**5. Relationship with the Overland Silk Road Pattern**

Figures 3c and 4a demonstrate that the in situ positive PV forcing over the Tibetan Plateau corresponds well with the positive PV anomaly in the atmosphere over the plateau. To identify the link of this positive PV anomaly with the upper layer general circulation, the 200 hPa meridional wind distribution regressed onto the iPV index is shown in Figure 6. Significant southerly and northerly anomalies are concentrated in the mid-latitudes of Eurasia from the western Eurasian continent to East Asia. The structure of the meridional wind anomaly is similar to the SRP which also appears as alternate southerly and northerly anomalies along the mid-latitude Asian westerly jet from western Europe to East Asia [14,16]. The correlation coefficient between the iPV and the index of SRP (SRPI) is as high as 0.59, exceeding the significance level of 0.01.

**Figure 6.** The distribution of the meridional wind anomaly at 200 hPa (shading, unit: m s<sup>−</sup>1) regressed onto the iPV index. Areas exceeding the 0.05 significance level are highlighted by black dots.

Previous studies have shown that, through the associated anomalous northerly wind over East Asia, the summertime SRP has varying degrees of influence on the circulation and the precipitation in the East Asian areas [4,15,18,19,29]. In order to better understand the relationship between plateau PV forcing and the onland SRP in the influence of the interannual intensity of the EASM, we conduct two sets of partial correlation analysis: one by removing the linear influence of SRPI from that of iPV and the other by removing the linear influence of iPV from that of SRPI, and the results are presented in Figures 7 and 8, respectively.

**Figure 7.** The distributions of circulation (vector, unit: m s<sup>−</sup>1) and geopotential height (shading, unit: gpm) at 200 hPa (**a**,**c**) and the 850 hPa circulation (vector, unit: m s−1) and precipitation (shading, unit: m day−1) (**b**,**d**) regressed onto the iPV index (**a**,**b**) and regressed onto the iPVr index (**c**,**d**). iPVr represents the remaining time series after removing the linear correlation with SRPI from the iPV index. Dotted regions indicate the geopotential height and precipitation exceeding the 95% confidence level.

It can be seen from Figure 7 that when the linear influence of the SRP is removed, the distributions of circulation and precipitation anomalies in the lower tropospheric layer change little (Figure 7d) compared with the original distributions (Figure 7b). The main changes in the upper troposphere circulation are the weakening and westward shifting of the anomalous cyclone to the north of the Tibetan Plateau and its upstream anticyclonic anomaly (Figure 7c), while the position and intensity of the anomalous anticyclone over East Asia (Figure 7a) is almost unchanged (Figure 7c). The associated anomalous northerly wind still prevails over East Asia. As mentioned in Section 4 and previous studies [4,15], the northerly wind anomaly is a key factor that induces the precipitation anomaly over the Jianghuai region. Consequently, the results shown in Figure 7 indicate that no matter whether there is the influence of the SRP or not, PV forcing over the Tibetan Plateau can directly influence the intensity of EASM. The correlation coefficient between iPVr and iEAM is 0.49, which is only 0.05 lower than that between iPV and iEAM, but still reaches a significance level of 0.01. On the contrary, when the influence of plateau PV forcing is removed from the Silk Road tele-correlation, the regressed upper tropospheric circulation undergoes remarkable changes (Figure 8c versus Figure 8a). The strong cyclone anomaly to the north of the plateau (Figure 8a) becomes significantly weak and shifts southward (Figure 8c). The anomalous westerly wind prevailing over the entire plateau platform (Figure 8a) becomes much weaker and moves southward to the south of the plateau (Figure 8c). The downstream anticyclone anomaly originally located over the Jianghuai region moves northeastward and is located over Northeast China. The associated anomalous northerly wind moves northeastward correspondingly. The Jianghuai region is controlled by the easterly anomaly in the upper troposphere (Figure 8c). In the lower troposphere (Figure 8b,d), the subtropical anticyclone circulation over the northwestern Pacific is weakened, and no apparent precipitation anomalies occur in the Jianghuai area. The correlation coefficient between SRPIr and the overall East Asian monsoon intensity index iEAM drops to 0.01, which is almost independent. These results indicate that the SRP cannot influence the EASM directly in July. PV forcing over the Tibetan Plateau may play a role in "bridging" the connection between the EASM and the SRP.

The above results suggest that PV forcing over the Tibetan Plateau can directly influence the East Asian monsoon's variability. Such connection between the EASM and the plateau PV forcing in July is affected little by the SRP, whereas the plateau PV forcing plays a key role in "bridging" the SRP and the EASM precipitation. If the plateau PV forcing disappears, the upstream SRP may not have a significant effect on the East Asian summer monsoon.

**Figure 8.** The distributions of circulation (vector, unit: m s<sup>−</sup>1) and geopotential height (shading, unit: gpm) at 200 hPa (**a**,**c**) and the 850 hPa circulation (vector, unit: m s−1) and precipitation (shading, unit: m day−1) (**b**,**d**) regressed onto the SRPI index (**a**,**b**) and regressed onto the SRPIr index (**c**,**d**). SRPIr represents the remaining time series after removing the linear correlation with iPV from the SRPI index. Dotted regions indicate the geopotential height and precipitation exceeding the 95% confidence level.

#### **6. Conclusions**

By integrating the PV substance and its local change equation over the global atmospheric volume bounded by an enclosed isentropic surface as the upper boundary, it shows that the global gross PV substance equals the integral of the PV circulation (PVC) at the earth's surface of the whole globe. That means the gross source of PV substance of the global atmosphere is located at the Earth's surface. EOF analysis of the surface PV circulation (SPV) over the Tibetan Plateau higher than 3 km in July indicates that PC2 can be used as an index to characterize effects on the EASM of the intrinsic plateau PV forcing. By making partial correlation and regression analysis, this paper further studies the influence mechanism of the plateau PV forcing on the interannual variability of the EASM and its relationship with that of the onland SRP over Eurasia. The main conclusions can be summarized as follows:

(1) When the second mode of SPV on the surface of the Tibetan Plateau platform is in the positive phase (Figure 2b), with positive SPV on its south and negative SPV on its north, a strong positive PV anomaly and strengthened westerly flow will develop in the troposphere over the plateau, forming a structure of zonal PV advection increasing with height in the troposphere over the downstream Jianghuai region, which is conducive to the generation of air ascent. The upper troposphere over East Asia is controlled by the strong positive anomaly of geopotential height due to

the eastward shifting of the South Asian High. The associated northerly anomaly favors the transport of positive PV anomaly to the Jianghuai region; whereas, in the lower troposphere, the anomalous southwesterly flow on the northwestern side of the enhanced western Pacific subtropical high transports not only abundant water vapor, but also negative PV anomaly to the Jianghuai region, forming a circulation background of PV advection increasing with height. This not only enhances the three-dimensional circulation anomaly of the East Asian monsoon in July, but also facilitates stronger precipitation along the Meiyu front;

(2) The link between East Asian monsoon variability and plateau PV forcing in July is influenced very little by the SRP. The latter mainly impacts the wave position and intensity in the upper troposphere to the west of the plateau, but has limited effect on the spatial distributions of circulation and precipitation downstream of the plateau. However, when the linear signal of plateau PV forcing is removed from the SRP sequence, the cyclone anomaly to the north of the plateau is significantly weakened and shifts southward. The westerly wind originally over the plateau becomes much weaker and shifts to the south of the plateau as well. At the same time, the anticyclone anomaly originally located over the Jianghuai region shifts to Northeast China. The Jianghuai region is controlled by the easterly anomaly in the upper troposphere, which weakens the circulation background of PV advection increasing with height. Consequently, the positive precipitation anomaly over the Jianghuai region becomes weak and the interannual variability of the SRP and the East Asian monsoon are no longer correlated. These results indicate that the SRP cannot influence the EASM directly in July. The plateau PV forcing plays a key role in "bridging" the influence of the SRP to the East Asian summer monsoon: the PV forcing over the Tibetan Plateau can modulate the influence of the SRP on the EASM by changing the position of the anticyclone anomaly in the upper troposphere downstream of the Tibetan Plateau which is critical for the development of air ascent and precipitation of the EASM. When the influence of plateau PV forcing is removed, this anticyclone anomaly is located over Northeast China, which has little impact on the EASM. However, when the influence of plateau PV forcing is considered, the anticyclone anomaly shifts to central China, contributing to a stronger EASM year. In other words, the influence of the SRP in the Eurasian region on the East Asian monsoon in July is inseparable from the involvement of the PV forcing over the Tibetan Plateau. In summary, it is the surface PV forcing of the Tibetan Plateau that directly and significantly affects the interannual variability of the EASM over the Jianghuai region.

**Author Contributions:** Y.L.: Conceptualization; validation; Funding acquisition; supervision; writing—review and editing. L.L.: Data curation; formal analysis; investigation; writing—original draft. G.W.: Conceptualization; Funding acquisition; writing—review and editing. T.M.: writing—review and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** The study was supported by the National Natural Science Foundation of China Projects (91937302) and the Strategic Priority Research Program of Chinese Academy of Sciences (XDB40030205).

**Data Availability Statement:** All datasets used in this study are publicly available. We would like to thank the Global Modeling and Assimilation Office and the Goddard Earth Sciences Data and Information Services Center for the dissemination of MERRA-2 reanalysis data (https://climatedataguide.ucar. edu/climate-data/nasas-merra2-reanalysis; accessed on 18 May 2023). The GPCP precipitation data are available from https://psl.noaa.gov/data/gridded/data.gpcp.html (accessed on 18 May 2023).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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