Spatiotemporal patterns of surface marine heatwaves in the northeastern North Atlantic

Abstract. Marine heatwaves (MHWs) are increasing globally, yet their long-term variability and trends remain poorly characterized across the high-latitude northeastern North Atlantic. We analysed daily sea surface temperature from the TOPAZ reanalysis during 1991–2025 to quantify the spatial characteristics, temporal trends and dominant modes of surface MHW variability across the North Sea, Norwegian Sea and Barents Sea and assessed associations with large-scale climate variability. We also examined MHW exposure within Norwegian particularly valuable and vulnerable areas (SVOs). MHWs occurred at an average frequency of 1.7 events yr^-1 and lasted 22.6 ± 6.6 days, with southern and coastal regions experiencing more frequent and intense events, whereas northern and sea ice-influenced regions experienced fewer but generally longer events. MHW frequency, cumulative intensity and annual MHW days increased over the study period, while trends in mean and maximum intensity were weaker and more spatially heterogeneous. The fraction of open water affected by MHWs increased by 21.7 % per decade, reaching 99 % in 2025. The two leading modes of detrended annual MHW day variability explained 53.5 % of the total variance and represented a coherent basin-scale pattern and a north–south dipole. Statistical associations with the East Atlantic pattern, previous summer North Atlantic Oscillation and previous year Atlantic thermal conditions suggest contributions from both atmospheric circulation and oceanic preconditioning. These lagged atmospheric and oceanic relationships suggest potential for improving MHW early-warning frameworks. MHW exposure was substantial within ecologically important SVOs. This increase in MHW frequency, persistence and extent across the region, has the potential to reduce thermal refugia and increase risks to fisheries, aquaculture and high-latitude marine ecosystems. These findings provide a regional baseline for ecological impact assessment and long-term monitoring, supporting ecosystem-based management and adaptation strategies in a rapidly changing subarctic region.