An assessment of the impact of Pacific Decadal Oscillation on autumn droughts in North China based on the Palmer drought severity index

This study assesses the impact of the Pacific Decadal Oscillation (PDO) on the severity of autumn droughts in North China (ADNC) during the period of 1962-2013. The ADNC are defined using the Palmer drought severity index (PDSI). The results show that the ADNC are more severe during negative PDO phases than during positive PDO phases. During the negative PDO phase, both the autumn precipitation and the Penman-Monteith reference evaporation (PET_pm) play roles in the significant severity of the ADNC. In contrast, during positive PDO phases, the droughts over North China are impacted to a greater degree by the climate conditions in previous seasons. Regression analysis shows that the atmospheric circulation anomalies associated with ADNC are quite different over North China under different PDO phases. Specifically, during negative PDO phases, anomalous low-pressure systems are observed over the East Asia-western Japan area, indicating a strengthened East Asian trough (EAT). Anomalous northerly winds bring cold and dry air from high-latitude inland regions to North China, leading to severe ADNC during this period. Meanwhile, significant descending motion is observed in the upstream region of the EAT and the area of North China, accompanied by reduced cloudiness and reduced precipitation over these regions. In contrast, during positive PDO phases, the ADNC-associated atmospheric circulation anomalies over North China are quite weak. Further analysis shows that during the negative phase of PDO years, the El Nino Southern Oscillation can impact the severity of ADNC through the centre of the Pacific-East Asian teleconnection pattern over northeastern Asia.