Importance of ocean dynamics in the onset and persistence of the 2013-15 and 2019-20 northeast Pacific marine heatwaves

Large-scale marine heatwaves in the mid-latitude northeast Pacific have garnered significant attention due to their vast spatial extent, intensity, prolonged duration, and detrimental impacts on marine ecosystems and fisheries. Contrary to the conventional understanding that surface heat fluxes drive their formation, here we show that the two severe and impactful multi-season marine heatwave events in 2013-15 and 2019-20 were caused by the large-scale northward displacement of warm subtropical waters into the typically colder subarctic region. These oceanic changes are mainly explained by wind-driven circulation changes and Sverdrup balance adjustment. The marine heatwave decay phase corresponds with anomalous northwesterly winds which transport cold, dry air, enhancing latent heat loss and leading to ocean surface cooling. The physical driver is linked to the Tropical/Northern Hemisphere (TNH) teleconnection pattern, which aligns with previous studies. Collectively, the characteristic interannual timescale of the oceanic dynamic response and TNH explains the multi-season persistence of these extreme events.