Relative impacts of sea ice loss and atmospheric internal variability on winter Arctic to East Asian surface air temperature based on large-ensemble simulations with NorESM2

To quantify the relative contributions of Arctic sea ice and unforced atmospheric internal variability to “warm Arctic, cold East Asia” (WACE), this study analyses three sets of large-ensemble simulations carried out by the Norwegian Earth System Model with a coupled atmosphere-land surface model, forced by seasonal sea ice conditions from preindustrial, present-day, and future periods. Each ensemble-member within the same set uses the same forcing but with small perturbations to the atmospheric initial state. Hence, the difference between the present-day (or future) ensemble-mean and the preindustrial ensemble-mean provides the ice-loss-induced response, while the difference of the individual members within the present-day (or future) set is the effect of atmospheric internal variability.

Results indicate that both present-day and future sea ice loss can force a negative phase of the Arctic Oscillation with a WACE pattern in winter. The magnitude of ice-induced Arctic warming is over four times larger than the ice-induced East Asian cooling, the latter with a magnitude that is about 30% of the observed cooling. Sea ice loss contributes about 60% (80%) of Arctic winter warming for the present-day (future) climate. Atmospheric internal variability can also induce a WACE pattern with comparable magnitudes between Arctic and East Asia. Ice-loss-induced East Asian cooling can easily be masked by atmospheric internal variability effects because random atmospheric internal variability may induce warming with larger magnitude. Observed WACE pattern occurs as a result of both Arctic sea ice loss and atmospheric internal variability, with the former  dominating Arctic warming and the latter dominating  East Asian cooling.