Drivers of the 2023 record shattering marine heat extreme in the North Atlantic

The year 2023 shattered numerous heat records globally and regionally. We here focus on the origins of the unprecedented and persistent warm sea surface temperature (SST) anomalies in the North Atlantic Ocean. Using a combination of multiple observational datasets and climate model large ensemble outputs, we show that the 2023 North Atlantic SST event is a manifestation of anthropogenically-forced warming, exacerbated by an extreme phase of internal climate variability and associated regional surface flux anomalies. The long-term stratification increase due to anthropogenically-driven ocean warming amplified the impacts of internal variability. At current global warming level, the 2023 event is assessed as a decadal-type occurrence for SST anomalies averaged across the entire North Atlantic basin, but it is estimated as a centennial-type event for the subtropics and the eastern basin. The distinctive horseshoe-shaped regional distribution of these anomalies is crucial to consider for correct communication and risk assessment in a warming climate.