Fully coupled climate models have long shown a wide range of Antarctic sea ice states and evolution over the satellite era. Here, we present a high-level evaluation of Antarctic sea ice in 40 models from the most recent phase of the Coupled Model Intercomparison Project (CMIP6). Many models capture key characteristics of the mean seasonal cycle of sea ice area (SIA), but some simulate implausible historical mean states compared to satellite observations, leading to large intermodel spread. Summer SIA is consistently biased low across the ensemble. Compared to the previous model generation (CMIP5), the intermodel spread in winter and summer SIA has reduced, and the regional distribution of sea ice concentration has improved. Over 1979-2018, many models simulate strong negative trends in SIA concurrently with stronger-than-observed trends in global mean surface temperature (GMST). By the end of the 21st century, models project clear differences in sea ice between forcing scenarios.Plain Language Summary Coupled climate models are complex computer programs that simulate the interaction of the atmosphere, ocean, land surface, and cryosphere. An important feature of the Southern Ocean is its sea ice cover, which typically expands in winter to cover an area comparable to that of Russia. Climate models have shown very different amounts of Antarctic sea ice coverage and very different trajectories of sea ice change in response to expected greenhouse gas emissions. This year, new coupled climate models released under the Coupled Model Intercomparison Project (CMIP6) will form the basis of the next IPCC assessment report. Here, we compare output from those models to satellite observations of the areal coverage of sea ice. As a whole, the models successfully capture some elements of the observed seasonal cycle of sea ice but underestimate the summer minimum sea ice area. Compared to results from the previous model generation (CMIP5), the range across models has reduced, and the location of sea ice agrees better with observations. Models project sea ice loss over the 21st century in all scenarios, but confidence in the rate of loss is limited, as most models show stronger global warming trends than observed over the recent historical period. Key Points:• CMIP6-mean Antarctic sea ice area is close to observations, but intermodel spread remains substantial • We find modest improvements in the simulation of sea ice area and concentration compared to CMIP5 • Most CMIP6 models simulate sea ice losses and stronger-than-observed GMST trends over 1979-2018

The Arctic winter sea-ice cover is in retreat overlaid by large internal variability. Changes to sea ice are driven by exchange of heat, momentum and freshwater within and between the ocean and the atmosphere. Using a combination of observations and output from the Community Earth System Model Large Ensemble, we analyze and contrast present and future drivers of the regional winter sea-ice cover. Consistent with observations and previous studies, we find that for the recent decades ocean heat transport though the Barents Sea and Bering Strait is a major source of sea-ice variability in the Atlantic and Pacific sectors of the Arctic, respectively. Future projections show a gradually expanding footprint of Pacific and Atlantic inflows highlighting the importance of future Atlantification and Pacification of the Arctic Ocean. While the dominant hemispheric modes of winter atmospheric circulation are only weakly connected to the sea ice, we find distinct local atmospheric circulation patterns associated with present and future regional sea-ice variability in the Atlantic and Pacific sectors, consistent with heat and moisture transport from lower latitudes. Even if the total freshwater input from rivers is projected to increase substantially, its influence on simulated sea ice is small in the context of internal variability.

The Arctic sea ice cover has decreased in all seasons and all regions since satellite observations started in 1979 (Comiso et al., 2017; Onarheim et al., 2018), serving as a visible manifestation of ongoing climate change (Serreze et al., 2007). These changes in sea ice cover potentially impact several aspects of climate in the Arctic and lower latitudes, including ocean circulation and hydrography (