How does the East Asian Summer Monsoon Behave in the Decaying Phase of El Niño during Different PDO Phases?

Abstract:

Modulation of Pacific Decadal Oscillation (PDO) on the behavior of the East Asian summer monsoon (EASM) in El Niño decaying years has been studied. When El Niño is in phase with PDO (El Niño/high PDO), the low-level atmospheric anomalies are characterized by an anticyclone around the Philippines and a cyclone around Japan, inducing an anomalous tripolar rainfall pattern in China. In this case, the western Pacific subtropical high (WPSH) experiences one-time slightly northward shift in July and then stays stationary from July to August. The corresponding anomalous tripolar rainfall pattern has weak sub-seasonal variations. When El Niño is out of phase with PDO (El Niño/low PDO), however, the anomalous Philippine anticyclone has a much larger spatial domain, thereby causing an anomalous dipole rainfall pattern. Accordingly, WPSH experiences clearly two-time northward shifts. Therefore, the related dipole rainfall pattern has large sub-seasonal variations. One pronounced feature is that the positive rainfall anomalies shift northward from the southern China in June to the central China in July and finally to the northern China in August. The different El Niño-EASM relationships are caused by the influences of PDO on the decaying speed of El Niño. During the high PDO phase, El Niño decays slowly and has a strong anchor in the north Indian Ocean warming, which is responsible for the anomalous EASM. Comparatively, during the low PDO phase, El Niño decays rapidly and La Niña develops in summer, which induces different EASM anomalies from that during the high PDO phase. Additionally, PDO changes El Niño behaviors mainly via modifying the background tropical winds.