Asymmetric Relationship between the Meridional Displacement of the Asian Westerly Jet and the Silk Road Pattern

Abstract

In previous work, a significant relationship was identified between the meridional displacement of the Asian westerly jet (JMD) and the Silk Road Pattern (SRP) in summer. The present study reveals that this relationship is robust in northward JMD years but absent in southward JMD years. In other words, the amplitude of the SRP increases with northward displacement of the jet but shows little change with southward displacement. Further analysis indicates that, in northward JMD years, the Rossby wave source (RWS) anomalies, which are primarily contributed by the planetary vortex stretching, are significantly stronger around the entrance of the Asian jet, i.e., the Mediterranean Sea–Caspian Sea area, with the spatial distribution being consistent with that related to the SRP. By contrast, in southward JMD years, the RWS anomalies are much weaker. Therefore, this study suggests that the RWS plays a crucial role in inducing the asymmetry of the JMD–SRP relationship. The results imply that climate anomalies may be stronger in strongly northward-displaced JMD years due to the concurrence of the JMD and SRP, and thus more attention should be paid to these years.

摘 要

在之前的工作中, 我们发现了夏季亚洲西风急流南北偏移与丝绸之路遥相关之间存在显著的联系. 而本研究则揭示出, 该相关关系存在很强的不对称性: 在急流偏北的年份里相关显著, 而在急流偏南的年份里则没有明显的关系. 换句话说, 丝绸之路遥相关的强度随急流北偏而增强, 但随着急流的南偏却变化甚微. 我们的进一步研究指出, 在急流偏北的年份, 罗斯贝波波源异常(其中行星涡度的伸缩项起主要作用)在亚洲急流入口附近的区域, 即地中海-里海附近非常强, 其空间分布也和丝绸之路遥相关有关的波源异常分布非常一致. 而在急流偏南的年份里, 罗斯贝波波源异常非常弱. 因此本文的结果表明, 罗斯贝波的波源异常在急流南北偏移和丝绸之路遥相关的不对称关系中起重要作用. 这些结果也说明: 由于急流偏北较强的年份里, 可能同时出现较强的急流偏北以及丝绸之路遥相关异常, 因此这些年份里也很可能出现较强的气候异常, 我们需要尤其注意这些年份的环流及气候变化动向.