Mechanisms of ENSO‐Modulated Precipitation Seasonality in Southeastern China and Implications for Stalagmite δ 18 O Records

Southeastern China (SEC) lies within the core region of the East Asian monsoon and exhibits pronounced hydroclimatic variability associated with the El Niño–Southern Oscillation (ENSO). Instrumental and modeling studies indicate that ENSO influences regional precipitation primarily through changes in seasonality rather than annual mean precipitation. However, it remains unclear whether SEC stalagmite δ 18 O records reliably capture this signal to extend reconstructions of ENSO‐related variability beyond the instrumental period. Here we combine instrumental observations and simulated data from the Isotope‐incorporated Global Spectral Model (IsoGSM) to characterize the modern hydroclimatic response to ENSO and evaluate a high‐resolution stalagmite δ 18 O record (YQ15‐1, 1987–2015 CE) from Yongquan Cave, SEC. Modern analyses show that ENSO modulates SEC hydroclimate mainly by altering the ratio of East Asian Summer Monsoon (EASM) to Non‐Summer Monsoon (NSM) precipitation. El Niño events generally reduce EASM precipitation and enhance NSM precipitation, lowering the EASM/NSM ratio and enriching precipitation δ 18 O (δ 18 O p ), whereas La Niña produces the opposite pattern. These seasonal changes are closely linked to ENSO‐related variations in large‐scale circulation, particularly shifts in the Western Pacific Subtropical High (WPSH) that regulate moisture transport and convective activity. The stalagmite YQ15‐1 δ 18 O record is broadly consistent with this ENSO‐driven precipitation seasonality, exhibiting positive (negative) δ 18 O anomalies during El Niño (La Niña) phases that align with the instrumental EASM/NSM precipitation ratio. These results indicate that stalagmite δ 18 O in SEC records the integrated seasonal response to ENSO, providing a process‐based framework for interpreting speleothem records in terms of past ENSO‐related hydroclimate variability.