Differences Between the CMIP5 and CMIP6 Antarctic Sea Ice Concentration Budgets

Compared to the Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models, the Antarctic sea ice area (SIA) has been improved in Phase 6 (CMIP6). However, the lack of knowledge about the reliability of sea ice dynamic and thermodynamic processes in the CMIP6 models still limits the accuracy of Antarctic sea ice projections. Here, by using a novel and systematic statistical metric, the performance of CMIP5 and CMIP6 models with near‐realistic SIAs was assessed. We found improvements in CMIP6 models relative to CMIP5. Moreover, forcing the sea ice‐ocean model with atmospheric reanalysis led to excessive ice convergence compared to the fully coupled ocean‐sea ice‐atmosphere model, although the SIA bias could be much smaller. This prevalent insufficient ice divergence in the models is highly correlated with the negative ice thickness bias, highlighting the importance of ice thickness in the correct simulation of sea ice dynamics. Plain Language Summary Current state‐of‐the‐art climate models do not reproduce the total area of Antarctic sea ice and its trends as observed. This impedes the use of climate models to understand changes in Antarctic sea ice over the past decades and to project its future. Separating how much of the simulated sea ice change is due to freezing and melting, and how much is due to ice transport allows us to identify physical causes for model biases, which helps us to optimize models. We examined the climate models that have simulated near‐realistic sea ice areas between 1991 and 2009 and found significant improvements in their simulation of sea ice processes in the latest generation of models compared to the previous generation, although there is still much room for further improvement. As sea ice thickness and velocity interact, a key limitation of the state‐of‐the‐art Antarctic sea ice simulation may stem from the general underestimation of ice thickness. This could be a critical issue to be targeted on the way toward increasingly skillful climate projections. Key Points The Antarctic sea ice concentration (SIC) budgets in CMIP6 have improved from CMIP5 The ice‐ocean coupled models have more realistic SIC budgets than fully coupled models, except for the excessive velocity convergence Underestimation of velocity divergence in the model was found to be strongly associated with underestimation of sea ice thickness