Contrasting sea ice thickness variability in Arctic pack ice and marginal ice zone through ocean–sea-ice reanalyses

Abstract. Rapid changes in the Arctic sea ice, including widespread thinning, loss of multiyear ice, and shifts in the seasonal cycle, provide compelling evidence of an ongoing transition toward a new Arctic climate state. While the expansion of the marginal ice zone (MIZ) has been extensively documented using concentration-based metrics, much less is known about how sea-ice thickness (SIT) variability differs between marginal and consolidated ice regimes, and how these differences relate to upper-ocean conditions. This study investigates the seasonal and interannual variability of SIT within the Arctic MIZ, and contrasts it with that of the more consolidated and less mobile pack ice, using a multi-decadal ocean–sea ice reanalysis ensemble together with satellite-derived and observation-based datasets. A regional framework is adopted to account for the strong spatial heterogeneity of Arctic sea ice and to complement the pan-Arctic analysis. Furthermore, the relationship between SIT and surface ocean conditions beneath the ice is examined. The Global Ocean Reanalysis Ensemble Product (GREP) from the Copernicus Marine Service provides a physically consistent, spatially and temporally complete framework that integrates sea-ice and ocean properties, enabling a coherent investigation of their coupled variability beyond what can be achieved using fragmented satellite observations or sparse in situ measurements alone. Analysis of the GREP ensemble reveals distinct seasonal and regional thinning patterns in the MIZ and pack ice. Since 1993, the MIZ has experienced a pan-Arctic July thinning trend of -0.14 m/decade, with regional trends ranging from -0.09 m/decade in the Barents-Kara seas sector to -0.26 m/decade in the Laptev-East Siberian seas sector. By contrast, pack ice exhibits stronger and more spatially uniform thinning throughout the year. Sea surface temperatures beneath the MIZ are systematically warmer and more variable than beneath pack ice, underscoring the importance of accounting for sea-ice heterogeneity when assessing Arctic change. These findings demonstrate that the expanding MIZ has become an increasingly important component of Arctic climate variability and that its evolution cannot be inferred from pack-ice behavior alone. Its strong regional sensitivity to oceanic forcing underscores the need to accurately represent ice-type variability and marginal-ice processes in climate models, both to constrain model biases and to improve predictions of the future Arctic sea-ice cover. Integrating ocean variables into this framework offers a robust pathway for identifying MIZ-specific indicators of the evolving Arctic climate state.