Projected amplification of summer marine heatwave intensity in the Northeast Pacific Ocean in a warming world

Marine heatwaves are on the rise: their frequency, intensity, and duration are expected to increase in a warming world. Yet it remains unclear whether local feedback processes could amplify extreme ocean temperatures. A prominent marine heatwave recently occurred in the Northeast Pacific Ocean in summer 2019, exhibiting the highest sea surface temperatures ever recorded in this area since the availability of satellite observations in 1979. Here, we use fully-coupled model experiments, termed “nudged storylines”, in which the evolution of large-scale winds in the free troposphere is nudged to the observed (reanalysed) one before and during the summer 2019 event, to generate close analogues of this record-breaking marine heatwave for past, present, and plausible future climates. We show in particular that future climate analogues of the marine heatwave may warm 50% more than what is expected from the projected global-mean ocean warming. Together with the rapid Northeast Pacific mean warming, air-sea feedback processes lead to a projected warming amplification of 1°C above the 1.9°C global-mean ocean temperature rise. Primary drivers of this amplification are a reduction in clouds and ocean mixed-layer depth, as well as anomalous air advection from fast-warming subpolar regions. Our results show that marine heatwave temperatures may warm substantially faster than the global and regional background temperature, increasing the stress on local ecosystems and fishery resources.