Space-time variability of heat content in the North European Basin based on ORAS5 reanalysis

The study investigates the spatiotemporal variability of heat content in the North European Basin (NEB) — the Barents, Greenland, and Norwegian Seas. The research is based on the ORAS5 (Ocean ReAnalysis System 5) reanalysis data for the period 1982–2024. The climatic vulnerability of the region, driven by Arctic amplification and the intense advection of warm Atlantic waters, necessitates a detailed investigation of heat redistribution mechanisms. The aim of the work is to quantify interannual changes, taking into account seasonal dynamics, to identify spatial patterns in the distribution of heat content trends for different layers (0–200 m, 200–300 m, 300–400 m, 400–500 m and 500–600 m) and to study the vertical distribution of heat content during the period of modern climate change. The analysis employed methods of linear regression, the coefficient of determination ( R 2) of the linear trend, and layer-wise averaging; the statistical significance of the trends identified was assessed using Student’s t-test. The most pronounced positive heat content trends ( R 2 > 0.5) are observed during the winter season in key advection zones of warm Atlantic waters: along the West Spitsbergen Current, over the Mohn Ridge, and within the Bear Island Trough. The Lofoten Basin stands out due to exceptionally high and persistent R 2 values (> 0.6 down to a depth of 600 m), explained by the dominant role of mesoscale eddies in deep heat penetration processes. The analysis revealed significant vertical transformations in the thermohaline structure of the NEB waters since the 2000s: in the Fram Strait, the 1.4 °C isotherm descended from ~400 m to ~650 m; in the Boreas Basin, the 0 °C isotherm descended from ~500 m to ~650 m; over the Mohn Ridge, the layer of water warmer than 2 °C thickened from ~200 m to ~300 m. Summer months show minimal R2 values in the central basins, reflecting the strong influence of seasonal stratification and enhanced turbulent mixing processes. The combination of changes observed — weakening of vertical stratification, intensification of meridional heat transport, and the progressive deepening and eastward spread of Atlantic-origin warm waters — serves as a key indicator of the accelerating “Atlantification” process of the NEB, fundamentally altering the regional heat balance. The results obtained highlight the decisive role of complex bathymetry and sustained advection in shaping the spatial patterns of heat accumulation within the basin. The patterns identified are of significant importance for forecasting thermohaline circulation and the sea ice regime of the Arctic under climate change conditions.