheric circulation is more strongly influenced concurrent sea ice conditions than those in preceding months. Thus, predictability of the winter circulation from autumn sea ice may be limited.