Diversity of the Drought–Flood Abrupt Alternation: Dynamic Processes during the Preflood Season over South China

Abstract This study investigates the atmospheric circulation anomalies and dynamic mechanisms driving drought–flood abrupt alternation (DFAA) events during the preflood season from 1979 to 2021 over South China (SC). Our results demonstrate that vertical dynamic effects are the primary drivers of these events, with distinct physical mechanisms across different stages. In drought-to-flood (DTF) events, enhanced negative (positive) zonal vorticity advection from the lower- to midtroposphere, caused by anticyclonic (cyclonic) circulation anomalies over SC during drought (flood) stages, plays a pivotal role in switching between anomalous descending and ascending motions. In contrast, in flood-to-drought (FTD) events, ascending motions during flood stages are primarily induced by zonal warm advection from the interaction between lower-level anticyclonic anomalies over the northern South China Sea and cyclonic anomalies over SC. During drought stages of the FTD events, descending motions are sustained by multiple factors: reduced positive meridional vorticity advection from the lower- to midtroposphere, upper-tropospheric cold advection by anomalous easterly winds, and lower-tropospheric cold advection linked to anomalous zonal temperature gradients—all associated with barotropic anticyclonic anomalies over SC. These insights enhance our understanding of the complex dynamics underlying DFAA events during the preflood season over SC, thereby offering potential opportunities for improved predictions of extreme drought and flood occurrences during this season. Significance Statement This study reveals the dynamic driving mechanisms of drought–flood abrupt alternation (DFAA) events during the preflood season over South China (SC), emphasizing the critical role of vertical dynamic effects. Drought-to-flood (DTF) events are primarily controlled by zonal vorticity advection, characterized by rapid transitions from anticyclonic to cyclonic conditions in the lower–midtroposphere, resulting in shifts in vertical motion. However, flood-to-drought (FTD) events are governed by warm and cold temperature advection, arising from changes between southwest moisture transport and barotropic anticyclonic dominance. These processes drive transitions between ascending and descending motions, leading to precipitation extremes. By elucidating these mechanisms, this study advances the understanding of preflood season dynamics and provides valuable insights for improving drought and flood prediction and management in SC.