Persistent Northeast Pacific marine heatwaves are sensitive to the seasonality of tropical and North Pacific dynamics

Northeast Pacific marine heatwaves occur year-round but are shaped by seasonally-varying dynamics including El Niño-Southern Oscillation (ENSO) and North Pacific atmosphere-ocean interactions. Using a data-driven cyclostationary linear inverse model constructed from 64 years of monthly sea surface temperature and height reanalyses, we demonstrate that longer-lived marine heatwaves preferentially begin in winter, when ENSO teleconnections and oceanic memory most strongly influence event amplification. Springtime subsurface storage and subsequent fall/winter reemergence of thermal anomalies drive extended persistence through wintertime re-intensification. While intense events can also begin in summer, without strong ENSO and reemergence dynamics they rarely persist. The relative importance of tropical forcing versus North Pacific upper-ocean dynamics varies by region and season, producing distinct marine heatwave “flavors” linked to different ENSO types or to internal North Pacific processes alone. These findings reveal how seasonal phase-locking drives marine heatwave evolution, with implications for predictability and marine ecosystem impacts. Persistent marine heatwave events in the Northeast Pacific preferentially begin in winter, with El Niño-Southern Oscillation and re-emergence influencing duration, according to empirical dynamical modelling of the Northeast Pacific.