Moisture source partitioning reveals how human influence shapes the Tibetan Plateau summer precipitation dipole pattern

The Tibetan Plateau (TP), often termed the “Asian Water Tower”, is a critical reservoir and regulator of the Asian hydrological cycle. In recent decades, summer precipitation over the TP has exhibited a pronounced South Drying-North Wetting dipole pattern, with profound implications for regional water security and ecosystem stability. Both externally advected and internally recycled precipitation may contribute to this pattern. However, their respective roles and the extent to which anthropogenic forcing has shaped their contributions remain unclear. Here, we use the WAM2layers moisture-tracking model to partition TP summer precipitation into externally sourced and internally recycled components, and to quantify how changes in precipitation frequency and intensity shape the dipole. We find that the dipolar pattern is primarily driven by changes in externally sourced precipitation, which strengthens precipitation in the north while inducing drying in the south, with internally recycled precipitation further amplifying southern aridification. Specifically, increases in the frequency of externally sourced precipitation events lead to a plateau-wide precipitation increase. However, this effect is offset over the southern TP by a concurrent decline in event intensity, thereby shaping northward moistening associated with the externally sourced component. Meanwhile, the reduction of internally recycled precipitation in the southern TP is primarily attributable to a decrease in event frequency, while increases in the north result from simultaneous enhancements in both frequency and intensity. Mechanistically, a weakened subtropical westerly jet, due to spatially uneven emissions of anthropogenic aerosols, strengthens the dipole by enhancing externally sourced precipitation intensity over the northern plateau while suppressing it in the south. By contrast, negative phases of the Interdecadal Pacific Oscillation mainly reduce the frequency of internally recycled precipitation in the south. These findings reveal that anthropogenic forcing and natural variability jointly shape the TP summer precipitation dipole trend.