Integrated large-scale circulation impact on rainy season precipitation in the source region of the Yangtze River

Monthly precipitation data at regular grids of 0.5 degrees x 0.5 degrees derived from observations during June-August 1961-2016 were used to reveal characteristics of large-scale circulations associated with rainy season precipitation over the source region of the Yangtze River (SRYR). The integrated impact of major influencing circulation patterns was examined by principal component analysis and composites. Results showed that the first rainy season precipitation mode associates with the Southern Oscillation Index (SOI) and the Pacific Decadal Oscillation (PDO), explaining 64% of spatial and temporal rainy season precipitation variance in the region. Composites of precipitation pattern under different phases of SOI and PDO revealed that the effect of PDO on precipitation varies with the SOI phase. When in phase with the SOI, PDO-induced precipitation anomalies are magnified. When out-of-phase, anomalies weaken or even disappear. Composites of moisture flux patterns show that large-scale atmospheric circulation affects the strength of westerlies that transport moisture to the study area and formation of convergence. In coming decades, the PDO is likely to continue in a negative phase with La Nina (positive SOI) events, implying more precipitation during the rainy season. Consequently, this knowledge can be used to improve decision making regarding water supply and flood risk management in the SRYR.