Climate models separated by the marginal ice zone: using CMIP6, large ensembles, and altimetric observations of waves in ice to evaluate sea ice model skill.

Abstract:

The marginal ice zone (MIZ) is a key region for understanding the forced response of sea ice, as it lies at the interface between ice, ocean, and atmosphere - as well as the interface of modern model developments, particularly related to the interaction of ocean waves and ice. Often described as regions where sea ice concentration is between 15 and 80%, the total extent of the concentration-based MIZ (MIZc) has changed little over the satellite period, especially compared to the large declines in overall sea ice extent and area. We will present the first use of satellite altimetry, using the new ICESAT-2 satellite, to directly record waves propagating through ice, and thereby a new, global, wave-based definition for the MIZ (MIZw). Globally, the two definitions are coupled in winter months but decoupled in summer when surface melting occurs far from wave-affected regions. MIZw can be readily used to evaluate the latest generation of sea ice models that include coupled wave-ice interactions. Yet MIZc also has its use: using output from the CLIVAR Large Ensemble Project and CMIP6, we will show that modern climate models do not plausibly estimate the sensitivity of Arctic sea ice area/extent to warming in almost any month of the year, even when controlling for internal variability and inter-model differences. However, Arctic MIZc is well-simulated by a subset of models, and can be used to cleanly distinguish between their past and future predictions of sea ice state.