Interdecadal Relationships between the Asian-Pacific Oscillation and Summer Climate Anomalies over Asia, North Pacific, and North America during a Recent 100 Years

Summertime relationships between the Asian-Pacific Oscillation (APO) and climate anomalies over Asia, the North Pacific, and North America are examined on an interdecadal time scale. The values of APO were low from the 1880s to the mid-1910s and high from the 1920s to the 1940s. When the APO was higher, tropospheric temperatures were higher over Asia and lower over the Pacific and North America. From the low-APO decades to the high-APO decades, both upper-tropospheric highs and lower-tropospheric low pressure systems strengthened over South Asia and weakened over North America. As a result, anomalous southerly-southwesterly flow prevailed over the Asian monsoon region, meaning stronger moisture transport over Asia. On the contrary, the weakened upper-tropospheric high and lower-tropospheric low over North America caused anomalous sinking motion over the region. As a result, rainfall generally enhanced over the Asian monsoon regions and decreased over North America.