What Percentage of Silk-Road Pattern Triggers Pacific–Japan Pattern through Rossby Wave Breaking?

Abstract

In this study, we investigate the rate at which the Silk Road pattern (SRP) with Rossby wave breaking (RWB) near the Asian jet exit causes the Pacific–Japan (PJ) pattern in boreal summer. In this study, the SRP case is detected using the two principal components of upper-tropospheric meridional winds over Eurasia and characterized by the presence of an upper-level anticyclonic anomaly over the Yellow Sea or near Japan. They are further classified into cases with and without RWBs. In the SRP case with RWB, the upper-level anticyclonic anomaly near the Asian jet exit has a more extended shape in the zonal direction and larger amplitude than in the case without RWB. In the composite, a wave train associated with the SRP appears over Eurasia, which is accompanied by the RWB near the Asian jet exit. The occurrence of RWB is associated with strong deceleration and diffluence in the basic state there. The RWB pro-motes enhanced convection on its southern side due to the intrusion of upper-level high potential vorticity toward the southwest, resulting in the formation of the PJ pattern. The excited PJ pattern in the composite has a dipole structure with cyclonic anomalies to the south and anticyclonic anomalies to the north. Approximately 60 – 70 % of the SRP case with RWB is accompanied by the PJ patterns. Conversely, in the case of the SRP without RWB, the composite represents a wave train structure over Eurasia but indicates neither enhanced convection south of the RWB nor PJ patterns. Approximately 40 – 50 % of the SRP case without RWBs is accompanied by the PJ patterns. Hence, the presence of RWBs increases the percentage of the formation of positive PJ patterns by a factor of 1.2 – 1.7, indicating that the RWB plays an important role in the excitation of PJ patterns.