Assessing record-breaking North Atlantic warming extremes in summer 2023 using reanalysis and Grand Ensemble simulations

The marine heatwave in the North Atlantic in summer 2023 set new temperature records and raised concerns about the impact of climate change on oceanic extreme events. This study examines this record-breaking marine heatwave with a focus on the subpolar North Atlantic by analysing ECMWF ERA5 reanalysis data and the Max Planck Institute Grand Ensemble CMIP6 version (MPI-GE CMIP6).

We demonstrate that due to a superposition of the global warming background state and natural variability, individual members of MPI-GE CMIP6 reproduce a North Atlantic summer heat wave within recent decades, which matches the strength of the observed 2023 heatwave. We assess possible atmospheric and oceanic drivers, including those not discussed in the literature so far, such as the atmospheric circulation state and associated surface heat flux in the preceding winter or the oceanic heat transport convergence across the subpolar North Atlantic. Our results indicate that for the subpolar North Atlantic processes related to oceanic and atmospheric variability have significantly contributed to the record observed and simulated heatwaves. Based on the historical and future scenarios of MPI-GE CMIP6, we suggest that both frequency and intensity of marine heatwaves in the North Atlantic will increase significantly, which may have various impacts on marine ecosystems and regional climate.