Analysis on the Dynamics of a Wave-like Teleconnection Pattern along the Summertime Asian Jet Based on a Reanalysis Dataset and Climate Model Simulations

摘要

The Silk Road pattern, a wave-like anomaly pattern observed along the summertime Asian jet, is one of the major teleconnection patterns that can influence the East Asian summertime climate. Our analysis based on a reanalysis (JRA-25) dataset confirms the conventional notion that the pattern has a characteristic of a free stationary Rossby wave train, with its horizontal wavenumber close to the stationary Rossby wavenumber determined by the mean intensity of the jet. However, our analysis reveals its more essential characteristic as a dynamical mode whose extraction of available potential energy from the baroclinic Asian jet is highly efficient for its self-maintenance. Our analysis also reveals high sensitivity of its barotropic energy conversion to subtle zonal asymmetries of the Asian jet, which can be regarded as a critical factor to anchor the strongest vorticity anomaly around the western jet core and thereby determine the preferred longitudinal phase alignment of the wave train as observed. In fact, singular value decomposition of a global baroclinic model linearized about the observed mean state for boreal summer leads to identification of a perturbation similar to the Silk Road pattern with respect to its structure and energetics. It is thus indicated that the configuration of the mean flow determines the dominant phase, as well as the meridional location and the wavenumber, of the Silk Road pattern.The aforementioned dynamical characteristics of the Silk Road pattern are found useful for assessing and interpreting the reproducibility of the pattern in the present-day climate simulated in climate models that participated in the phase 3 of the Coupled Model Intercomparison Project (CMIP3). The pattern tends to be identified as the dominant mode of upper-tropospheric meridional wind variability as observed in such models that can reproduce the mean Asian jet realistically, including its zonal structure, which confirms the dynamics of the Silk Road pattern revealed in our observational analysis. On the basis of our analysis, a metric is proposed for assessing the models' reproducibility of the pattern.