How NSIOD and El Niño Jointly Modulate Spring Precipitation Over Southeastern China: Mechanisms Revealed by Observations and CESM Simulations

Using observational data (1979–2024) and simulations from the Community Earth System Model (CESM1.0.4), this study investigates how the Negative Southern Indian Ocean Dipole (NSIOD) and El Niño jointly modulate spring precipitation over Southeastern China. Results show that under the co‐occurrence of NSIOD and El Niño, a synergistic response is triggered, generating extensive and intense positive precipitation anomalies over Southeastern China, with the regional mean anomaly reaching +1.626 mm day −1 , which exceeds the sum of the anomalies induced by each event individually. Under their combined influence, significant sea level pressure anomalies around 25° S–35° S enhance cross‐equatorial flow, strengthen the South China Sea‐Southeast Asia moisture channel and accelerate the onset of the South China Sea Summer Monsoon. Simultaneously, sea surface temperature (SST) warming in the equatorial central‐eastern Pacific induces anomalous subsidence over the western Pacific via a local adjustment of the Walker Circulation, which in turn intensifies the western North Pacific subtropical high. These conditions establish persistent anomalous southwesterlies along China's southeastern coast, forming a steady warm moisture transport belt. Quantitative analysis reveals that concurrent NSIOD and El Niño events produce the strongest 500 hPa geopotential height anomalies over the key western North Pacific region, with a regional mean of +14.0 gpm, as well as the strongest moisture convergence over Southeastern China, with a regional mean of −22.243 × 10 −5 kg m −2 s −1 . Additionally, anomalous meridional wind convergence within 20° N–30° N drives ascending motion, which aligns with the anomalous upper‐level outflow branch of the Hadley circulation and further enhances regional convection. Supported by both observations and CESM experiments, these interconnected processes collectively promote the most extensive and intense positive spring precipitation anomalies over Southeastern China during the co‐occurrence of NSIOD and El Niño events.