Dominant synoptic-scale circulation patterns and their contributions to the increasing wet-season precipitation over the three-river source region

The Three-River Source Region (TRSR), the core of the “Asian Water Tower”, plays a critical role in Asian water cycle and sustains the water security of billions downstream. Using the self-organizing map method, this study identifies four synoptic patterns (SP1–SP4) during the wet seasons during 1979–2020 over TRSR and investigates the precipitation characteristics, physical mechanisms, and contributions to the total precipitation increase under different synoptic patterns. Results show that SP2 dominates in occurrence frequency and accounts for 36.41% of total precipitation. Although less frequent, SP1 and SP4 produce higher rainfall intensities, accounting for 28.25% and 19.90% of the total precipitation, respectively. SP3 has the lowest frequency and weakest precipitation, contributing only 15.44%. Mechanistically, SP1 features strong moisture transport and uplift linked to deep monsoon trough and westerly ridge, yielding the highest rainfall. SP4, influenced by a strong westerly trough combined with a monsoon trough, produces the second-highest rainfall. In contrast, SP2 and SP3 are linked to weaker monsoon activity and moisture supply, leading to limited precipitation. Trend analysis reveals that the total precipitation increase is largely driven by SP2 and SP3 via increased occurrence frequency and precipitation intensity, the latter fueled by increased precipitable water and enhanced upward motion. Conversely, SP4 exerts a negative contribution, as its declining frequency and precipitation intensity are driven by reduced moisture. This study provides a circulation-pattern-based framework for understanding the heterogeneous wetting of TRSR, offering essential guidance for interpreting past changes, improving future projections, and informing regional water management and climate adaptation.