Maintenance Mechanism of Rossby Wave Breaking and Pacific-Japan Pattern in Boreal Summer

To reveal a maintenance mechanism for Rossby wave breaking (RWB) east of Japan and Pacific–Japan (PJ) pattern, which are triggered due to quasi-stationary Rossby wave propagation along the Asian jet, the past 44 RWB cases east of Japan are analyzed using a reanalysis dataset. A comparison between the composites of seven persistent and seven non-persistent cases, which are classified based on the duration of the RWB and PJ pattern, indicates that the persistent case demonstrates stronger and longer-lived quasi-stationary Rossby wave propagation along the Asian jet. The subsequent stronger RWB in the persistent case causes the consequential formation of the more enhanced PJ pattern through the stronger high potential vorticity intrusion toward the subtropical western North Pacific. The persistent case further demonstrates a persistent northward-tilting vertical structure of the anomalous anticyclone east of Japan, accompanied by the enhanced anomalous warm air advection in the lower to middle troposphere north of the anomalously extended North Pacific Subtropical High associated with the PJ pattern. The diagnosis of the Q-vector and partial correlation analysis indicate that the anomalous warm air advection in the middle troposphere is closely associated with dynamically induced anomalous ascent from Japan to the east by an adiabatic process. Enhanced anomalous moisture flux convergence from Japan to the east, which is due to the moisture inflow along the fringe of North Pacific Subtropical High from the subtropical western North Pacific, also causes the anomalous ascent over the region by a diabatic process. A simple correlation analysis reveals nearly equivalent associations of the adiabatic and diabatic factors with the anomalous ascent. The anomalous ascent contributes to enhanced and persistent RWB through a negative vorticity tendency due to vortex squashing in the upper troposphere, which further contributes to the enhanced and persistent PJ pattern in the persistent case.