Transient Behavior of the Asian Summer Monsoon Anticyclone Associated With Eastward Eddy Shedding

The Asian monsoon anticyclone (AMA) exhibits a trimodal distribution of sub-vortices and the western Pacific is one of the preferred locations. Amplification of the western Pacific anticyclone (WPA) is often linked with eastward eddy shedding from the AMA, although the processes are not well understood. This study investigates the dynamics driving eastward eddy shedding associated with the emergence of the WPA in the upper troposphere and lower stratosphere on synoptic scales. Using reanalysis data during 1979–2019, our composite analysis reveals that amplified WPA events are tied to the upstream Silk Road (SR) wave-train pattern over midlatitude Eurasia as identified in previous studies. The quasi-stationary eastward propagating eddies result from baroclinic excitation along the westerly jet, as identified by coherent eddy heat fluxes and weakening of the low-level temperature gradient. The upper-level westerly jet is important in determining the longitudinal phase-locking of wave trains, which are anchored and amplify near the jet exit. Occasionally enhanced convection near the Philippines also triggers anticyclonic eddies that propagate upward and northeastward via the Pacific-Japan (PJ) pattern, forming the WPA in the upper troposphere. Correlation analysis suggests that the SR and PJ mechanisms are not physically correlated.