Climate modes synergistically influence marine heatwaves in the North Sea

Abstract. Global shelf seas have experienced unprecedented marine heatwaves (MHWs) in recent decades. Although MHWs have been extensively studied at the global scale, their regional variability and underlying mechanisms remain poorly understood, particularly in shelf seas influenced by multiple climate modes. Here, we examine MHW variability in the Northeastern Atlantic shelf using a correlation-based k-means clustering approach. Two distinct subregions with contrasting seasonal patterns are identified. In winter, the southern North Sea experiences increased MHW frequency, intensity, and duration. This enhancement is linked to a positive East Atlantic Pattern, which intensifies westerly winds and enhances warm Atlantic inflow through both atmospheric and oceanic pathways. In contrast, the northern North Sea shows enhanced MHW frequency and duration in summer, while MHW intensity weakens. This summer response is modulated by Atlantic Multidecadal Variability, with its positive phase strengthening Pacific–Atlantic connections via Rossby wave propagation, altering cloud cover and surface radiative forcing. A shallow mixed layer, enhanced stratification, and circulation-induced upwelling favor frequent and persistent but less intense summer MHWs. This north–south contrast demonstrates that different combinations of atmospheric and oceanic processes shape MHW variability across the shelf, providing a diagnostic and mechanistic framework for understanding regional MHW variability and its potential predictability.