Preceding Factors of Summer Asian-Pacific Oscillation and the Physical Mechanism for Their Potential Influences

The authors explore the preceding factors of summertime Asian-Pacific Oscillation (APO) using observations and output from the NCEP Climate Forecast System version 2 (CFSv2). Results show that the winter and spring sea surface temperatures (SSTs) in the tropical central-eastern Pacific (TCEP) and the spring sea level pressure (SLP) over the north Indian Ocean (NIO) are significantly correlated with summer APO. The preceding TCEP SST anomaly tends to exert a delayed impact on summer APO through the following process. The previous winter TCEP SST anomaly persists until spring and results in SLP anomaly over the NIO in spring. The latter induces a vertical motion anomaly over the western Tibetan Plateau, which alters spring rainfall and underlying soil moisture in situ, further modulating local surface air temperature during the following summer and hence the summer APO. The CFSv2 has high skills in predicting the winter and spring TCEP SST and the spring NIO SLP and successfully captures the observed relationships of TCEP SST and NIO SLP with summer APO. This result explains why the CFSv2 is capable of predicting the summer APO teleconnection by several months in advance.