Aggravation of Record-Breaking Drought over the Mid-to-Lower Reaches of the Yangtze River in the Post-monsoon Season of 2019 by Anomalous Indo-Pacific Oceanic Conditions

The Mid-to-Lower Reaches of the Yangtze River (MLRYR) suffered an extreme drought in the post-monsoon season of 2019, contemporaneous with a strong Central Pacific (CP) El Nino and a super positive Indian Ocean Dipole (pIOD) event. The present work shows that CP El Nino-related Pacific Sea Surface Temperature Anomalies (SSTAs) weakened the western North Pacific anticyclone to prevent moisture transport to the MLRYR and explained 60% of the drought intensity. The super pIOD with an extremely cold SSTA in the tropical Southeastern Indian Ocean contributed 40% of the drought amplitude via an atmospheric teleconnection. The Indian Ocean cold SSTAs first strengthened the post-monsoon rainfall and enhanced diabatic heating over South Asia, leading to baroclinic circulation anomalies with induced descending motion over the MLRYR. The aggravated dry conditions there ultimately broke the historical drought record for the period since 1979. Plain Language Summary In the post-monsoon season of 2019, the Mid-to-Lower Reaches of the Yangtze River (MLRYR) experienced a record-breaking drought, which severely disrupted water supplies and affected the planting of crops. At the same time, a super positive Indian Ocean Dipole (pIOD) event occurred, along with a central Pacific (CP) El Nino in the tropical Pacific. The present study indicates that in addition to the CP El Nino, the extremely cold SSTAs in the tropical Southeastern Indian Ocean associated with the super pIOD event was also an important factor in the record-breaking drought event. This factor first shifted the intertropical convergence zone northward to intensify the post-monsoon rainfall and its released condensation heating over South Asia. Then, a vertically baroclinic circulation was stimulated to strengthen a descending motion over the MLRYR via an atmospheric teleconnection. On the other hand, the tropical Pacific warm SSTAs related to the strong CP El Nino weakened the western North Pacific anticyclone, which reduced the moisture supply to the MLRYR. In this way, both the pIOD and CP El Nino events jointly resulted in the record-breaking MLRYR drought in 2019 and explained approximately 40% and 60% of this extreme drought, respectively.