Physical understanding of the extreme global temperature jump in 2023.

Global surface air temperature reached unprecedented heights in early boreal fall 2023, surpassing the previous record for year-to-year temperature increase by a significant margin. We attribute most of this temperature jump to the onset and maturing stages of the 2023 El Niño, with some contributions from the North Atlantic. Using a process-based analysis from multiple observational datasets, we show that the uniqueness of the 2023 event arose from the La Niña-like ocean-atmosphere state on which it developed. This background favoured (1) a steep year-to-year increase of Sea Surface Temperature, particularly in mean atmospheric subsidence regions, reducing low-cloud cover and giving rise to a record-breaking change in the radiative budget; (2) anomalously sustained precipitation over high sea surface temperatures in the Western Pacific, fuelling unusual diabatic heating compared to canonical El Nino events. This altogether led to an exceptionally early increase in tropical tropospheric temperature in boreal fall, ultimately influencing the jump in temperature at the global scale.