Hydrographic Changes and Water Mass Distribution in the Norwegian Sea, 1995–2023

Intermediate and deep water of the Norwegian Sea comprise overflow water that feeds the deepest limb of the Atlantic Meridional Overturning Circulation. Here, we investigated the variability and sources of intermediate and deep-water masses in the Norwegian Sea from 1995 to 2023, using repeat hydrographic sections and a combination of ship-based measurements and Argo data. Intermediate waters exhibited substantial warming, with rates of approximately 0.1°C decade⁻¹ at ∼1,000 m depth, while deep waters below 2,000 m warmed at 0.05°C decade⁻¹. Salinity variability was prominent at intermediate depths, and the Norwegian Sea Arctic Intermediate Water (NSAIW) layer, originating from the Greenland Sea Arctic Intermediate Water (GSAIW), became progressively thinner and shallower. The most pronounced thinning occurred during the 2000s, when the GSAIW became warmer and with properties more similar to the NSAIW. In the Norwegian Sea a prominent intermediate layer with salinity below 34.90 psu disappeared about 2010 due to increased salinity, but reemerged in the past 2–3 years as a distinct 200 m thick layer above the NSAIW, originated from the southern Greenland Basin. These changes highlight the sensitivity of the Nordic Seas to climate variability with implications for regional and global ocean circulation.