Synergistic Impacts of Climate Variabilities on Marine Heatwaves in Shelf Seas

Global shelf seas have experienced unprecedented marine heatwaves (MHWs) during the past decades. Despite state-of-the art forecast systems demonstrate skillful prediction capability of MHWs in the El Niño and tropical regions, their poorer skill in other regions necessitates a more complete mechanistic understanding of MHWs at the relevant spatio-temporal scales. Here we investigated synergistic impact of climate variabilities on integrated cumulative intensity of MHWs (MHW-ICI) in the Northeastern Atlantic shelf, one of the regions with significant climate variabilities. In winter, MHW-ICI increases only during concurrent positive phases of the North Atlantic Oscillation (NAO) and East Atlantic Pattern (EAP), which generate southwesterly winds that enhance warm water transport into the southern North Sea and strengthen downwelling in the northeastern region. When the NAO and EAP are in negative or opposing phases, easterly wind anomalies prevail and weaken MHW-ICI. In summer, Atlantic Multidecadal Variability (AMV) modulates the relative importance of NAO and El Niño-Southern Oscillation (ENSO) on North Sea MHW-ICI. During positive AMV phases, strengthened ENSO-Atlantic connections enhance MHW-ICI, while during negative phases, NAO dominates by influencing regional heat flux. These findings highlight the synergistic impacts of climate variabilities on MHWs in shelf seas and offer insights for improving MHW predictions.