Influence of the Pacific Decadal Oscillation on the Relationship between El Nino and the Northeast Asian Summer Monsoon

The influence of the Pacific decadal oscillation (PDO) on the relationship between El Nino and the northeast Asian summer monsoon (NEASM) is examined using observational datasets for the period of 1979-2007. When El Nino occurs during the boreal winter (December-February), the amount of rainfall over northeast Asia is usually above normal during the following summer (June-August). This relationship between El Nino and the NEASM is intensified when El Nino and the PDO are in phase during the previous winter. However, when El Nino and the PDO are out of phase, the relationship is weakened. The authors argue that the PDO can constructively or destructively interfere with the summer rainfall response over northeast Asia to El Nino. They follow the hypothesis that the summer rainfall over northeast Asia could be separated into two components, that is, the tropics-related component and the extratropics-related component. Then they argue that the PDO could modulate the relationship between El Nino and the NEASM through changes in the extratropics-related rainfall, which is associated with the atmospheric circulation, such as the Eurasian pattern. The conditional composites show that when El Nino and the PDO are in phase, the Eurasian-like pattern acts to enhance the extratropics-related rainfall over northeast Asia, resulting in the strengthening of the NEASM. In contrast, the Eurasian-like pattern acts to reduce the extratropics-related rainfall when El Nino and the PDO are out of phase, resulting in the weakening of the NEASM.