Decadal strengthening of the preceding-winter ENSO influence on subsequent pre-flood-season precipitation over southern China. Part I: Amplified El Niño teleconnection due to enhanced convective sensitivity under a warmer mean state

The pre-flood season (PFS; April–June) marks the onset of the rainy season over southern China (SC). Using observational and reanalysis datasets, we show that the relationship between El Niño-Southern Oscillation (ENSO) and subsequent PFS precipitation over SC is nonstationary, exhibiting a pronounced strengthening since the mid-1990s. This two-part study investigates the mechanisms responsible for the strengthened influences of El Niño and La Niña, respectively. Part I specifically elucidates the physical causes behind the intensified impact of El Niño. We find that this decadal enhancement primarily arises from the warmer background sea surface temperature (SST) over the equatorial Pacific. Although El Niño-related SST anomalies are comparable between P1 (1959–1995) and P2 (1996–2021), the precipitation response over the Niño4 region is substantially stronger in P2 because of the warmer background. This enhanced convective heating induced an intensified low-level cyclonic anomaly to its west, whose northeasterly anomalies bolster the climatological trade winds and induce more pronounced SST cooling over the western North Pacific (WNP). The resulting suppressed convection over the WNP stimulates a stronger anomalous low-level anticyclone over the South China Sea-WNP (SCS-WNP), enhancing southwesterly moisture transport into SC and thereby producing wetter PFS conditions. In contrast, the Niño4 convection response and the associated circulation anomalies are weaker in P1, yielding a weaker SCS-WNP anticyclone and a negligible impact on SC PFS precipitation. These results underscore the pivotal role of the background mean state in modulating El Nino’s teleconnections, and the strengthened impact of El Niño could help improve the prediction skill of the SC PFS precipitation.