Why was the 2023 jump in global temperature so extreme?

Global surface air temperature reached unprecedented heights in early boreal fall 2023, surpassing the previous record for the largest year-to-year temperature increase by a significant margin. We attribute the majority of the temperature jump to the physical specificities observed in the onset and maturing stages of the 2023 El Niño event combined with additional influences from the North Atlantic. Using a process-based approach applied on a combination of observational datasets, we show that the uniqueness of the 2023 event can be largely related to the La-Niña-like ocean-atmosphere background state upon which it developed. This resulted in (1) a record-breaking change in the radiative budget over the Indo-Pacific basin due to a steep year-to-year increase of SSTs, particularly in mean subsidence regions, reducing low cloud cover at the onset of the 2023 event; and (2) an extreme and exceptionally early increase in tropical tropospheric temperature in boreal fall relative to past strong El-Niños, due to unusual diabatic heating fueled by abnormally sustained precipitation over regions of high SSTs. Our study contributes to a better understanding of the interactions between interannual internally-driven processes and a changing background state, which is crucial to isolate the fingerprint of anthropological forcing in ongoing trends.