东亚副热带季风季节循环年际变化对华北夏季降水的影响

Interannual Variation in the Seasonal Cycle of the East Asian Subtropical Monsoon and Its Impact on Summer Rainfall in North China

由于受到东亚副热带夏季风复杂变化的影响,华北夏季降水的年际变化预测一直是我国季节气候预测的难点。本文采用1979~2020年中国站点日降水数据和CRA-40大气再分析资料,通过谐波分解、MV-EOF分析等统计方法,研究了东亚副热带季风季节循环年际变化对华北夏季降水的影响。结果表明:在气候态上,华北降水相关的季节循环在850 hPa风场上主要表现为东南风和西南风主导两种模态,其中,西南风主导的降水在7月初达到峰值,而东南风主导的降水在7月底达到峰值。在年际尺度上,季节循环前两个主模态中的华北降水虽然表现为区域一致性变化特点,但在夏季先后受到西南风和东南风的影响。通过对季节循环华北夏季风雨季的开始时间(P1)、峰值时间(P2)、结束时间(P3)、持续时间(Dur)和振幅(Amp)与夏季(6~8月)降水年际变化分析,我们发现东南风主导的季节循环的P1、P3和Amp与华北夏季降水异常呈显著正相关,而P2和Dur与华北夏季降水异常呈负相关。西南风主导的季节循环的P2、P3和Amp与华北夏季降水异常均呈显著正相关。其中,东南风季节循环的位相(P1、P2、P3)变化与夏季西南风的强度有关,而其幅度(Amp)变化主要取决于东南风的强度。由于东南风主导的夏季降水开始于每年的4~5月份,其建立的早晚为6~8月夏季降水的季节预测提供了一个新的参考指标。

Owing to the complicated changes in the East Asian summer monsoon and its chaotic influence,the accurate forecast of summer rainfall anomalies in North China is challenging.To understand the origin of summer rainfall anomalies in this region,we applied the daily observed rainfall data,CRA-40 atmospheric reanalysis,harmonic analysis,and MV-EOF analysis for examining the impact of the seasonal cycle of the East Asian subtropical summer monsoon(EASSM)on the summer rainfall anomalies in North China during 1979–2020.Our results show that,in climatology,the rainfall-related seasonal cycle of winds at 850 hPa in North China exhibits two dominant modes:southwesterly wind and southeasterly wind,which peak in early and late July,respectively.On interannual time scale,although the rainfall of first two modes exhibits consistent change,it is successively affected by southwest and southeast wind anomalies.Based on the analysis of the onset date(P1),peak date(P2),retreat date(P3),duration(Dur),and amplitude(Amp)of the seasonal cycle modes during the rainy season and the summer monsoon rainfall(from June to August)anomalies in North China,we found that the P1,P3 and Amp of the southeast wind mode have a significant positive correlation with the summer rainfall anomalies in North China,while P2 and Dur are negatively correlated with the summer rainfall anomalies in North China.The P2,P3,and Amp of the southwest wind mode show significantly positive correlations with the summer rainfall anomalies in North China.The phase(P1,P2,and P3)variations of southeast wind are related to the intensity of the southwest wind in summer,whereas the variation of Amp mainly depends on the intensity of the southeast wind.Since the summer rainfall dominated by the southeast wind starts from April to May,the establishment of the EASSM related to the rainfall in North China provides a new index for the seasonal forecasts of summer rainfall in North China.