Recently Intensified Extreme Precipitation in Late Spring in the Hengduan Mountains

Using multiple sources of daily precipitation datasets, ERA5 reanalysis data, and HadISST sea surface temperature data from 1979 to 2024, this study identifies a significant increasing trend in May precipitation over the Hengduan Mountains (HM). The contribution of extreme precipitation (R95p) to total precipitation (PRCPTOT) increased at a rate of 1.48% (10yr)⁻¹. A regime shift occurred around 1998, after which PRCPTOT and R95p increased by 18.2% and 46.9%, respectively. This increase is primarily driven by vertical and horizontal moisture advection. Thermodynamic effects (increased moisture) dominate the southwestern Yunnan portion of HM, while dynamic effects (anomalous ascent) are more prominent in its Tibetan portion. Furthermore, R95p exhibits higher sensitivity to specific humidity than PRCPTOT, causing its contribution to total precipitation to rise from 17.76% to 22.07% post-1998. These changes are linked to the phase shifts of the Atlantic Multidecadal Oscillation (AMO) to positive and the Pacific Decadal Oscillation (PDO) to negative in the late 1990s. This combination triggered wave activity flux, establishing a "high-level divergence, low-level convergence" structure over HM and the Bay of Bengal. This structure facilitated the early establishment of the onset of the Bay of Bengal Summer Monsoon (BOBSM). Three pathways—BOBSM-induced cyclonic anomalies, enhanced upper-level westerlies, and southeasterly flow from the South China Sea—channeled moisture into HM. These results highlight the potential of AMO and PDO as interdecadal predictors for water resource management in this critical "water tower" region.