Intraseasonal Variations of the East Asian Summer Monsoon in El Niño Developing Years and La Niña Years under Different Phases of the Pacific Decadal Oscillation

Based on observational and reanalysis datasets of 1957-2017, this study compares the impact of different phases of the Pacific Decadal Oscillation (PDO) on the seasonal and intraseasonal variation of the East Asian Summer Monsoon (EASM) in El Niño developing years and La Niña years. The results show that the PDO positive (negative) phase reinforces El Niño developing (La Niña) behaviors mainly by modifying the background EASM circulation, precipitation, and intraseasonal variation. When El Niño is in phase with the PDO (El Niño/high PDO), the PDO amplifies the warm SST (sea surface temperature) anomalies signal of the central and eastern Pacific during the summer; in addition, it strengthens the mid-high latitudes circulation anomalies of the East Asia continent and the Northwest Pacific through the cold SST state. In La Niña years, the negative PDO phase promotes the tropical air-sea interaction and also causes circulation anomalies in mid-high latitudes, such as an anti-cyclonic anomalies in Northeast Asia. Regarding intraseasonal variations, during June in El Niño developing years, there exists stable northerly anomalies from the anti-cyclonic circulation east of Lake Baikal, which induces negative geopotential height anomalies over East Asia. From July, the circulation is adjusted, a cyclonic anomalies appears in the eastern ocean of Japan, and the northerlies, and negative geopotential height anomalies are strengthened. In August, with the EASM seasonal march and the development of El Niño, a cyclonic anomalies occurs in the western Pacific Ocean, the subtropical geopotential height further reduces, and WPSH (western Pacific subtropical high) retreats eastward. In La Niña years, the circulation anomalies are weak in June. The main differences begin with July, characterized by a large-scale cyclonic anomalies control in the Northwest Pacific Ocean and a significant East Asia-Pacific teleconnection pattern, which eventually weakens the WPSH and retreats it eastward. As a result, during the period 1957-2017, less summer precipitation existed in both cases over East Asia, especially in northern China.