Why was the 2023 jump in global temperature so extreme?

Global surface air temperature (GSAT) reached unprecedented heights in 2023. The record of year-to-year temperature increases was surpassed by a significant margin, especially in early boreal fall. We attribute the majority of this seasonal jump to the onset and maturing stages of the 2023 El Niño event. Using a process-based analysis of multiple 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 steep year-to-year increase of Sea Surface Temperature (SST), particularly in mean atmospheric subsidence regions, leading to extreme reduction of low-cloud-cover and giving rise to a record-breaking change in the radiative budget over the central and eastern Indo-Pacific; (2) anomalous sustained precipitation over climatological high SSTs in the Western Pacific, fueling unusual diabatic heating and an exceptionally early increase in tropical tropospheric temperature in boreal fall, ultimately influencing the GSAT jump with an additional contribution from the North Atlantic.
Our study improves the understanding of the interactions between interannual internally-driven processes and changes in mean climate background state, which a changing background is crucial to assess the evolution and modulation of anthropogenically-driven trends.