Climatological View on Double Ridges Process of the Western Pacific Subtropical High and Its Relation to Quasi-10-day Oscillation of Monsoon Trough

The subtropical high experiences double ridges process(DRP) nearly every year,of which the phase-locking and geographical preference result in the similar behavior in the western Pacific Subtropical High(WPSH) on a climatological basis.In this study,the 43-year 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 for several days before its vanishment and by the maintenance of the original one in the northern portion of WPSH.The onset of new-born ridge for each case is closely associated with the anomalous activities of monsoon trough,e.g.,intermittent enhancement and eastward extension with a timescale of 8-10 days.The sudden development of monsoon trough during late July to late September usually leads to the phase-locking of WPSH DRP.Further studies show that the 8-10-day oscillation of monsoon trough is due to a westward-propagating quasi-10-day oscillation from the western Pacific and a northward-propagating quasi-10-day oscillation from the equatorial region.The monsoon trough tends to reinforce and extend eastward when those two oscillation disturbances overlap each other in a preferable phase(e.g.,westerly phase) over the South China Sea region(10°N-15°N,110°E-120°E).