A new concept: double ridges process of West Pacific subtropical high

Under specific external environment and surrounding circulations, a northern and a southern line are available at the same longitude in subtropical region on a synchronous basis, though the subtropical high has characteristic of single ridge in most areas at most of time. We define this phenomenon as 'the double ridges process'. It plays important roles on West Pacific Subtropical High (WPSH) discontinuous northward advancement and southward retreat, further on the second Meiyu period of Changjiang River basin. Furthermore, the moisture transport passage is changed when the process persists. The moisture is transported northward and converged to the north of the ridges, leading to two rainbelt formed. Subtropical high experiences double ridges process (DRP) nearly every year, of which the phase-locking and geographical preference result in DRP occurrence in the WPSH on a climatological basis. In this study, satellite data and the 43-yr mean climatology of NCEP/NCAR Reanalysis datasets are employed to examine the characteristics of WPSH DRP during the period of May to October. Six cases are found to occur during the period of late July to late September, indicating the preference time of the occurrences of WPSH DRP. Unexceptionally, each of them is characterized by a new-born ridge in the south of WPSH which persists several days before its vanishment and by the maintanence of the original one in the northern portion of WPSH.