Compensating Biases and a Noteworthy Success in the CMIP5 Representation of Antarctic Sea Ice Processes

Coupled Model Intercomparison Project phase 5 (CMIP5) climate models simulate a wide range of historical sea ice areas. Even models with areas close to observed values may contain compensating errors, affecting reliability of their projections. This study focuses on the seasonal cycle of sea ice, including analysis of model concentration budgets. Many models have insufficient autumn ice growth, leading to large winter biases. A subset of models accurately represent sea ice evolution year‐round. However, comparing their winter ice concentration budget to observations reveals a range of behaviors. At least one model has an accurate ice budget, which is only possible due to realistic ice drifts. The CMIP5 generation of model physics and resolution is therefore structurally capable of accurately representing processes in Antarctic sea ice. This implies that substantially improved projections of Antarctic dense ocean water formation and ice sheet melting are possible with appropriate subsetting of existing climate models. Plain Language Summary Antarctic sea ice simulated by numerical climate models generally exhibits large differences compared to satellite‐based estimates. Therefore, attempts to use these models to understand how Antarctic sea ice—and as a result Antarctic climate, sea level rise, and ocean circulation—will change in the future are severely compromised. This study looks beyond standard measures of sea ice such as the area of ocean covered by ice. Instead, we calculate how much of the change in sea ice over the course of a year is caused by local melting and freezing on the one hand and ice transport (for example by winds and ocean currents) on the other. Of an already small number of models which do show a realistic amount of sea ice, we identify that some do so for the wrong reasons. However, there is one model using established model components which shows realistic behavior. This model is able to match observations of sea ice drift. This demonstrates that the existing climate models are capable of representing Antarctic sea ice without further increases in sophistication. Focusing on such models for climate projections may improve the reliability of projections of the future. Key Points Some CMIP5 models capture Antarctic sea ice area, but quantifying their dynamic and thermodynamic processes reveals serious deficiencies One model is right for the right reasons; realistic Antarctic sea ice can be simulated using existing model physics and r