What Percentage of Silk-Road Pattern Trigger Pacific– 2 Japan Pattern through Rossby Wave Breaking?

In this study, we investigate the rate at which the Silk-Road pattern (SRP) with Rossby 30 wave breaking (RWB) near the Asian jet exit causes the Pacific–Japan (PJ) pattern in boreal 31 summer. Here, the SRP case is detected using the two principal components of upper- 32 tropospheric meridional winds over Eurasia and characterized by the presence of an upper- 33 level anticyclonic anomaly over the Yellow Sea or near Japan. They are further classified 34 into cases with and without RWBs. In the SRP case with RWB, the upper-level anticyclonic anomaly near the Asian jet exit 36 has more extended shape in the zonal direction and larger amplitude than in the case without 37 RWB. In the composite, a wave train associated with the SRP appears over Eurasia, which 38 is accompanied by the RWB near the Asian jet exit. The occurrence of RWB is associated 39 with strong deceleration and diffluence in the basic state there. The RWB promotes 40 enhanced convection on its southern side due to the intrusion of upper-level high potential 41 vorticity toward the southwest, resulting in the formation of the PJ pattern. The excited PJ 42 pattern in the composite has a dipole structure with cyclonic anomalies to the south and 43 anticyclonic anomalies to the north. Approximately 60–70% of the SRP case with RWB is 44 accompanied by the PJ patterns. the PJ patterns. Hence, the presence of RWBs increases the percentage of the formation of positive PJ patterns by a factor of 1.2 to 1.7, indicating that the RWB plays an important role in the excitation of PJ patterns.


56
The Silk Road pattern (SRP) is the dominant teleconnection pattern along the Asian jet in 57 boreal summer (Lu et al. 2002, Enomoto et al. 2003. The SRP can be extracted as the  Pacific subtropical high over mainland Japan (e.g., Lu and Dong 2001). 78 From a lag composite analysis of the RWB near Japan occurred in past decades, 79 Takemura and Mukougawa (2020a) showed that there is a dynamical process in which  Niño southern oscillation (ENSO), the basin-wide SST warming in the Indian Ocean (e.g., 96 Xie et al. 2009,2016), and Pacific meridional mode of SST (Chiang and Vimont 2004, 97 Takaya 2019). They also showed that the PJ pattern without RWB is related to convective  On the other hand, although Takemura and Mukougawa (2020a) showed from their 102 composite analyses that SRP can excite PJ pattern through RWBs, a quantitative evaluation 103 of the ability of SRP to excite PJ patterns has not yet been performed. Hence, in this study, 104 we will evaluate the percentage of SRP cases where RWBs around Japan cause positive 105 PJ patterns, and examine the difference in the probability of PJ pattern formation with and 106 without RWBs. This approach is important for re-examining and highlighting the role of RWB 107 in the connectivity between SRP and PJ patterns (Takemura and Mukougawa 2020a). To 108 accomplish this task, we follow the method adopted by Takemura and Mukougawa (2022) 109 to perform lag composite analysis of the SRP case with and without RWBs.  (Kobayashi et al. 2015). Here, anomaly is defined as the difference from the climatology.

116
The climatology is obtained by a 60-day low-pass (Lanczos; Duchon 1979) filtered daily 117 mean for the 30-year period from 1981 to 2010. To extract the low-frequency components, 118 including quasi-stationary Rossby waves, a five-day running average was applied to the 119 daily data. In order to smooth the relative vorticity field horizontally, a triangular truncation 120 (T24) retaining total wavenumber 24 is used to eliminate disturbances with horizontal scales 121 smaller than the synoptic eddies. The statistical significance of the composited anomalies 122 was assessed by two-tailed Student's t-test. The variable t is defined as = ′ ̅ /√ 2 /( − 1), 123 where ′ ̅ is the composited anomalies, is the standard deviation, is the number of 124 cases. The variable obeys a Student's t-distribution with − 1 degrees of freedom.

125
The propagation of Rossby wave packets was analyzed using the wave activity flux (WAF) 126 defined by Takaya and Nakamura (2001). The horizontal WAF is defined as follows: 128 where is the zonal wind, is the meridional wind, is the climatological horizontal wind cases, respectively. Here, "WB" and "ZN" stand for "wave breaking" and "zonal", respectively.

165
Since the longitude range defining the WB index is from near Japan to its east, the WB index  To examine the diffluence and deceleration of the basic flow near the exit of the Asian jet, 176 which is a precondition for RWB (Colucci 2001), the stretching deformation of the basic state 177 ( ) was derived from the horizontal wind according to Mak and Cai (1989) and Bluestein 178 (1992) as follows:   On the other hand, the composited anomalies of 39 ZN/SRP1+ case are shown in Fig. 6.

231
In the upper troposphere, as in the case of WB/SRP1+, the SRP along the Asian jet gradually 232 amplifies from day -4 to day 0 (Figs. 6d, 6g, and 6j). However, the upper-level anomalous 233 anticyclone over northeastern China is weaker than the WB/SRP1+ case (Fig. 5j). The lack 234 of development of an anticyclone, which does not have a zonally-elongated shape as in the 235 case of WB/SRP1+, is consistent with the lack of RWB in the region and scattered 236 convection to the south of Japan from day -2 to day +2 (Figs. 6h, 6k, and 6n) To compare the horizontal structure of the Asian jet with and without RWB, the difference 245 in on day 0 between the WB/SRP1+ case and the ZN/SRP1+ case is shown in Fig. 7a.

246
The stretching deformation of the WB/SRP1+ case is significantly negative, and its 247 magnitude is larger than the ZN/SRP1+ case in the vicinity and west of the region where 248 RWBs occur, indicating favorable conditions for RWB generation in the WB/SRP1+ case. Japan associated with the RWB are seen from day -4 to day 0 (Figs. 8d, 8g, and 8j).

267
On the other hand, Figure 9 shows the composite anomalies for the 53 ZN/SRP2-cases.

268
Although SRP amplification is clearly seen along the Asian jet in the upper troposphere from