Circulation changes of Western North Pacific warm pool in tropical cyclone season

The Western North Pacific (WNP) warm pool undergoes a timing shift of velocity potential that provides a conducive environment for tropical cyclones (TCs) to form. As a result, several TCs form in the WNP yearly with a peak in August. However in August 2014, the WNP had a quiet August with only one TC generation compared with the average of 5.7 TCs from 1950 to 2014.

Using 850 mb velocity potential (χ) anomalies, we analyze the 30~90 days intraseasonal oscaillation that induces TC formation in the WNP. It was found that the WNP lower-level convergent center has shifted from August to July, whiles divergent anomalies have begun dominating August since 1998. Further EOF analyses indicate the intra-seasonal oscillation has undergone an interdecadal change, related to the timing shift in the recent decade. For the longer than 30 years time scale, the EOF analyses also show a timing shift from mid August to late July that enhances the intra-seasonal changes. Diagnostic analyses point out these changes may link to Pacific subtropical gyre strengthening that correlated to anthropogenic climate warming and Western Pacific SST warming.