Probabilistic associations between meteorological drought and compound moisture sources-transport anomalies in the middle and lower Yangtze River

Moisture-transport anomalies are closely associated with meteorological drought, but the distinct and compound roles of source-region supply and pathway variability remain insufficiently quantified. Using CMFD meteorological data and FNL-driven FLEXPART simulations for 2005–2024, this study developed a multi-index conditional-probability framework for the middle and lower reaches of the Yangtze River (MLYR). Three standardized 10-day indices represented source-region precipitation contribution (SPC), moisture pathway intensity (MPI), and moisture pathway frequency (MPF). Gaussian copulas were used to estimate conditional drought probabilities under individual and paired moisture anomalies. Climatologically, Pathways 2 and 3 accounted for 45.4% of total pathway transport, while the Northwest Pacific and southern China–Indochina Peninsula together contributed 35.9% of source moisture. However, negative anomalies in the South China Sea and Pathway 2 were associated with higher individual drought probabilities than those in the Northwest Pacific and Pathway 3, despite their smaller climatological shares. Across 210 pair-matched configurations, compound conditional drought probability exceeded the stronger constituent single-anomaly probability in 181 cases (86.2%), with a median increase of 7.55 percentage points. After calendar-year-block permutation and BH-FDR correction, ΔP was significantly greater than zero in 164 pairs, and all nine top-ranked combinations exceeded 50% conditional probability. These results show that mean contribution magnitude was not a reliable indicator of anomaly-conditioned drought association and that elevated drought probability is frequently, but not universally, associated with concurrent weakening across moisture-transport components. The framework provides a process-oriented basis for diagnosing drought-relevant transport configurations and can be adapted to other basins after region-specific recalibration and independent validation.