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

## TL;DR

This study shows that ocean dynamics, not surface heat, caused major marine heatwaves in the northeast Pacific from 2013-15 and 2019-20.

## Contribution

The paper identifies wind-driven ocean dynamics as the primary cause of multi-season marine heatwaves in the northeast Pacific.

## Key findings

- Warm subtropical waters moved northward due to wind-driven circulation changes, causing marine heatwaves.
- Anomalous northwesterly winds during decay phases increased latent heat loss and cooled the ocean surface.
- The TNH teleconnection pattern is linked to the persistence of these heatwave events.

## Abstract

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.

The study shows that gyre-scale ocean dynamics, driven by wind stress curl anomaly, were the main cause of the 2013-15 and 2019-20 northeast Pacific marine heatwaves via poleward warm water advection, while surface heat fluxes explain their decay.