Role of the Low-Latitude Quasi-Biweekly Oscillation in the Extreme Persistent Heavy Rainfall in the Mei-Yu Season over the Middle and Lower Reaches of the Yangtze River

Abstract

Abstract Persistent heavy rainfall events (PHREs), as a result of the interaction among multiscale systems, are prone to continuously affect the middle and lower reaches of the Yangtze River valley (MLYR). Based on the observation and reanalysis data, a total of 41 PHREs during the mei-yu seasons of 1979–2020 are first identified over the MLYR and divided into prolonged (over 5 days) and normal (3–5 days) groups. The contributions of quasi-biweekly-scale and synoptic-scale components to the abovementioned two types of PHREs are analyzed. Prolonged PHREs are dominated by the quasi-biweekly component of precipitation (QBW_Pr), while normal PHREs depend on synoptic-scale components (SS_Pr). The quantitative moisture budget indicates that the favorable environment for the QBW_Pr of prolonged PHREs is associated with the interaction between background moisture and quasi-biweekly circulation disturbances. Approximately 80% of prolonged PHREs occurred in phases 6–8 of the boreal summer QBWO (BSISO2) life cycle. The large-scale meridional vertical circulation along the South China Sea (SCS)–MLYR, and the westward-propagating suppressed convection at low latitudes, accompanied by the substantial northward and upward moisture supply, significantly promotes and maintains the QBW_Pr for prolonged PHREs. In contrast, the QBWO signals are relatively insignificant in the evolution of normal PHREs. The impact of the low-latitude QBWO on the prolonged PHREs is further confirmed by sensitivity experiments with RegCM4.9. As the QBWO are weakened on the southeast lateral boundary near the SCS, the vertical circulation and moisture transport clearly diminish along the SCS–MLYR. Consequentially, precipitation reduces visibly during prolonged PHREs. Significance Statement Persistent heavy rainfall events (PHREs), as the reflection of the interaction among multiscale circulation systems, characterized by high intensity and wide coverage, are prone to cause severe flooding and death in the middle and lower reaches of the Yangtze River valley (MLYR). This study aims to investigate the connection between PHREs and different time scales of atmospheric variability, thus elucidating the crucial factors and the influencing mechanisms for the prolonged PHREs during the mei-yu season. Our results reveal that the prolonged PHREs are dominated by the quasi-biweekly component of precipitation, while normal PHREs depend on the synoptic-scale component. The quasi-biweekly precipitation of prolonged PHREs is promoted and maintained by the meridional vertical circulation along the South China Sea (SCS)–MLYR, which is accompanied by the westward-propagating SCS QBWO-related suppressed convection at low latitudes. This study highlights the specific role of the low-latitude QBWO to the formation of prolonged PHREs relative to normal PHREs.