Projected amplification of summer marine heatwaves in a warming Northeast Pacific Ocean

Marine heatwaves are expected to become more frequent, intense, and longer-lasting in a warming world, yet the role of feedback processes in amplifying or dampening extremes is unclear. Using coupled climate simulations with observed 2017–2020 atmospheric flow imposed (nudged storylines), the study examines how the record-breaking 2019 Northeast Pacific marine heatwave would have unfolded in preindustrial, present-day, and +4 °C warmer climates. Air–sea interactions—including reductions in low clouds and mixed-layer depth, and air advection from fast-warming subpolar regions—modulate warming rates during the heatwave. In a +4 °C climate, global oceans are +1.9 °C warmer than present, while the Northeast Pacific warms by +2.3–2.7 ± 0.25 °C regionally. Event-specific feedbacks further amplify intensity and spatial extent beyond these thresholds, yielding +2.9 ± 0.15 °C warming above present. Such amplification would heighten stress on marine ecosystems and fisheries.