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.

Surface and Upper-Level Features Associated with Wintertime Cold Surge Outbreaks in South Korea

The surface and upper-level features associated with a sharp drop ofwintertime daily temperature over South Korea is investigated in this study. This sharp drop indaily temperature is called a cold surge and is one of the most hazardous weather phenomena in EastAsian winters. An upper-level baroclinic wave of 60° wavelength propagating eastward at a phasespeed of 12° longitude per day across the continent of northern China from the west of Lake Baikaltoward the eastern coast of China causes the outbreak of cold air over South Korea. The coolingassociated with the upper-level baroclinic jvave is found at all altitudes under the geopotentialheight-fall center near the tropopause. The development in the ridge seems to derive the earlyevolution of the eastward-propagating sinusoidal wave, whereas the trough is connected directly withthe tropospheric temperature-drop. An enhancement of the wintertime East Asian jet stream after theoutbreak of a cold surge is a response to the steep temperature gradient associated with thedeveloping baroclinic wave.

Decadal Changes in Climatological Intraseasonal Fluctuation of Subseasonal Evolution of Summer Precipitation over the Korean Peninsula in the mid-1990s

Decadal changes in the subseasonal evolution and the phase-locked climatological intraseasonal fluctuation of summertime rainfall over the Korean Peninsula before and after the mid-1990s are investigated.The activity and the migration speed of the monsoon rain band over the East Asian region are altered in the recent decade,resulting in the drier conditions in late spring and the earlier onset of Changma.In early August when a climatological monsoon break was clear in the earlier decade,the precipitation has increased dramatically with a meridional coherency.The response to the enhanced convection over the South China Sea and southeastern China provides a favorable condition for more precipitation in early August through the changes in moisture transport and tropical cyclone passage.