1.Institute of Atmospheric Sciences, Fudan University;2.Tianjin Climate Center;3.State Key Laboratory of Severe Weather and Institute of Climate System, Chinese Academy of Meteorological Sciences
Based on the homogenized daily precipitation in North China and ECMWF-ERA5 reanalysis data for 1961-2018, and a new monitoring standard that takes precipitation and the position of the Western Pacific subtropical high ridge into consideration, we calculated the rainy season precipitation in North China (RSPNC) and onset/ending date with the new monitoring method, and discussed the climatic characteristics of the water-vapor transport and associated interdecadal variations in precipitation and moisture budget. The temporal and spatial variations in water-vapor transport and 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 in each year, so as the periods of occurrence of the rainy season and the intraseasonal variation. (2) Precipitation is determined by large-scale atmospheric moisture transport and associated convergence. The critical four water-vapor pathways maintained the RSPNC including Indian monsoon, East Asian monsoon, trans-equatorial airflow between 110°E and 120°E, and mid-latitude westerlies near 40°N. (3) The RSPNC and water-vapor budget displays similar interdecadal variations, and abrupt climate changes occurred in 1977, 1987 and 1999, respectively, featuring a "reduction-increase-reduction" phase. The RSPNC is strongly correlated with the net water-vapor budget within the domain of North China. (4) The intensity of water-vapor flux and the arriving timing exert significant impacts on the amount of precipitation. The distribution patterns of water-vapor flux anomalies in rainy decades and rainless decades are distinct: in the rainy decades, the 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-high latitude westerlies in North China, and the water-vapor diverges more strongly than that in normal years. In terms of intraseasonal process, water-vapor fluxes are stronger in amplitude, reach North China earlier, weaken later, converge stronger, and have a longer lifetime. While in the rainless decades, the anomalous cyclonic circulation dominates the Northeastern China, 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 obviously strengthened; accordingly, the intraseasonal process shows the opposite characteristics. (5) Considering the four boundaries of water vapor transport, the southern and weastern 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 boundary, 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 swithes to input from northern boundary, which is essentially distinct from that in the rainy decades.