欧亚土壤湿度异常对北半球大气环流的显著影响

用44 a ERA40再分析资料的土壤湿度和大气环流变量场, 研究持续性的欧亚大陆土壤湿度异常对后期北半球大尺度大气环流的反馈作用。首先,运用经验正交函数分解去除ENSO遥相关及趋势影响后,分析了欧亚大陆中高纬度土壤湿度变率主要模态的季节变化特征,及相对应主分量时间序列显示的土壤湿度异常的衰减时间,结果表明土壤湿度异常的主要模态在全年都表现出很好的连续性。其次,对不同季节的连续3个月的月平均土壤湿度和500 hPa高度场进行滞后最大协方差分析,研究欧亚地区中高纬度土壤湿度异常与北半球大气环流异常之间的线性耦合。第一最大协方差模态的结果表明:全年的主导信号是大气强迫土壤湿度的变化,但在冬季和夏季,大气中类似于负位相北极涛动的环流型与之前月份(最长达4个月)土壤湿度的持续变化显著相关。最后,基于土壤湿度变率中心的回归分析也证实了秋季和春季欧亚土壤湿度,特别是北非副热带,欧亚内陆和西伯利亚地区的土壤湿度异常,分别与其后的冬季和夏季的大气环流显著相关。欧亚大陆土壤湿度异常超前大气环流的信号,将有助于改善冬季和夏季北半球季节气候预报能力。

Significant influence of Eurasian soil moisture anomaly on Northern Hemisphere atmospheric circulation

By using data of soil moisture and atmospheric circulation variables field from the 44 a Medium Range Weather Forecasting (ECMWF) reanalysis dataset of European Centre, the possible feedback of persistent changes of Eurasian soil moisture on lagged large-scale atmospheric circulation was investigated. Firstly, after removing the effects of trends and teleconnections of the ENSO, empirical orthogonal function (EOF) analysis was used to examine the seasonal variations characteristics of the leading mode of the soil moisture variation rate at mid-high level in Eurasia and its decay time-scale in the associated principle component time series. Result showed that the leading EOF modes of Eurasian soil moisture anomalies exhibited considerable persistence through the whole year. Secondly, a lagged maximum covariance analysis on the monthly soil moisture and 500 hPa geopential height field for three consective months in different season was applied to investigate linear covariance between soil moisture anomaly at mid high level over Eurasia and atmospheric circulation anomaly in the northern hemisphere, it was concluded that the dominant signal was the atmospheric forcing on soil moisture anomaly throughout the whole year, but in winter and summer, the atmospheric signal resembling the negatively polarized Arctic Oscillation (AO) was significantly correlated with persistently changing soil moisture anomalies in Eurasia continent up to the four months in the leading MCA modes. Lastly, regression analysis based on the soil moisture anomaly centers from the MCA covariance patterns confirmed that spring (autumn) soil moisture in the regions of subtropical North Africa, western Europe and Siberia was significantly correlated to summer (winter) atmospheric circulation. The leading time of Eurasian soil moisture anomaly helps improve predictability of winter and summer atmospheric variability.