Internal variability effect doped by climate change drove the 2023 marine heat extreme in the North Atlantic

The year 2023 shattered numerous heat records both globally and regionally. We here focus on the drivers of the unprecedented warm sea surface temperature (SST) anomalies which started in the North Atlantic Ocean in early summer and persisted later on. Evidence is provided that 2023 should be interpreted as an extreme event in a warmer world because of superimposed internal variability on top of human forcing, which altogether, made the 2023 event all-time high due to extreme air-sea surface fluxes in the subtropics and eastern basin. The effect of internal variability has been considerably boosted by the long-term ocean stratification increase due to combined anthropogenically-driven ocean warming and multidecadal variability. The 2023 event would have been impossible to occur without anthropogenically-driven climate change but at the current warmer background climate state, it is assessed as a decadal-type event when considering the full North Atlantic ocean and a centennial event in the subtropics and eastern basin. Considering the regional distribution of anomalies is crucial for risk assessment in a warming climate.