Relationship between the Interdecadal Variation of Rainy Season Precipitation and Water Vapor Transport in North China

We calculated the rainy season precipitation in North China (RSPNC) and the onset/ending dates through a new monitoring method based on the homogenized daily precipitation in North China and 1961–2018 ERA5 reanalysis data, and a new monitoring standard that considers precipitation and the position of the Western Pacific subtropical high ridge. Moreover, we analyzed the climatic characteristics of water vapor transport and associated interdecadal variations in precipitation and moisture budget. The temporal and spatial variations in water vapor transport and the associated impact on RSPNC were further investigated. The main results can be summarized as follows: (1) The onset/ending dates of the rainy season in North China are distinct each year; therefore, the periods of the rainy season and the intraseasonal variation are also distinct. (2) Precipitation is determined by large-scale atmospheric moisture transport and the associated convergence. The critical four water vapor pathways, including Indian monsoon, East Asian monsoon, transequatorial airflow between 110°E and 120°E, and mid-latitude westerlies near 40°N, maintained the RSPNC. (3) The RSPNC and water vapor budget exhibits similar interdecadal variations, and abrupt climate changes occurred in 1977, 1987, and 1999, featuring a “reduction–increase–reduction” phase. The RSPNC is strongly correlated with the net water vapor budget within the North China domain. (4) The intensity of water vapor flux and the arriving time significantly affect the precipitation amount. The distribution patterns of water vapor flux anomalies in rainy decades and rainless decades are distinct: In the rainy decades, anomalous anticyclonic circulation dominates the Northwest Pacific, and the northward water vapor transport is strong, which converges with the eastward water vapor transport over mid to high latitude westerlies in North China, and the water vapor diverges more strongly than that in normal years. In terms of intraseasonal processes, water vapor fluxes are stronger in amplitude, reach North China earlier, weaken later, converge stronger, and last longer. In the rainless decades, anomalous cyclonic circulation dominates Northeast China, the Korean Peninsula, and the area around the Sea of Japan, and it turns into a weaker-than-usual northward water vapor transport, and the water vapor divergence is considerably strengthened. The intraseasonal process shows the opposite characteristics. (5) Considering the four boundaries of water vapor transport, the southern and western boundary water vapor inputs are the largest and the second-largest, respectively. Their interdecadal variations are critical for the interdecadal variation of the RSPNC. In rainy decades, there are stronger inputs of water vapor from the southern and western boundaries but strong output from the north boundary; however, in rainless decades, water vapor inputs are weak from the southern and western boundaries, and the output switches to input from the northern boundary, which is essentially distinct from the case in the rainy decades.