Record-breaking western North Pacific subtropical high during 2023

The western North Pacific subtropical high (WNPSH) exhibited record-breaking enhancement from June to December 2023. However, the driving factors behind this exceptional anomaly remain unclear. Here we report that such prominently intensified WNPSH was mainly driven by extreme sea surface temperature (SST) warming across tropical three oceans. Using a physically empirical model with leave-one-out cross-validation method, we successfully reconstructed key features of the WNPSH variations, including its area, intensity, and western ridge point. Quantitative results show that natural interannual SST variability and global warming together explain approximately 80% of the total WNPSH anomalies in 2023, where the SST variability contribute about 60% of the explained anomalies, more than the 40% contributed by global warming. And the multi-model ensemble yields results that are consistent with observations. The warmest SST anomalies in tropical North Atlantic and western Indian Ocean intensify the 2023 WNPSH extension and strength while the tropical eastern Pacific contributes to the westward displacement of the western ridge point. We highlight the highly resonant effects of natural variability and anthropogenic forcing, which considerably amplified the climate impacts of the 2023 WNPSH, such as leading to record-breaking heat in Asia and tropics, excessive rainfall in the North Indian Ocean, and historic lowest tropical cyclone activity frequency in the western North Pacific. Our results provide some new insights into the ongoing hot debate on the exceptional climate conditions in 2023.