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

XUEJUAN REN, XIUQUN YANG, AND CUIJIAO CHU

School of Atmospheric Sciences, Nanjing University, Nanjing, China

(Manuscript received 27 April 2009, in final form 22 November 2009)

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 upper-

level 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.

1. Introduction

In the midlatitude troposphere the jet stream and the

associated synoptic-scale transient eddy activities (STEA)

are special features of earth’s atmospheric motions. The

jet stream is considered the large-scale circulation, while

the STEA can be measured by the synoptic-scale filtered

daily data. Both are strongest in the upper troposphere.

In terms of the climatological seasonal variation, the jet

stream and STEA demonstrate increasing (decreasing)

variations in their intensity. At the same time, they also

show northward–southward (westward–eastward) move-

ment in its spatial distribution harmoniously, however,

with a large discrepancy in regional characteristics

(Whittaker and Horn 1984; Chang 2001; Hoskins and

Hodges 2002, 2005; Nakamura and Shimpo 2004; Newton

2004; Wernli and Schwierz 2006; Mesquita et al. 2008).

Many studies focus on the seasonal-to-interannual

variations of the STEA and the associated zonal wind

fields over the oceanic regions in the Northern Hemi-

sphere, resulting from their level of activity and their

obvious connection with weather processes and the

time-mean flow. Over the North Pacific the wintertime

jet stream intensity and STEA have an anticorrelated

relationship, which has been extensively studied using

observations and models (Nakamura 1992; Chang 2001;

Chang and Guo 2007; Chang 2009; Son et al. 2009).

Over the East Asian continent the STEA is dramati-

cally reduced compared with the oceanic regions, re-

sulting from the topographic effect and strong surface

friction (Hoskins and Hodges 2002; Chang et al. 2002;

Chang 2009; Son et al. 2009). Therefore, the seasonal

variation of the STEA over the East Asian region has

received relatively less attention. Statistical analyses on

the surface cyclone activity in the midlatitude East

Asian continent have shown a clear seasonal cycle, with

the most frequent cyclones occurring in the spring sea-

son (Whittaker and Horn 1984; Asai et al. 1988; Chen

et al. 1991; Wernli and Schwierz 2006; Wang et al. 2009).

The interesting questions to ask are whether the STEA

Corresponding author address: Dr. Xuejuan Ren, School of At-

mospheric Sciences, Nanjing University, Nanjing, 210093 China.

E-mail: renxuej@nju.edu.cn

3222 J O U R N A L O F C L I M A T E VOLUME 23

DOI: 10.1175/2009JCLI3225.1

� 2010 American Meteorological Society

over East Asia is also more vigorous in spring than in

other seasons; and, if so, what are the processes re-

sponsible for this seasonal variation? The traditional

approach to address this issue is to investigate the vari-

ation of the time-mean (over a month or a season) flow.

This is because the level of activity of the transient

eddies in the midlatitude bands is primarily related to

the strong baroclinicity of the time-mean flow. Follow-

ing Eady (1949) and Hoskins and Valdes (1990), the

baroclinicity is mainly determined by the vertical shear

of the time-mean wind fields and the static stability be-

tween two pressure levels. The main body of the STEA

over East Asia is located to the north of the Tibetan

Plateau (TP), where the land surface is dominated by the

Gobi Desert, with little vegetation cover. During spring

season, the rapid increase in surface sensible heat flux

and low-level turbulent diffusion in this region may lead

to large fluctuations of low-level static stability (Xu et al.

2006; Song et al. 2009; Zhang et al. 2009). Consequently,

the seasonal cycle of the baroclinicity over East Asia may

be associated with both the vertical shear of the time-mean

wind field and the static stability. In this study, we will try

to demonstrate that the distinct land–air interaction is

partially responsible for the seasonal cycle of baroclinicity

and STEA over East Asia, thus leading to a sharp contrast

to the oceanic region.

Our interest in exploring the relationship between the

STEA and jet stream over East Asia is also motivated by

a schematic diagram showing the jet stream distributions

in the Northern Hemisphere [Fig. 2 of Lee and Kim

(2003), originally illustrated by Riehl (1962) and Palmen

and Newton (1969)]. According to this diagram and other

studies (e.g., Mahlman 1973; Bals-Elsholz et al. 2001;

Nakamura and Shimpo 2004; Gallego et al. 2005; Koch

et al. 2006), a state of two jets—the subtropical jet and

the polar front jet (also referred to as the subpolar jet or

the eddy-driven jet) in the mid- to high latitudes—is a

general characteristic in both hemispheres. Over the

East Asian region in the cool seasons, the two-jet state

consists of the East Asian subtropical jet (EASJ) and the

East Asian polar front jet (EAPJ) lying zonally along

the southern and poleward sides of the TP, respectively.

After the confluence over the East Asian coastal waters,

the jet extends eastward into the oceanic region (Sheng

1986; Lee and Kim 2003; Newton 2004; Koch et al. 2006).

By examining the upper-level time-mean (e.g., monthly

or seasonal mean) wind fields in the respective winter

seasons of both hemispheres, one can identify a notable

feature over the North Atlantic region and at the lati-

tudes of Australia and New Zealand in the Southern

Hemisphere. That is, a state of two jets with a noticeable

minimum in the westerlies between the two jet branches

is presented (Black and Dole 2000; Bals-Elsholz et al.

2001; Nakamura and Shimpo 2004). However, over East

Asia the EASJ is stronger than the EAPJ. The intensity

of the time-mean wind gradually decreases from the

EASJ to the EAPJ region. Thus, it is difficult to define

an area between the EASJ and EAPJ where the mini-

mum in time-mean westerlies is located. The EASJ is

persistent and dominant in both the time-mean and

high-frequency fields. Hence, its physical characteristics

and seasonal-to-interannual variations have been ad-

dressed by using the pentad-mean and monthly-mean

wind fields (Kuang and Zhang 2005; Zhang et al. 2006).

Comparatively, less attention has been paid to the cli-

matological features of the EAPJ, because of its tran-

sient characteristics (Lee and Kim 2003) and an indistinct

geographical border between the EASJ and EAPJ. In

some recent studies, the daily data are used for identify-

ing the jet cores and for counting the frequency of the

jet cores in a certain season (or month; see Koch et al.

2006; Strong and Davis 2007, 2008; Schiemann et al.

2009). These studies suggested that the time-mean wind

field can well depict the subtropical jet, but that it is not

suitable for the more transient polar front jet stream.

Therefore, the daily or higher-frequency data should also

be used for a detailed description of the EAPJ seasonal

variation. In this study, the 6-hourly wind fields are ana-

lyzed for identifying the jet cores and the geographical

border between the EASJ and EAPJ. Furthermore, the

6-hourly data are used to investigate the seasonal cycle of

the EAPJ and its congruent relationship with the STEA

over East Asia.

One robust feature of EASJ’s seasonal cycle is the

rapid transition and location change of the jet core as-

sociated with the East Asian summer monsoon activity

(Wang and LinHo 2002; Lim et al. 2002; Chen et al. 2004;

Li et al. 2004; Zhang et al. 2006; Wu et al. 2007). Ac-

cording to previous studies, the EASJ demonstrates

a northward jump from the south flank to the north flank

of the TP in passing from the late spring to early summer

(Li et al. 2004; Zhang et al. 2006; Schiemann et al. 2009).

As the counterpart of the EASJ, the behaviors of the

EAPJ and the associated STEA during late spring and

early summer are still unclear. We shall try to address

this issue when examining the seasonal cycle of the EAPJ

and STEA. Specifically, we shall illustrate a harmonious

activity of the EAPJ and STEA from April to June, when

they both display a northward propagation.

In summary, the main questions to be addressed in

this paper are the following:

1) What is the geographical border between the EASJ

and EAPJ? What are the seasonal cycle, and the as-

sociated behavior of EAPJ, baroclinicity, and STEA

over East Asia? In particular, what is the harmonious

15 JUNE 2010 R E N E T A L . 3223

activity of the EAPJ and STEA during the transition

period from spring to summer?

2) What is the relationship among the dramatically in-

creased surface sensible heat in the midlatitude

bands to the north of the TP, the strong atmospheric

baroclinicity and the active STEA in spring?

The next section contains the descriptions of the data

and analysis methodology. The general features of the

seasonal marching of the EASJ, EAPJ, and STEA are

presented in section 3. In section 4 we discuss why the

baroclinicity and the STEA are more robust in spring.

Section 5 provides a summary of conclusions and topics

for further discussion.

2. Data and analysis methodology

The atmospheric data are from the 40-yr European

Centre for Medium-Range Weather Forecasts (ECMWF)

Re-Analysis (ERA-40; Uppala et al. 2005). The data have

a horizontal resolution of 2.58 3 2.58 in longitude and

latitude and cover the period from 1 September 1957 to

31 August 2002. We use the 6-hourly and monthly-mean

fields, as well as the data averaged for each pentad. The

pentad data are created according to the following pro-

cedure: In each calendar month, the first 5 pentads are

obtained by nonoverlapping, consecutive 5-day averaging

applied to the first 25 days; and the last pentad is obtained

by averaging the remaining days. Thus, every month (year)

consists of a total of 6 (72) pentads.

The number of the jet cores per month at each grid

point is calculated using the 6-hourly upper-level wind

fields. A jet core is regarded to exist, and the corre-

sponding latitude and longitude are recorded, if (1) the

wind speed jVj is equal or greater than 30 m s21 and (2)

the wind speed is the local maximum of the surrounding

24 grid points. The above counting procedure repeats

for every day, month, and year. To extract the transient

fluctuations associated with migratory synoptic-scale

disturbances at periods of 2.5–6 days, a bandpass-filtered

technique (Murakami 1979) is applied to the 6-hourly

dataset at the 300-hPa level. STEA is measured by the

variance of v92. The overbar represents averaging over

a specified period, and the prime denotes perturbations

with periods of ;(2.5–6) days. The regions of the mid-

latitude ocean, over which abundant STEA span zon-

ally, are referred to as storm tracks (Blackmon 1976).

The midlatitude STEA is primarily attributed to the

atmospheric baroclinicity. An accurate measure of the

baroclinicity is the Eady growth rate maximum (Eady

1949; Hoskins and Valdes 1990). It is defined as sBI

5

0.31 f (›jVj/›z)N�1, where f is the Coriolis parameter, N

the Brunt–Vaisala frequency, V the time-mean horizontal

wind, and z the vertical height. In general, the growth rate

is calculated in the lower level of the atmosphere because

the baroclinic development primarily occurs in the lower

troposphere (Lunkeit et al. 1998). Therefore, the pressure

levels of 850 and 700 hPa are used for computing sBI in

this study.

3. An overview of seasonal variations in EASJ,EAPJ, STEA, and sBI

a. Wind fields and number of the jet cores

Figure 1 shows the climatological distributions of the

wind fields at 300 hPa over the East Asian–North Pacific

regions for each of the four seasons. During the cool

seasons (spring, autumn, and winter), the zonally pro-

longed EASJ south of 408N is characterized by the

strong westerly wind, while the EAPJ poleward side of

the TP is anchored by the westerly and northwesterly

wind. In winter and autumn the wind speeds gradually

decrease from the EASJ to the EAPJ regions, with no

indication of there being a split between the two that can

be taken as a geographical border between the two jets.

In spring the wind field displays a similar pattern as that

in the winter; however, it shows an area of weak wind

over the TP region. In summer, the EASJ is located along

the north flank of the TP with significantly reduced in-

tensity, accompanied by the disappearance of the EAPJ

in the 508–708N band.

Figure 2 displays the distribution of the number of the

jet cores at 300 hPa for each season. During seasons other

than summer, the jet cores are concentrated mainly in two

lobes, corresponding to the EASJ and the EAPJ regions.

The dominant one is trapped zonally on the south flank of

the TP, parallel with the other one on the poleward side of

the TP. A distinct area with the minimum jet cores ap-

pears between the two lobes. This area extends zonally

from the west side to the east side of the TP, indicating a

clear geographical border between the two jet core lobes.

This area can also be considered as the border between

the EASJ and EAPJ in the cool seasons, especially in

wintertime. In summer, accompanied with the northward

jump of the EASJ, the south lobe is unanimously located

along the north side of the TP. The number of the jet

cores in the north lobe is dramatically reduced in the re-

gion of 508–658N.

b. STEA and sBI

The climatological distributions of the STEA at 300 hPa,

and the Eady growth rate maximum sBI between 700 and

850 hPa for the four seasons are shown in Fig. 3. Two

branches of the STEA are located over the East Asian

landmass. The strong northern branch is located in the

3224 J O U R N A L O F C L I M A T E VOLUME 23

broad midlatitude bands and extends southeastward,

while the weak southern branch is zonally elongated

over the south flank of the TP and joins the northern

branch over the East Asian coastal region. The STEA

is much suppressed over the TP region. For the sea-

sonal cycle aspect, the STEA in the northern branch is

stronger in spring, autumn, and winter and weaker in

summer, while the south branch of the STEA appears

only in winter and spring. Accompanied by the two-jet

state over East Asia, the baroclinicity over the landmass

shows two noticeable bands. The dominant one is lo-

cated on the poleward side of the TP; it has a spring

maximum and a summer minimum, resembling the local

STEA. The other sBI band on the equatorward side

of the TP appears only in spring and winter, with dra-

matically smaller values than that on the poleward side

of the TP.

c. Latitude–pentad section of STEA,zonal wind, and sBI

We further examine the details of seasonal marching

of the synoptic-scale eddy activities and their associated

large-scale flow in East Asian, especially in the EAPJ

region. We attempt to show that both the STEA and

EAPJ (also sBI) demonstrate northward propagation

from April to June. It is one of the distinct features of

the STEA over East Asia, compared with that over the

North Pacific. The latitude–pentad sections of the clima-

tological STEA, upper-level zonal wind fields, and low-

level sBI are shown in Figs. 4a, 5a, and 6a, respectively.

The longitude bands of the section span over the East

Asian region. To provide a convenient comparison with

the features over the oceanic region, the sections over the

North Pacific are depicted in Figs. 4b, 5b, and 6b, re-

spectively. Figure 4a shows that the main body of the

STEA (located poleward of 408N and especially in the

458–708N band) increases in strength from early spring

and is continuously prosperous until the middle of June.

Accompanied with this intensification, the center of the

main STEA band conspicuously shifts poleward in May

and June. It begins to get weaker after the middle of

June, and reaches minimum strength in July and August.

After reviving in early September, the STEA over East

Asia goes into the second active period of the year. The

STEA over the North Pacific (Fig. 4b) also demonstrates

gradual northward movement from winter to summer.

However, the extent of north–south shifting of STEA

over the North Pacific is dramatically smaller than that

over the East Asian region.

In cool seasons, the EASJ dominates the subtropical

region over East Asia (Fig. 5a). It occupies the north

side of the TP in July–August and retreats to the south

FIG. 1. Climatological distribution of the wind vector and isotach (m s21) at 300 hPa for (a) spring, (b) summer,

(c) autumn, and (d) winter. The orographic isoline of 3 km corresponding to the main body of the Tibetan Plateau is

denoted (thick dark line).

15 JUNE 2010 R E N E T A L . 3225

side of the TP after September. The features of the

northward–southward movement of the EASJ shown in

Fig. 5a are generally consistent with the results of

Schiemann et al. (2009). This previous study has exam-

ined in detail the transitions of EASJ in spring and au-

tumn. Hence, here we focus on the EAPJ evolution. In

contrast to the strong and quasi-steady EASJ, the EAPJ

poleward of 408N is highly transient, and displays de-

tectable subseasonal variations (Fig. 5a). However, it is

still notable that the EAPJ is more intensified in April

and May. In early June, accompanied by a gradual de-

crease in wind intensity, the belt of strong EAPJ quickly

migrates northward and dominates the higher latitudes.

After the migration, the EAPJ is dramatically weakened

in July and August. Its intensity recovers at midlatitudes

in October and November. Over the oceanic region, the

westerly jet stream is located in the subtropical region

steadily during cool seasons and only moves northward

slightly during summer (Fig. 5b).

Consistent with many previous studies, the evolution

of sBI over the oceanic area (Fig. 6b) does not match

that of the local STEA (Fig. 4b). The STEA in Fig. 4b is

robust in spring but not in winter, though sBI (also the

zonal wind shown in Fig. 5b) reaches its peak in winter,

demonstrating the suppression of the Pacific storm track

in midwinter (Nakamura 1992). In the EAPJ region in

Fig. 6a, conversely,sBI exhibits a marked maximum in

spring accompanied with a prosperous STEA during the

same period. The band with large sBI values shown in

Fig. 6a also expands northward in May and June, similar

to the STEA behavior shown in Fig. 4a. In July and

August, the values of sBI decrease rapidly in both the

EASJ region and the mid- to high latitudes. In autumn,

large values of sBI reappear in the midlatitude bands

(Fig. 6a).

4. Why are the STEA and sBI over East Asiaprosperous in spring?

The analysis in section 3 demonstrates that both the

upper-level STEA and lower-level baroclinicity pole-

ward of the TP are prosperous in spring. This is another

distinct feature of the STEA over East Asia, compared

with that over the North Pacific. In this section, we at-

tempt to show that the seasonal variation of the land–air

interaction on the poleward side of the TP region, es-

pecially the sensible heat flux on the land surface, is

partly responsible for the spring prosperity of the local

STEA and sBI.

As mentioned early, the value of sBI is mainly de-

termined by the vertical wind shear ›jVj/›z and the

Brunt–Vaisala frequency N in the lower-level tropo-

sphere. The detailed seasonal marching of ›jVj/›z and N

are summarized by the latitude–pentad sections in Fig. 7.

FIG. 2. Climatological distribution of the number of jet cores at 300 hPa for (a) spring, (b) summer, (c) autumn,

and (d) winter.

3226 J O U R N A L O F C L I M A T E VOLUME 23

Figure 7a suggests that, in the EAPJ region, the seasonal

cycle of the wind shear does not completely match that of

sBI shown in Fig. 6a. For example, sBI in the EAPJ region

has a spring maximum, though the local wind shear rea-

ches its maximum in late autumn and winter. The reason

for the spring maximum of sBI is that sBI is also related to

the seasonal cycle of the local N. The values of N over the

East Asian midlatitude landmass show a robust seasonal

variation (Fig. 7c). It decreases rapidly from about 1.1 at

the beginning of March to less than 0.8 at the end of May.

Thus, the dramatically decreased N contributes to the large

local sBI in spring jointly with the slightly weaker (com-

pared with that in late autumn and winter) but still sig-

nificant wind shear in the EAPJ region. In contrast, the

values of N over the oceanic region are relatively large

(Fig. 7d). The values are bigger than unity even during the

warm seasons, indicating that there is a comparatively

stable atmosphere over the ocean. On the other hand, the

amplitude of N over the oceanic region during the entire

year is small and nearly negligible. Therefore, the vertical

wind shear shown in Fig. 7b is responsible for the seasonal

cycle of sBI in the oceanic region shown in Fig. 6b.

FIG. 3. Climatological distributions of the STEA (contours, m2 s22) at 300 hPa and the Eady growth rate maximum

sBI between 700 and 850 hPa (color shading, day21) for (a) spring, (b) summer, (c) autumn, and (d) winter.

FIG. 4. Latitude–pentad section of climatological STEA at 300 hPa for the longitude bands of (a) 808–1208E and

(b) 1608E–1308W. The contour intervals are 15 m2 s22 for (a) and 30 m2 s22 for (b).

15 JUNE 2010 R E N E T A L . 3227

The rapid decrease of N in the EAPJ region in spring

indicates a decrease in local static stability. This could be

caused by either of the following two factors: a decrease

in the difference in potential temperature (Du), or an in-

crease in the thickness (Dz) between two pressure levels.

Figure 8 shows the latitude–pentad sections of Du and the

Dz between the 700- and 850-hPa levels. Clearly, over

both the landmass and the oceanic regions, the thickness

between the two levels increases steadily from January to

July and then decreases gradually afterward. In contrast,

the evolution of the Du over the landmass bears close re-

semblance to that of N, suggesting that the rapid decrease

in N to the north of the TP in spring is mostly attributed to

the local decrease in Du.

Most of the EAPJ region is arid or semiarid with

relatively small rainfall climatologically. This region is

desert or semidesert, with scattered vegetation cover. In

spring, the land surface in this region is bare soil with

little soil moisture (Xu et al. 2006). With the rapid in-

crease in solar radiation in spring, the land surface is

warmed up and then heats the low-level atmosphere

mainly through the surface sensible heat flux. At the same

time, the intensified turbulent diffusion can transport the

heat from the lower to higher levels (Qian et al. 2006;

Song et al. 2009). The upper panels in Fig. 9 show the

latitude–pentad sections of the surface sensible heat over

the East Asian landmass and the western North Pacific.

The rapid decrease in N in the EAPJ region in spring

(Fig. 7c) is accompanied by a dramatic increase in the

local sensible heat flux (Fig. 9a). The lower panels in Fig. 9

depict time evolutions of the air temperature at 850 and

700 hPa for the two regions. Figure 9c shows that over

the landmass as the air is heated via the sensible heat

flux in spring; the air temperature at 850 hPa increases

more rapidly than that at 700 hPa. This leads to a rapid

decrease in Du between 850 and 700 hPa (Fig. 8a), and

a simultaneous significant decrease in static stability in

the lower troposphere (Fig. 7c). In contrast, over the oce-

anic region the temperature difference between the two

levels shows weaker changes in the whole year (Fig. 9d),

FIG. 5. As in Fig. 4, but for zonal wind at 300 hPa. The contour interval is 6 m s21. The longitude bands are

(a) 808–1208E and (b) 1308–1808E.

FIG. 6. As in Fig. 4, but for the Eady growth rate maximum sBI. The contour interval is 0.2 day21. The longitude bands

are (a) 808–1208E and (b) 1308–1808E.

3228 J O U R N A L O F C L I M A T E VOLUME 23

though the seasonal variation of the sensible heat flux is

significant (Fig. 9b). This leads to smaller variations in

Du and N (Figs. 7d and 8b).

To further investigate the relationship between the

STEA, sBI, wind vertical shear, N, Du, and surface

sensible heat flux in the EAPJ region in different sea-

sons, Fig. 10 shows the scatter diagrams between each

pair of the elements from the above list. The first two

rows in Fig. 10 are for the EAPJ region, and the third

row is for the North Pacific region. Climatologically, a

simple quasi-linear relationship is found between STEA

and sBI in the EAPJ region. That is, the strong sBI in

the EAPJ region corresponds to active local STEA

(Fig. 10a). Figures 10b,c tell that in the EAPJ region, sBI

is mainly determined by local wind shear in winter, au-

tumn, and summer. However, in spring the relationship

between sBI and wind shear is not obvious, and sBI is

jointly affected by wind shear and N. This result is in

distinct contrast to that in the oceanic region where the

seasonal cycle of the wind shear is always responsible for

the local sBI (see Figs. 10h,i). In the EAPJ region, the

overall seasonal marching of N shows a quasi-linear re-

lationship with that of Du (Fig. 10d). The relationship

between the sensible heat flux and Du shows a sea-

sonal dependence to a certain extent (Fig. 10e). The long

oblique blue-dotted lines in Figs. 10e,f demonstrate that,

FIG. 7. Climatological latitude–pentad sections of (a),(b) the vertical wind shear between 850 and 700 hPa (with

a contour interval of 0.6 3 102 day21), and (c),(d) the Brunt–Vaisala frequency N (with a contour interval of 0.1 3

103 day21). Longitude bands of (left) 808–1208E and (right) 1308–1808E are shown.

FIG. 8. As in Fig. 7, but for the difference in potential temperature between 700 and 850 hPa (Du; thin contours and

shaded area, K), and the thickness between the two levels (Dz; thick contours, gpm).

15 JUNE 2010 R E N E T A L . 3229

from March to May, Du and N decrease robustly ac-

companied with the rapid increase in surface sensible

heat flux. Thus, the large local sBI is maintained in spring.

The long, oblique yellow lines for autumn in Figs. 10e,f

demonstrate an opposing process, that is, an unfavorable

condition for the local sBI. In Figs. 10b,c, the two blue

dots near the purple ones (representing summer) corre-

spond to the last two pentads of May, when the atmo-

spheric circulation begins to transform from spring to

summer patterns. Thus, the relationship between the two

variables bears some resemblance to that in summer.

How the atmospheric boundary processes influence

the eddy activity is an interesting issue. Recently, Zhang

et al. (2009) discussed the equilibrium response of eddy

activity to different boundary layer processes by using

a plain multilevel quasigeostrophic channel model with

interactive static stability and a simplified parameteriza-

tion of atmospheric boundary layer physics. Their model

results show that boundary processes, for example, the

turbulent vertical heat transport and the surface heat flux,

play important roles in baroclinic eddy equilibration.

This is carried out through modifying the mean flow and

maintaining the mean flow available energy. These pro-

cesses result in more active eddy activities. The model

results of Zhang et al. (2009) are confirmed in this study

using the reanalysis data. Namely, in the EAPJ region,

the rapid increase in the sensible heat flux in spring

and the associated boundary processes (the decrease in

Du between 700 and 850 hPa, and the increase in the

turbulence transport) lead to the decrease in the static

stability and the increase in sBI at the lower level. This is

favorable to the active STEA in spring.

5. Conclusions and discussion

In this paper we attempt to investigate the seasonal

variations of the jet streams and the associated synoptic-

scale eddy activities over the East Asian landmass, with

an emphasis on the seasonal marching of the East Asian

polar front jet and the STEA. We try to explore an ex-

planation for the spring prosperity of the STEA and low-

level baroclinicity in the EAPJ region.

Through counting the number of jet cores in the

6-hourly wind fields combined with the monthly fields,

the geographical border between the EASJ and EAPJ is

identified. This border lies at the latitudes of the northern

part of the Tibetan Plateau. In the summer season as a

consequence of the northward jump of the EASJ and the

disappearance of the EAPJ, the two-jet state is replaced

by a single jet pattern of the EASJ over East Asia. Cor-

responding to the two-jet state in the cool seasons, two

branches of the STEA and baroclinicity dominate over

the East Asian landmass. The active northern branch is

located to the north of the TP and the weak southern

branch is zonally elongated over the south flank of the

TP. Associated with the single jet pattern in the warm

season, the relatively weakened STEA belt is located in

the mid- to high-latitude bands, while the STEA and the

FIG. 9. As in Fig. 4, but for sensible heat flux (W m22). The longitude bands are (a) 808–1208E and (b) 1308–1808E.

Time evolution of the air temperature (8C) at 850 (dotted line) and 700 hPa (solid line) for the regions of

(c) 408–608N, 808–1208E, and (d) 308–608N, 1308–1808E. The long dashed lines in (c) and (d) denote the temperature

difference between 850 and 700 hPa.

3230 J O U R N A L O F C L I M A T E VOLUME 23

baroclinicity along the south flank of the TP disappear

with the absence of the strong local westerlies. The STEA

over the TP region is suppressed throughout the entire

year.

By examining the latitude–pentad sections of the cli-

matological STEA, and upper-level zonal wind fields and

low-level baroclinicity in the EAPJ region, it is revealed

that the STEA in the EAPJ region demonstrates spring

FIG. 10. Scatter diagrams showing the relationship between sBI (day21) and (a) STEA (m2 s22), (b) vertical wind

shear (day21 3 10 2), (c) N (day21 3 103); (d) N and Du (K), (e) Du and sensible heat flux (W m22), and (f) N and

sensible heat flux. The variables in (a)–(f) are averaged within the 458–608N, 808–1208E area. (g)–(i) Same as (a)–(c),

respectively, but averaged for the North Pacific region, defined as 308–608N, 1608E–1308W for STEA and 308–608N,

1308–1808E for sBI, wind shear, and N.

15 JUNE 2010 R E N E T A L . 3231

prosperity accompanied by the strong local baroclinicity.

The STEA is weakened in summer conspicuously, ac-

companied by the disappearance of the EAPJ at mid-

latitudes. During the transition from late spring to early

summer, the main STEA band over East Asia migrates

poleward in conjunction with the EAPJ shifting in the

same direction.

The seasonal variation of the land–air interaction to

the north of the TP region, especially the rapid increase

in the sensible heat flux in spring and the associated

boundary processes, is partially responsible for the spring

prosperity of the local baroclinicity and the STEA. This is

because in the EAPJ region N dramatically decreases,

while the wind shear is slightly weaker but still significant.

This results in strong local baroclinicity in spring, which is

favorable for the active STEA over the landmass.

Recently, the influence of the eddy-seeding effect from

upstream on the North Pacific storm-track variations is

mentioned by several studies (Orlanski 2005; Chang and

Guo 2007). To address such an issue one needs a de-

scription of the seasonal marching of the synoptic-scale

eddy activities over East Asia. Our study meets such a

need. Furthermore, the two-jet state in the upper level and

its close relationship with the eddy activities are funda-

mental dynamical issues that cannot be overemphasized in

the midlatitude bands. The results of this study enrich the

knowledge of the two-jet structure over East Asia and the

seasonal marching of EAPJ with the local baroclinicity

and eddy activities. However, there are still open ques-

tions that need in-depth investigation. For instance, what

is the dynamical connection between the EAPJ and STEA

over East Asia? Can far-upstream eddy activities over

the European region influence the seasonal variation of

the STEA over East Asia via downstream development?

Clearly, further investigations are needed to better un-

derstand these dynamic processes.

Acknowledgments. This work is jointly supported by

the National Natural Science Foundation of China un-

der Grants 40775044 and 40730953, and by the National

Public Benefit Research Foundation of China under

Grants GYHY200706005 and GYHY200806004. We

thank the two anonymous reviewers for their insightful

criticism and suggestions that led to significant im-

provement in our manuscript. We would also like to

thank Dr. Youyu Lu and Dr. Yaocun Zhang for their

helpful discussions. The ERA-40 data are obtained from

the ECMWF data server.

REFERENCES

Asai, T., Y. Kodama, and J.-C. Zhu, 1988: Long-term variations of

cyclone activities in East Asia. Adv. Atmos. Sci., 5, 149–158.

Bals-Elsholz, T. M., E. H. Atallah, L. F. Bosart, T. A. Wasula,

M. J. Cempa, and A. R. Lupo, 2001: The wintertime Southern

Hemisphere split jet: Structure, variability, and evolution.

J. Climate, 14, 4191–4215.

Black, R. X., and R. M. Dole, 2000: Storm tracks and barotropic

deformation in climate models. J. Climate, 13, 2712–2728.

Blackmon, M. L., 1976: A climatological spectral study of the 500 mb

geopotential height of the Northern Hemisphere. J. Atmos. Sci.,

33, 1607–1623.

Chang, E. K. M., 2001: GCM and observational diagnoses of the

seasonal and interannual variations of the Pacific storm track

during the cool season. J. Atmos. Sci., 58, 1784–1800.

——, 2009: Diabatic and orographic forcing of northern winter

stationary waves and storm tracks. J. Climate, 22, 670–688.

——, and Y. Guo, 2007: Dynamics of the stationary anomalies

associated with the interannual variability of the midwinter

Pacific storm track—The roles of tropical heating and remote

eddy forcing. J. Atmos. Sci., 64, 2442–2461.

——, S. Lee, and K. L. Swanson, 2002: Storm track dynamics.

J. Climate, 15, 2163–2183.

Chen, S. J., Y.-H. Kuo, P.-Z. Zhang, and Q.-F. Bai, 1991: Synoptic

climatology of cyclogenesis over East Asia, 1958-1987. Mon.

Wea. Rev., 119, 1407–1418.

Chen, T.-C., S.-Y. Wang, W.-R. Huang, and M.-C. Yen, 2004:

Variation of the East Asian monsoon rainfall. J. Climate, 17,

744–764.

Eady, E. T., 1949: Long waves and cyclone waves. Tellus, 1, 33–52.

Gallego, D., P. Ribera, R. Garcia-Herrera, E. Hernandez, and

L. Gimeno, 2005: A new look for the Southern Hemispheric

jet stream. Climate Dyn., 6, 607–621.

Hoskins, B. J., and P. J. Valdes, 1990: On the existence of storm

tracks. J. Atmos. Sci., 47, 1854–1864.

——, and K. I. Hodges, 2002: New perspectives on the Northern

Hemisphere winter storm tracks. J. Atmos. Sci., 59, 1041–1061.

——, and ——, 2005: A new perspective on the Southern Hemi-

sphere winter storm tracks. J. Climate, 18, 4108–4129.

Koch, P., H. Wernli, and H. C. Daves, 2006: An event-based jet-

stream climatology and typology. Int. J. Climatol., 26, 283–301.

Kuang, X., and Y. Zhang, 2005: Seasonal variation of the East

Asian subtropical westerly jet and its association with the

heating field over East Asia. Adv. Atmos. Sci., 22, 831–840.

Lee, S., and H. Kim, 2003: The dynamical relationship between

subtropical and eddy-driven jet. J. Atmos. Sci., 60, 1490–1503.

Li, C., J.-T. Wang, S.-Z. Lin, and H.-R. Cho, 2004: The relationship

between East Asian summer monsoon activity and northward

jump of the upper westerly jet location. Chin. J. Atmos. Sci.,

28, 642–658.

Lim, Y.-K., K.-Y. Kim, and H.-S. Lee, 2002: Temporal and spatial

evolution of the Asian summer monsoon in the seasonal cycle

of synoptic fields. J. Climate, 15, 3630–3644.

Lunkeit, F., K. Fraedrich, and S. E. Bauer, 1998: Storm tracks in

a warmer climate: Sensitivity studies with a simplified global

circulation model. Climate Dyn., 14, 813–826.

Mahlman, J. D., 1973: On the maintenance of the polar front jet

stream. J. Atmos. Sci., 30, 544–557.

Mesquita, M. D. S., N. G. Kvamstø, A. Sorteberg, and

D. E. Atkinson, 2008: Climatological properties of summer-

time extra-tropical storm tracks in the Northern Hemisphere.

Tellus, 60A, 557–569.

Murakami, M., 1979: Large-scale aspects of deep convective ac-

tivity over the GATE area. Mon. Wea. Rev., 107, 994–1013.

Nakamura, H., 1992: Midwinter suppression of baroclinic wave

activity in the Pacific. J. Atmos. Sci., 49, 1629–1642.

3232 J O U R N A L O F C L I M A T E VOLUME 23

——, and A. Shimpo, 2004: Seasonal variations in the Southern

Hemisphere storm tracks and jet streams as revealed in a re-

analysis dataset. J. Climate, 17, 1828–1844.

Newton, C. W., 2004: Associations between twice-yearly oscillations

of the North Pacific cyclone track and upper-tropospheric cir-

culations over the Eastern Hemisphere. Mon. Wea. Rev., 132,

348–367.

Orlanski, I., 2005: A new look at the Pacific storm track variability:

Sensitivity to tropical SSTs and to upstream seeding. J. Cli-

mate, 18, 1367–1390.

Palmen, E., and C. W. Newton, 1969: Atmospheric Circulation

Systems. International Geophysics Series, Vol. 13, Academic

Press, 603 pp.

Qian, Z., Y. Cai, J. Liu, Z. Liu, D. Li, and M. Song, 2006: Some

advances in dust storm research over China-Mongolia areas.

Chin. J. Geophys., 49, 83–92.

Riehl, H., 1962: Jet streams of the atmosphere. Department of

Atmospheric Science, Colorado State University, Tech. Rep.

32, 177 pp.

Schiemann, R., D. Luthi, and C. Schar, 2009: Seasonality and in-

terannual variability of the westerly jet in the Tibetan Plateau

region. J. Climate, 22, 2940–2957.

Sheng, C., 1986: An Introduction to the Climate of China. Beijing

Science Press, 85–89 pp.

Son, S.-W., M. Ting, and L. M. Polvani, 2009: The effect of to-

pography on storm-track intensity in a relatively simple gen-

eral circulation model. J. Atmos. Sci., 66, 393–411.

Song, Y., W. Guo, and Y. Zhang, 2009: Numerical study of impacts

of soil moisture on the diurnal and seasonal cycles of sensible/

latent heat fluxes over semi-arid region. Adv. Atmos. Sci., 26,319–326.

Strong, C., and R. E. Davis, 2007: Winter jet stream trends over the

Northern Hemisphere. Quart. J. Roy. Meteor. Soc., 133, 2109–

2115.

——, and ——, 2008: Variability in the position and strength of

winter jet stream cores related to Northern Hemisphere tele-

connections. J. Climate, 21, 584–592.

Uppala, S. M., and Coauthors, 2005: The ERA-40 Re-analysis.

Quart. J. Roy. Meteor. Soc., 131, 2961–3012.

Wang, B., and LinHo, 2002: Rainy season of the Asian–Pacific

summer monsoon. J. Climate, 15, 386–398.

Wang, X., P. Zhai, and C. Wang, 2009: Variations in extratropical

cyclone activity in northern East Asia. Adv. Atmos. Sci., 26,471–479.

Wernli, H., and C. Schwierz, 2006: Surface cyclones in the ERA-40

dataset (1958–2001). Part I: Novel identification method and

global climatology. J. Atmos. Sci., 63, 2486–2507.

Whittaker, L. M., and L. H. Horn, 1984: Northern Hemisphere

extratropical cyclone activity for four mid-seasonal months.

J. Climatol., 4, 297–310.

Wu, G., and Coauthors, 2007: The influence of mechanical and

thermal forcing by the Tibetan Plateau on Asian climate.

J. Hydrometeor., 8, 770–789.

Xu, X., K. L. Jason, Z. Lin, and H. Chen, 2006: An investigation of

sand-dust storm events and land surface characteristics in China

using NOAA NDVI data. Global Planet. Change, 52, 182–196.

Zhang, Y., X. Kuang, W. Guo, and T. Zhou, 2006: Seasonal evolu-

tion of the upper-tropospheric westerly jet core over East Asia.

Geophys. Res. Lett., 33, L11708, doi:10.1029/2006GL026377.

——, P. H. Stone, and A. Solomon, 2009: The role of boundary

layer processes in limiting PV homogenization. J. Atmos. Sci.,

66, 1612–1632.

15 JUNE 2010 R E N E T A L . 3233