Recent Change in Summer Rainfall over the Tibetan Plateau: the Roles of External Forcing and Internal Variability

During the recent decades, summer precipitation has increased over the western and northeastern Tibetan Plateau (TP) and decreased over the southeastern TP. This study investigates the effects of external forcing and internal variability on summer rainfall changes over the TP during 1979-2013. It is shown that the variations in atmospheric circulation and precipitation over the TP are largely contributed by the anthropogenic forcing and oceanic boundary conditions and that the Interdecadal Pacific Oscillation (IPO) plays an important role in TP summer rainfall changes on decadal time scales. The sea surface temperature (SST) anomalies in the tropical central-eastern Pacific induce a wave train consisting of anomalous cyclones and anticyclones over Eurasia. The anomalous anticyclone to the northeast TP weakens westerlies, facilitating the convergence of water vapor over the TP. Further analysis indicates that the IPO-related SST anomalies contribute partly to the uncertainty in simulations of summer rainfall changes over the TP in recent decades using a 40-member ensemble of the Community Earth System Model (CESM). Consequently, the IPO can be used as one of the predictive factors of TP summer rainfall on decadal time scales.