Weakened El Niño/La Niña Influence on the East Asian Summer Monsoon Under Pliocene Warmth: Insights From Model Sensitivity Experiments

The impact of the El Niño–Southern Oscillation (ENSO) on East Asian summer monsoon (EASM) precipitation under sustained warming remains uncertain. Through Pliocene‐like simulations, we demonstrate that under a permanent El Niño–like phase, an eastward‐shifted Walker Circulation triggered a low‐level anticyclone over the South China Sea and prompted a southward shift of the westerly jet via a Rossby wave train. This circulation configuration facilitated the influx of abundant moisture into East Asia, consequently enhancing regional precipitation, particularly in southern China. In contrast, under a permanent La Niña–like phase, a westward‐intensified Walker Circulation generated a low‐level cyclone over the East China Sea and a coastal anticyclone, strengthening moisture transport into East Asia, with the most pronounced increases in precipitation occurring in central‐eastern China. We attribute the consistent wetting trends in East Asia across both ENSO phases to the increased moisture‐holding capacity of a warmer atmosphere, which enhances rainfall efficiency irrespective of differing circulation patterns. Our findings indicate a paradigm shift in the El Niño/La Niña–monsoon relationship, transitioning from the contemporary dipole/tripole precipitation response to a regime in which both ENSO phases result in a broad, monopole‐like increase in East Asian rainfall in a warming climate. This suggests a weakened El Niño/La Niña influence on the EASM precipitation under Pliocene‐like warming conditions. These evolving El Niño/La Niña–monsoon dynamics must be considered when interpreting proxy records from past warm periods and projecting future climate impacts.