夏季黄河流域降水气候特征及其与大气环流的关系

Spatio-Temporal Variations of Summer Rainfall over Yellow River Valley and Its Association with Atmospheric Circulation

本文基于1958—2015年夏季黄河流域55个观测站降水量和NCEP/NCAR再分析1高度场等资料,使用MannKendall突变检验、合成分析和Monte Carlo检验等气候统计方法,分析了黄河流域58年夏季降水量的气候变化特征,以及导致其变化的大气环流成因。58年期间,黄河流域夏季降水量总体呈减少趋势,尤其在河套北部有显著性减少趋势,其主要原因是欧亚中高纬度等压面升高、西风带减弱所致;1975年和1996年是黄河流域夏季降水的两个明显年代际气候变化转折点,在1958—1975年期间,黄河流域夏季降水量年际变化大,异常偏多和偏少年出现频次较高,期间欧亚中高纬度及其以南包括黄河流域地区高度场偏低,主要受高空低压系统和较强冷空气影响;在1976—1995年期间,黄河流域大部降水偏多,其主要环流成因为乌拉尔山阻塞高压发展、贝加尔湖到东北亚一带受负高度距平控制高空槽加深,同时,来自南方的暖湿气流输送增强;到1996—2015年最近20年间,乌拉尔山北部环流高度场偏低、里海至贝加尔湖再到东北亚一带高度场一致偏高,黄河流域一带西风带强度和冷空气势力均较弱,流域受高压影响导致大部区域降水偏少。不同时期黄河各流域段降水量与中高纬度阻塞高压以及与西北太平洋副热带高压的相关关系分析进一步说明了上述结论。

With the recent 58 summers data from 1958--2015, including precipitation from 55 observing stations over the Yellow River Valley and height field of NCEP/NCAR reanalysis 1 data etc. , the charac- teristics of interannual variations for the summer rainfall in the Yellow River Valley and their causes asso- ciated with the synchronous variations of atmospheric circulations are analyzed by Mann-Kendall test, composite analysis and Monte-Carlo test methods in this paper. The results show that the reasons why the summer rainfall in the Yellow River Valley has been decreasing since 1958, in particular, significantly de- creasing trend in the northern bend of Yellow River, are derived from the rising isobaric surface and weakened westerlies over the mid-high latitude of Eurasia. The climatic changes of the 58-year rainfall in Yellow River Valley is divided into three different periods by two abrupt points in 1975 and 1996 according tonatural changes in itself. During the period of 1958--1975, the low pressure circulations with stronger cold air dominated the Yellow River Valley and led to the larger interannual variations of anomalous precipi- tation whatever the above and below normal. In the phase 1976--1995, the reasons why the above normal rainfall happened to most sections of Yellow River Valley, are the development of Ural blocking high, and the negative height anomalies in troposphere dominated the regions from Baikal to Okhotsk, creating a deep trough over the Yellow River Valley. In addition, the warm and moist flow from South became stron- ger. In the recent 20 years, 1996--2015, most regions of Yellow River Valley suffered less rainfall, mainly due to the decreased heights of circulations over the northern Ural, and meanwhile the isobaric surface rising together over the areas from Caspian Sea to Baikal and to Okhotsk accompanied with weakened wes- terlies, which resulted in the weakened cold air and enhanced high pressure circulations controlling the Yellow River Valley. Moreover, the correlation analyses of rainfall in four basins of the Yellow River Val- ley with mid high latitude blocking highs and the subtropical high in three different decades evidence the above conclusion further.