Weakening of Interannual Variability in the Summer East Asian Upper-tropospheric Westerly Jet since the Mid-1990s

In this study, we found that the intensity of interannual variability in the summer upper-tropospheric zonal wind has significantly weakened over Northeast Asia and the subtropical western North Pacific (WNP) since the mid-1990s, concurrent with the previously documented decrease of the westerly jet over North China and Northwest China. Corresponding to this weakening of zonal wind variability, the meridional displacement of the East Asian westerly jet (EAJ) manifested as the leading mode of zonal wind variability over the WNP and East Asia (WNP--EA) before the mid-1990s but not afterward. The energetics of the anomalous pattern associated with the meridional displacement of the EAJ suggests that barotropic energy conversion, from basic flow to anomalous patterns, has led to the weakening of the variability in the EAJ meridional displacement and to a change in the leading dominant mode since the mid-1990s. The barotropic energy conversion efficiently maintained the anomalies associated with the variability in the EAJ meridional displacement during 1979--1993 but acted to dampen the anomalies during 1994--2008. A further investigation of the energetics suggests that the difference in the patterns of the circulation anomaly associated with either the first leading mode or the meridional displacement of the EAJ, i.e., a southwest--northeast tilted pattern during 1979--1993 and a zonally oriented pattern during 1994--2008, has contributed greatly to the change in barotropic energy conversion.