West Pacific Subtropical High Double-ridges Process And Its Relationship With Subtropical High Discontinuous Meridional Movement

NCEP/NCAR reanalysis dataset, China stations data and TRMM rainfall data are used to document the interesting phenomenon defined as West Pacific Subtropical High (WPSH) double-ridges. Dynamics and thermodynamic characteristics are analyzed to explore the onset processes and affecting factors of the WPSH double-ridges, as well as its possible impact on WPSH progress northward and retrogress southward on a short-and-middle-term basis, especially discontinuous movement. Corresponding physical mechanism is discussed by means of subtropical high dynamics theory. Main conclusions are made as follows:1. The new concept of WPSH double-ridges is illustrated in system and depth. Diagnosis analysis in combination with modeling method is employed to show the universality, geographical preference and phase locking of the phenomena. It is also represented well in the model, indicating that this phenomenon may be one natural shape of general circulation variation. Appearance of the double-ridges changes the water vapor transportation channels and forms two rain-belts. End of the double-ridges plays important role on WPSH discontinuous progress northward and retrogress southward.2. Onset process of WPSH double-ridges is indicated on a climatic basis. It could be concluded into five types: equatorial buffer zone moving northward, South China Sea monsoon trough progressing eastward, Middle Pacific subtropical high extending westward, westerly ridge or continental anticyclone suppressing southward and the original WPSH ridge breaking into two parts and separating with each other.3. The WPSH double-ridges is much more frequent during stronger Asian summer monsoon year. Furthermore, the formation of the WPSH double-ridges is controlled by 10-20-day oscillation of South China Sea summer monsoon, and further modified by 30-60-day oscillation. On this account, the formation of WPSH double-ridges is closely related with Asian monsoon or monsoon trough aviation and intraseasonal oscillation.4. One type of WPSH double-ridges process is analyzed to find possible mechanism of maintenance. This type of WPSH double-ridges process presents as that the new-born ridge in the southern flank of subtropical high develops and takes place of the original ridge in the northern flank of WPSH finally, leading to the WPSH anomaly southward than usual. The formation, maintenance and disappearance of this type of double-ridges processes have close relationship with propagation of the easterly waves from southeast to northwest on the periphery of the southern flank of subtropical high, as well as the reaction with the WPSH. It may change the distribution of vertical motion in the subtropical high regions, and further strengthen the southern ridge which replaces the northern ridge later. This type of WPSH double-ridges process provides foreboding for subtropical high ridge discontinuous retrogression southward and be attributed for WPSH forecast.