Prolonged seasonal drought events over northern China and their possible causes

摘要

The features and possible causes of prolonged seasonal drought (PSD) events over northern China (NC) during 1960-2012 are explored in this study using the multi-scale standardized precipitation evapotranspiration index (SPEI). The results show that PSD events (i.e., drought events with a timescale of 9 months or less) have had a relatively high probability of occurrence from spring to autumn during the past five decades. The results of the empirical orthogonal function (EOF) analysis show that the leading mode of the SPEI09 in autumn reflects a broad pattern of drought in NC, which confirms the existence of the PSD events. Furthermore, we conclude that two factors, teleconnection patterns in the mid-latitudes of Eurasia and transitions from La Nina to El Nino, are mainly responsible for the PSD events. On the one hand, the associated teleconnection patterns in the upper-level troposphere show an Eurasian (EU)-like pattern in spring, the Silk Road (SR) pattern or the circumglobal teleconnection (CGT) pattern in summer, and a Scandinavia (SCA)-like pattern in autumn, respectively. Associated with these teleconnection patterns, descending anomalies are detected in the whole atmosphere over NC in all seasons. On the other hand, the transition from La Nina (winter) to El Nino (the following autumn) also exerts effects on the PSD through modulating the Walker circulation and the meridional vertical circulation along the coast of East Asia. However, this process is more effective in spring relative to summer and autumn. In addition, physical connections likely exist among these factors, and their combined effects, together with the insufficient moisture supply, regulate the PSD events over NC.