Diversity of the drought–flood abrupt alternation: dynamic processes during the pre-flood season over South China

This study investigates the atmospheric circulation anomalies and dynamic mechanisms driving drought–flood abrupt alternation (DFAA) events during the pre-flood 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 middle troposphere, 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 middle troposphere, 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 pre-flood season over SC, thereby offering potential opportunities for improved predictions of extreme drought and flood occurrences during this season.