Spatiotemporal Evolution of Marine Heatwaves and Sea Ice Thickness in the Nordic Seas

This study investigates rapid Arctic Ocean changes driven by global warming, focusing on the Barents Sea, Greenland Sea, and Norwegian Sea region (68–82° N, 5–60° E). Using the Marine Heatwave (MHW) index and three sea ice thickness (SIT) datasets, we systematically analyze the joint variability of MHWs and SIT over the period 1985–2025. The results show a pronounced stepwise intensification of MHWs over the past four decades, with the decadal mean category increasing from 0.05 during 1985–1994 to 0.38 during 2015–2025, representing an overall sevenfold increase, and reaching a record-high annual mean of 0.68 in 2025. Climatological analysis further indicates that MHW intensity is markedly elevated during the warm season (May to August) and late autumn (October to November), peaking in November at approximately 0.25. During these periods, interannual standard deviations commonly exceed 0.20, highlighting not only strong background heatwave conditions but also enhanced temporal variability. Based on the 1993–2024 monthly climatology derived from multi-source SIT data, sea ice thickness generally ranges from 0.6 to 1.5 m during the cold season (January to May), increases toward late winter and spring, rapidly thins during the melt season to summer minima of approximately 0.4 to 0.5 m, and partially recovers to about 0.4 to 1.1 m during late autumn and early winter. Interannual variability is most pronounced during the melt season, with summer standard deviations reaching 0.24 to 0.37 m, indicating a high sensitivity of marginal ice zones to atmospheric and oceanic forcing. Overall, MHWs exhibit a strong, accelerating long-term intensification across the study region, forming a persistently elevated heatwave background over recent decades. In contrast, SIT shows only modest thinning or episodic fluctuations, with magnitudes substantially smaller than the increase in MHW intensity. Systematic differences among SIT datasets in absolute thickness, seasonal amplitude, and interannual variability reflect contrasting assumptions in data assimilation, satellite retrievals, and ice rheology. These results underscore the importance of multi-dataset intercomparison for quantifying uncertainties and resolving regional differences in ice type when assessing coupled heat and ice variability in Arctic marginal seas.