The impact of mid-high-latitude Eurasian ISO on soil moisture anomaly in North China during boreal summer

The feature of the intraseasonal oscillation (ISO) over mid-high-latitude Eurasia and its impact on the soil moisture (SM) in North China during boreal summer are investigated. The dominant pattern of the ISO over mid-high-latitude Eurasia in boreal summer is obtained by an empirical orthogonal function analysis. It is characterized by a southeastward propagation with a quasi-barotropic structure. Based on the ISO phases, it is found that the southeastward-propagating ISO can significantly affect the vertical motion and the precipitation-related moisture condition, and thus change the SM anomaly in North China. A diagnosis of the moisture budget reveals that the moisture variation related to the SM anomaly is mainly attributed to the advection of the mean moisture by the meridional ISO flow which comes from mid-high latitudes. Moreover, the probability density functions of the SM anomaly calculated by subseasonal to seasonal models prove the significant regulation of the mid-high-latitude ISO on the SM anomaly. The above results inspire us to use the southeastward-propagating ISO signals as the predictors of the extended-range SM anomaly prediction by a lag-regression model. The results show that the forecast using the mid-high-latitude ISO signals improves the prediction skill of the SM anomaly, which is better than that using local predictors.