基于自动识别的近三十年华北地区冬季冷锋活动异常及其可能成因

利用1989—2018年ERA5再分析资料，使用适用于欧亚大陆的冷锋两步客观识别算法，得到冬季冷锋活动数据集。在此基础上，分析了近30年来华北地区冬季冷锋的活动特征，并讨论了冷锋活动异常的可能成因。结果表明:（1）华北地区是东亚大陆上冬季冷锋频数最多、活动最强的区域，且华北地区冷锋活动强度有明显的年际变化；（2）华北地区冷锋活动强年，从北大西洋到欧亚大陆上空存在一个明显的欧亚遥相关（EU）波列正位相，华北地区上空500 hPa为负位势高度异常，表明东亚大槽增强，相应的西伯利亚高压偏强；（3）欧亚大陆北部边缘海区为异常暖平流时，华北地区冷空气活动增强，冷锋活动也增强；（4）大西洋北部的海面温度（SST）异常可能通过激发对流层中、上层的罗斯贝波，该罗斯贝波向东传播经欧洲—西西伯利亚传至中国华北地区，从而影响华北地区的冷锋活动强度。 

Winter cold front activities and associated circulation characteristics in North China in the recent 30 years detected by an automatic identification method

In this study, the dataset of winter cold front activities in Eurasia is obtained by applying the two-step cold front identification method to the ERA5 reanalysis data for the period from 1989 to 2018. Moreover, the characteristics of winter cold front activities in North China in the recent 30 years are analyzed, and possible mechanisms for cold front activity anomalies are discussed. The results show that North China is the key area with the highest frequency of cold fronts in the East Asian continent. The frequency index of cold front activities in North China has an obvious interannual variation. In the years of strong cold front activities in North China, the Eurasia teleconnection pattern from North Atlantic to Eurasia is obviously in positive phase, and negative geopotential height anomalies appear at 500 hPa over North China. It indicates an enhancement of the East Asian trough and a stronger Siberian High in surface. When anomalously warm advection prevails over the ocean in the northern margin of Eurasia, cold air activities and the frequency of cold fronts both increase in North China. The SST anomalies in the northern Atlantic Ocean may trigger Rossby waves in the middle and upper troposphere, which propagate eastward and reach North China via Europe and western Siberia, affecting the frequency of cold fronts in North China.