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.