Abstract:

El Niño event has a significant impact on global climate, especially on regional droughts and floods, being an important source of signals for climate prediction. In order to understand the impact of El Niño on summer droughts and floods in North China, based on the monthly precipitation data of more than 2 400 stations from 1961 to 2022, the monthly sea surface temperature data reconstructed by NOAA and the reanalysis circulation data from NCEP/NCAR, this paper comprehensively studies the relationship between interannual variability of El Niño and summer droughts and floods in North China and its impact mechanism by using seasonal evolution empirical orthogonal function decomposition (SEOF), regression reconstruction of circulation anomalies, circulation composition analysis. The main results are as follows: (1) There is a significant negative correlation between the summer precipitation in North China and the El Niño state in summer of the current year. That is, El Niño begins to appear in the spring, develops in the summer and has a strong intensity, which results in less summer precipitation in North China and is prone to drought. Further analysis shows when the La Niña states in spring, summer, autumn and winter in the previous year change to El Niño states in spring, summer, autumn and winter in the current year, the summer precipitation in North China is significantly less and prone to drought. (2) El Niño affects summer precipitation in North China mainly by regulating the high and low level circulation. The circulation patterns of the 200, 500 and 850 hPa induced by El Niño in the summer of that year are consistent with the circulation patterns of the drought summer years in North China. (3) When the upper westerly jet at 200 hPa over North China and its north side is weak, the ascending motion caused by upper air disturbance will be obviously weak. The position of the western Pacific subtropical high at 500 hPa is southward, and North China is controlled by a circulation pattern of “high in the west and low in the east”. The low trough system moves eastward at a fast speed, which is not conducive to maintaining long-term precipitation processes in North China. The tropical Indian summer monsoon and the East Asian subtropical summer monsoon at 850 hPa are relatively weak, and North China lacks effective sources of water vapor transport. This high and low level circulation configuration will result in less precipitation in summer in North China, making it prone to drought.

Key words: El Niño events, interannual variability, North China, summer drought, circulation anomaly

摘要：

厄尔尼诺（El Niño）事件对全球气候尤其区域旱涝有重大影响，是开展气候预测的重要信号来源。为认识厄尔尼诺对华北夏季旱涝的影响规律和改进预测技术，基于1961—2022年2 400多测站月降水量、NOAA重构的月海表温度数据和NCEP/NCAR再分析环流数据等，采用季节演变经验正交函数分解（SEOF）、环流异常回归重构、环流合成分析等方法，综合分析厄尔尼诺事件年际变化与华北夏季旱涝的关系及其影响机制。结果表明：（1）华北夏季降水与当年夏季El Niño状态呈显著负相关，即当年春季El Niño开始出现，夏季发展且强度较强，则华北夏季降水显著偏少，易发生干旱。进一步分析发现，海表温度从上年春、夏、秋、冬La Niña型转为当年春、夏、秋、冬El Niño型时，华北地区夏季降水显著偏少，易发生干旱。（2）El Niño主要通过调节高、低空环流影响华北夏季降水，其诱发的当年夏季200、500、850 hPa环流型与华北夏季干旱年环流型一致。（3）当200 hPa华北地区及北侧西风急流偏弱时，由高空扰动造成的上升运动会明显偏弱；500 hPa西太平洋副热带高压位置偏南，华北处于“东低西高”环流型控制下，低槽系统东移速度快，不利于华北维持长时间降水过程；850 hPa热带印度夏季风、东亚副热带夏季风偏弱，华北缺乏有效的水汽输送。这种高、低空环流配置会造成华北夏季降水偏少，易发生干旱。

关键词: 厄尔尼诺事件, 年际变化, 华北, 夏季干旱, 环流异常