The combined effect of multiple water vapor transport channels can better reflect the variability in summer precipitation over North China

Water vapor plays an important role in the cloud precipitation process. In summer, the water vapor over North China is mainly delivered by westerlies and East Asian summer monsoon (EASM), but the former is often overlooked. In this study, using empirical orthogonal function analysis, we found that the summer precipitation as well as the water vapor transported by westerlies and the EASM showed substantial variability in North China during 1979-2019. Additionally, the corresponding principal component of water vapor transport is significantly correlated with the time series of precipitation, indicating the combined water vapor flux transported by westerlies and the EASM dramatically affects precipitation in North China. Further analysis revealed that the combined water vapor transport is driven by the circumglobal teleconnection pattern. Using numerical simulations, we quantified the relative effects of water vapor transported by westerlies and the EASM and explored the mechanisms involved. Sensitivity experiments demonstrated that with a 50% reduction in westerly-driven water vapor transport, the precipitation in North China will decrease by 19.59%, and if the water vapor transported by the EASM is likewise halved, the precipitation will decrease by 38.31%. Moreover, when the combined westerly and EASM-driven water vapor transport is reduced by 50%, the precipitation will decrease by 48.90%, which is more severe than the effect of reducing either water vapor transport individually. Therefore, the effect that the water vapor transported by westerlies has on precipitation in North China cannot be ignored and should be considered together with the water vapor transported by the EASM.