Response of winter climate and extreme weather to projected Arctic sea-ice loss in very large-ensemble climate model simulations&#160;
Authors: Ye Kunhui , Woollings Tim , Sparrow Sarah , Watson Peter , Screen James 
Published: March 2024

Arctic sea-ice loss and amplified Arctic warming have been one striking signature of climate change, which have important impacts on climate variability in the Arctic and mid-low latitudes. Climate modeling including the Polar Amplification Model Intercomparison Project (PAMIP) has been a powerful tool for investigating the effects of Arctic sea-ice loss in a changing climate. However, existing climate model simulations including individual climate models from the multi-model/ensemble PAMIP project have relatively small ensemble sizes that may not allow a robust separation of forced response, particularly the response of extremes, to Arctic sea-ice loss from internal variability. Therefore, our confidence in the response to projected Arctic sea-ice loss in climate change is reduced. This has led to two unanswered important questions: (1) what ensemble sizes are needed for robust detection of extremes, as well as seasonal-mean responses to projected Arctic sea-ice loss? and (2) is the response dependent on resolution?To address the challenge, we have performed very large (~2000 members) initial-condition ensemble climate simulations, using both low (~90 km) and high (~60 km) resolutions, with prescribed boundary conditions (i.e., sea surface temperature and sea-ice concentration) taken from the PAMIP project, to advance understanding of mean climate and extreme weather responses to projected Arctic sea-ice loss under 2&#176;C global warming above preindustrial levels. We have run these simulations with the Met Office Hadley Centre global atmospheric model Version 4 on the University of Oxford&#8217;s innovative distributed computing project (Climateprediction.net). These simulations better sample internal atmospheric variability and extremes for each model compared to those from the PAMIP, and also allow studying the resolution-dependence of the response to projected Arctic sea-ice using a larger ensemble. Analysis of these simulations suggests that the mean climate response is mostly consistent with that from the PAMIP multi-model ensemble, including tropospheric warming, reduced midlatitude westerlies and storm track activity, an equatorward shift of the eddy-driven jet and increased mid-to-high latitude blocking. The response of temperature and precipitation extremes largely follows the seasonal-mean response. However, East Asia is a notable exception in showing an increase in severe cold temperature extremes in response to the projected Arctic sea-ice loss. Two resolutions of the same model exhibit significant differences in the stratospheric circulation. This does suggest resolution-dependence of the response but we consider that the difference in the stratospheric response weakly modulates the tropospheric response.We highlight that our very large-ensemble simulations have allowed rigorous sub-sampling to address the challenge of obtaining a robust forced response to projected Arctic sea-ice loss. The sub-sampling confirms that large ensembles (e.g. >=400) are needed to robustly estimate the seasonal-mean large-scale circulation response, and very large ensembles (e.g., >=1000) for regional climate and extremes. The reduction in uncertainty of the response with ensemble size is very well predicted by standard error analysis, guiding the design of future large ensembles.&#160;