Relationship between the Southern Indian Ocean Dipole and ENSO and their effect on summer precipitation in China

Based on reanalysis data from 1979 to 2021, this study explores the spatial distribution of the Southern Indian Ocean Dipole (SIOD) and its individual and synergistic effects with the El Niño–Southern Oscillation (ENSO) on summer precipitation in China. The inverse phase spatial distribution of sea surface temperature anomalies (SSTAs) in the southwest and northeast of the southern Indian Ocean is defined as the SIOD. Positive SIOD events (positive SSTAs in the southwest, negative SSTAs in the northeast) are associated with La Niña events (Central Pacific (CP) type), while negative SIOD events (negative SSTAs in the southwest, positive SSTAs in the northeast) are associated with El Niño events (Eastern Pacific (EP) type). Both SIOD and ENSO have certain impacts on summer precipitation in China. Precipitation in the Yangtze River basin decreases, while precipitation in southern China increases during pure positive SIOD (P_PSIOD) events. During pure negative SIOD (P_NSIOD) events, the changes in precipitation are exactly the opposite of those during P_PSIOD events, which may be due to differences in the cross-equatorial flow in the southern Indian Ocean, particularly in low-level Australian cross-equatorial flow. When positive SIOD and CP-type La Niña events occur simultaneously (PSIOD+La_Niña), precipitation increases in the Yangtze–Huaihe River basin, while it decreases in northern China. When negative SIOD and EP-type El Niño events occur simultaneously (NSIOD+El_Niño), precipitation in the Yangtze–Huaihe River basin is significantly lower than during P_NSIOD events. This is caused by differences in water vapor originating from the Pacific Ocean during different events. 摘要 基于1979年至2021年的再分析数据, 本文探讨了南印度洋偶极子 (SIOD) 的空间分布及其与厄尔尼诺-南方涛动 (ENSO) 对中国夏季降水的独立和协同影响. 南印度洋西南部和东北部海表面温度异常 (SSTAs) 的反相位空间分布被定义为 SIOD. 正SIOD事件 (西南部正SSTAs, 东北部负SSTAs) 多伴随La Niña事件, 且主要为中太平洋 (CP) 型; 而负SIOD事件 (西南部负SSTAs, 东北部正SSTAs) 则多伴随El Niño事件, 且主要为东太平洋 (EP) 型. SIOD和ENSO对中国夏季降水均有一定影响. 纯正SIOD (P_PSIOD) 事件期间, 长江流域降水减少, 而华南降水增加. 纯负SIOD (P_NSIOD) 事件期间, 降水变化与P_PSIOD事件相反, 这可能与越赤道气流 (特别是澳大利亚低空越赤道气流) 有关. 当正SIOD与CP型La Niña事件同时发生时 (PSIOD+La_Niña), 江淮流域降水增加, 而华北降水减少. 当负SIOD与EP型El Niño事件同时发生时 (NSIOD+El_Niño), 江淮流域降水明显低于P_NSIOD事件期间, 这主要归因于不同事件背景下太平洋水汽输送的差异.