ENSO and IOD contributions to seasonal meteorological droughts over the Yangtze River basin

The Yangtze River basin (YRB) has experienced frequent extreme drought events in recent years. Many studies have explored the impact of anomalous tropical sea surface temperatures on seasonal meteorological droughts over the YRB, yet two critical issues remain: (1) the regimes of the El Nino-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) causing YRB droughts; (2) the timing and spatial pattern of YRB droughts caused by ENSO and IOD. To address these knowledge gaps, we employed a comprehensive investigation. First, the Severity-Area-Density (SAD) method is applied to identify seasonal drought events over the YRB during 1971-2022. Second, we utilize composite analysis to specify the effects of ENSO and IOD. Our findings indicate that YRB droughts are significantly increased in the August of combined Eastern Pacific (EP) El Nino and positive IOD developing years, October of Central Pacific (CP) El Nino developing years, January of EP La Nina years, and April of CP El Nino decaying years. Third, we conduct numerical simulations and conclude that ENSO and IOD events contribute to the YRB droughts by modulating large-scale circulation patterns, triggering the formation of cyclones and anticyclones over the Western North Pacific (WNP) and the Tibetan Plateau, and affecting the westerlies and other water vapour transport processes. These results on the physical mechanisms causing seasonal drought in the YRB can guide drought monitoring and prediction in the region.