Climate Variabilities Synergistically Influence Marine Heatwaves in the North Sea

Abstract. Global shelf seas have experienced unprecedented marine heatwaves (MHWs) in recent decades. Although state-of-the-art forecast systems show skilful prediction of MHWs in tropical regions, their limited performance elsewhere highlights the need for a more complete mechanistic understanding at regional scales. Here, we examine MHWs in the Northeastern Atlantic shelf, a region strongly influenced by multiple climate variabilities. Using a correlation-based k-means clustering approach, we identified two distinct subregions with contrasting seasonal patterns. The southern North Sea (Cluster 1) exhibits increased MHW frequency, intensity, and duration in winter, primarily associated with a positive East Atlantic Pattern that typically follows a negative North Atlantic Oscillation in late autumn. These conditions intensify westerly winds and enhance warm Atlantic inflow through both atmospheric and oceanic pathways. In contrast, the northern North Sea (Cluster 2) shows enhanced MHW frequency and duration in summer, driven by teleconnections across multiple ocean basins. The Atlantic Multidecadal Variability modulates these linkages, with its positive phase strengthening Pacific-Atlantic connections via Rossby wave propagation. This north-south contrast demonstrates that different combinations of atmospheric and oceanic processes shape MHW variability across the shelf, providing a physical basis for improving regional MHW prediction.