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.