The relationship between the North Atlantic Oscillation and the Silk Road pattern in summer

Abstract The Silk Road pattern (SRP), which is the leading mode of upper-tropospheric meridional wind anomalies over the midlatitude Eurasia, has been widely used to explain the impacts of the summer North Atlantic Oscillation (SNAO) on East Asian climate. However, the relationship between the SNAO and SRP has not been fully elaborated yet. This study classifies the SNAO into two categories according to whether it is closely associated with the SRP or not: the strongly-linked category and weakly-linked category, on the interannual timescale. The SNAO of the strongly-linked category is featured by its concentrated and significant southern pole over the northwestern Europe, and corresponding significant negative (positive) precipitation and upper-tropospheric wind convergence (divergence) anomalies over the northwestern Europe. The wind convergence (divergence) anomalies directly induce the positive (negative) planetary vortex stretching anomalies, which contribute overwhelmingly to positive (negative) Rossby wave source anomalies of the northwestern Europe. These Rossby wave source anomalies, acting as disturbances, further inspire circulation anomalies of surrounding regions, including meridional wind anomalies over the Caspian Sea, which are crucial for the SRP formation. Resultantly the downstream SRP are triggered. All these essential features responsible for a strong SNAO-SRP linkage are weak for the weakly-linked category. The SNAO-SRP correspondence on the inter-decadal timescale is also discussed, and generally similar results are found. Results suggest the importance of shapes for the SNAO southern pole (including the location, the space extent, and the intensity) in determining whether the SNAO can closely link the SRP. Therefore, the shapes of SNAO southern pole should be concerned in the discussion on SNAO’s remote impacts.