华北雨季降水年代际变化与水汽输送的联系

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

本文基于1961~2018年华北地区均一化逐日降水资料和ECMWF(欧洲中期天气预报中心)ERA5全球再分析环流资料,采用一种综合考虑降水量和西太平洋副热带高压脊线影响的雨季监测标准,计算了华北雨季起讫日期和降水量,在此基础上讨论了华北雨季季节内进程的水汽输送特征。重点分析了降水量与水汽收支的年代际变化关系,揭示了水汽输送的时空变化规律及其对华北雨季降水的影响。研究结果表明:(1)华北雨季每年的起讫日期不同,从而每年雨季发生时段和季节内进程不同。(2)降水的形成与水汽输送及其辐合密切相关,有四个水汽通道维持华北雨季降水,即印度季风水汽、东亚季风水汽、110°E~120°E之间越赤道气流向北的水汽输送和40°N附近中纬度西风带水汽。(3)华北雨季降水和净水汽收支具有相似的年代际变化特征,分别在1977、1987、1999年发生突变,总体呈现“减—增—减”的阶段性变化趋势,两者位相转变相关性很强。(4)水汽输送的强弱和到达华北时间的早晚均对雨季降水多寡有重要影响。华北多雨年代与少雨年代水汽通量有明显的差异,主要表现在:在多雨年代,西北太平洋为反气旋式环流异常,偏南水汽强盛,并且与中高纬西风带异常偏西水汽汇聚于华北,华北地区水汽辐合偏强;考虑季节内进程,水汽到达华北的时间早、强度大,停留时间长、辐合强,减弱的时间晚;而在少雨年代,我国东北地区、朝鲜半岛及日本海附近为气旋式环流异常,华北地区由南向北的水汽输送偏弱,水汽辐散明显加强;季节内进程表现出与多雨年代相反的特征。(5)考虑华北地区四个边界的水汽收支,南边界和西边界有最大、次大水汽输入,二者的年代际变化是影响雨季降水年代际变化的重要因素。在多雨年代,南边界和西边界水汽净输入很强,但北边界的输出也很强;在少雨年代,南边界和西边界水汽净输入很弱,但北边界转为输入,这是区别于多雨年代的重要特征。

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.