Subseasonal Zonal Oscillation of the Western Pacific Subtropical High during Early Summer

This study examines the features and dynamical processes of subseasonal zonal oscillation of the western Pacific subtropical high (WPSH) during early summer, by performing a multivariate empirical orthogonal function (MVEOF) analysis on daily winds and a diagnosis on potential vorticity (PV) at 500 hPa for the period 1979–2016. The first MV-EOF mode is characterized by an anticyclonic anomaly occupying southeastern China to subtropical western North Pacific regions. It has a period of 10–25 days and represents zonal shift of the WPSH. When the WPSH stretches more westward, the South Asian high (SAH) extends more eastward. Above-normal precipitation is observed over the Yangtze–Huaihe River (YHR) basin. Suppressed convection with anomalous descending motion is located over the subtropical western North Pacific. The relative zonal movement of the SAH and the WPSH helps to establish an anomalous local vertical circulation of ascending motion with upper-level divergence over the YHR basin and descending motion with upper-level convergence over the subtropical western Pacific. The above local vertical circulation provides a dynamic condition for persistent rainfall over the YHR basin. An enhanced southwest flow over the WPSH’s western edge transports more moisture to eastern China, providing a necessary water vapor condition for the persistent rainfall over the YHR basin. A potential vorticity diagnosis reveals that anomalous diabatic heating is a main source for PV generation. The anomalous cooling over the subtropical western Pacific produces a local negative PV center at 500 hPa. The anomalous heating over the YHR basin generates a local positive PV center. The above south–north dipolar structure of PV anomaly along with the climatological southerly flow leads to northward advection of negative PV. These two processes are conducive to the WPSH’s westward extension. The vertical advection process is unfavorable to the westward extension but contributes to the eastward retreat of the WPSH.