Seasonal Variations of the Synoptic-Scale Transient Eddy Activity and Polar Front Jet over East Asia

Abstract

Seasonal variations of the synoptic-scale transient eddy activity (STEA) and the jet streams over East Asia are examined through analysis of the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data. Extracted from the 6-hourly upper-level wind fields, the distribution of the jet core numbers exhibits a distinct geographical border for the East Asian subtropical jet (EASJ) and the East Asian polar front jet (EAPJ) at the latitudes of the northern Tibetan Plateau (TP). In the cool seasons, two branches of the STEA and low-level baroclinicity exist over the East Asian landmass, accompanied by the two-jet state of the EASJ and EAPJ. In the warm seasons, a single jet pattern of the EASJ along the north flank of the TP is accompanied by the weakened STEA over the mid-to high latitudes of East Asia. Further analysis shows two distinct features of the seasonal variations of the STEA over East Asia, compared with that over the North Pacific. First, during the transitional period of April-June, the main STEA band over East Asia migrates northward dramatically, in conjunction with the EAPJ shifting in the same direction. Second, both the upperlevel STEA and the lower-level baroclinicity poleward of the TP are prosperous in spring. The relationship between the STEA, baroclinicity, vertical wind shear, and static stability in the EAPJ region in different seasons is further investigated. It is found that in addition to the time-mean wind fields, the rapid increase in the sensible heat flux poleward side of the TP region in spring and the associated boundary layer processes are partially responsible for the spring prosperity of the local baroclinicity and the STEA.

Observed Influences of Autumn–Early Winter Eurasian Snow Cover Anomalies on the Hemispheric PNA-like Variability in Winter

“…3a-3c. The below-normal (above normal) air temperatures in the TP (midlatitude Eurasia) due to the increased (reduced) albedo act to enhance (weaken) the local meridional temperature gradient in the vicinity of the East Asian subtropical jet (East Asian polar front jet) (Ren et al 2010) and then increase (reduce) the strength of the zonal westerly wind. Indeed, projections of the 300-hPa zonal wind in DJF on the MCA-snow cover time series associated with Figs.…”

Structure and dynamics of decadal anomalies in the wintertime midlatitude North Pacific ocean–atmosphere system

“…The midlatitude atmosphere has strong baroclinicity and strong internal variability, in which transient eddies accompanied with jet stream and storm track develop vigorously (Ren et al 2010;Chu et al 2013;Liu et al 2014). The synoptic transient eddies can transport heat and momentum flux in the atmosphere, and thus can redistribute the heat and momentum in space.…”

Influence of Upper Tropospheric Disturbances on the Synoptic Variability of Precipitation and Moisture Transport over Summertime East Asia and the Northwestern Pacific

“…In the summertime, a track extends from the eastern part of China and the Yellow Sea to the North Pacific over the region of the Japanese islands, and another track extends eastward from the north of the Tibetan Plateau (Whittaker and Horn 1984;Chen et al 1991;Mesquita et al 2008). However, their activity is lower than in the cool seasons (Whittaker and Horn 1984;Chen et al 1991), and the baroclinicity and poleward heat transport associated with them are significantly weaker (Nakamura 1992;Ren et al 2010;Yanase et al 2014). Therefore, the effect of baroclinic synoptic eddies on summertime weather systems, and in particular on precipitation, is not obvious and is poorly characterized.…”