the interannual variability of summer upper-tropospheric temperature over east asia

The Interannual Variability of Summer Upper-Tropospheric Temperatureover East Asia

LIXIA ZHANG AND TIANJUN ZHOU

LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

(Manuscript received 23 November 2010, in final form 10 March 2012)

ABSTRACT

By using 55-yr NCEP–NCAR reanalysis data, two dominant interannual variability modes of summer upper-

tropospheric (500–200 hPa) temperature over East Asia are identified. The first empirical orthogonal function

(EOF1)mode in its positive sign features amonopole cooling anomaly, while the secondmode (EOF2) features

a meridional dipolemode, with the positive (negative) center located south (north) of 358N. The EOF1 (EOF2)

mode is associated with ENSO developing (decaying) summers. They are the result of dynamical tele-

connections remotely induced by ENSO and local moist processes. During the El Nino developing summer, the

Indian summer monsoon precipitation decreases and forces the Silk Road teleconnection pattern at 200 hPa,

featuring an anomalous cyclone over the East Asian continent. Coupled with the anomalous northerly wind in

easternChina at 850 hPa, rainfall over north (south) China is suppressed (enhanced). The anomalous cyclone in

the upper troposphere, associated vertical motion, and precipitation contribute to the heat and vorticity balance

and maintain the monopole cooling. In the El Nino decaying summer, driven by the combined effects of a local

SST anomaly and remote warm SST anomaly forcing from the Indian Ocean, precipitation is reduced over the

western PacificOcean. Less latent heat is released and forces the Pacific–Japan teleconnection pattern along the

EastAsian continent, inducing a tripolar rainfall anomaly over EastAsia. The tripolar precipitation and vertical

motion anomalies and the zonal extended cyclonic anomaly in the upper troposphere provide the heating and

momentum flux balance and maintain the temperature anomaly pattern during the ENSO decaying summer.

1. Introduction

The East Asian summer monsoon (EASM) is an im-

portant component of the global monsoon system and

plays an important role in global climate variability. The

distinctive topography of East Asia produces unique

features of the EASM (Ding 1994; Webster et al. 1998;

Wang et al. 2001). The summer monsoon results from the

differential heating between Eurasian landmass and the

adjacent oceans (Webster et al. 1998). Multiscale vari-

ability of the EASM is greatly affected by the changes of

land–sea thermal contrast. Changes of land–sea thermal

contrast have been used to explain the interdecadal vari-

ability of the EASM (Ding et al. 2008; Li et al. 2010) and

to measure its predictability (Zhou and Zou 2010). Un-

derstanding the mechanism of tropospheric temperature

change is crucial to the prediction of land–sea thermal

contrast and thereby the monsoon variability.

The interdecadal variability of tropospheric temper-

ature over East Asia has been well documented. For

example, during the period from 1950 to 2000, contrary

to the warming trend elsewhere, a strong tropospheric

cooling trend was prominent over East Asia during July

and August that contributes to the tendency toward in-

creased droughts in northern China andmore floods along

the Yangtze River valley (Yu et al. 2004; Yu and Zhou

2007). Further analysis reveals that the tropospheric

cooling over East Asia is a regional manifestation of

an interdecadal variability mode of tropospheric tem-

perature change across the entire subtropical Northern

Hemisphere, which exhibits a significant cooling center

over East Asia and two warming centers over the North

Atlantic and North Pacific Oceans (Zhou and Zhang

2009). The interdecadal upper-tropospheric cooling is also

evident in spring (Yu and Zhou 2007), which is associated

with an anomalous meridional cell with descending mo-

tions in the latitudes (268–358N) and low-level northerly

winds over southeastern China (228–308N, 1108–1258E),

Corresponding author address: Dr. Tianjun Zhou, LASG, In-

stitute of Atmospheric Physics, Chinese Academy of Sciences,

Hua-Yan-Li No. 40, BeichenWest St., ChaoyangDistrict, P.O. Box

9804, Beijing 100029, China.

E-mail: [email protected]

1 OCTOBER 2012 ZHANG AND ZHOU 6539

DOI: 10.1175/JCLI-D-11-00583.1

� 2012 American Meteorological Society

leading to deficient rainfall over South China (Xin et al.

2006).

In contrast to sufficient studies of interdecadal variabil-

ity, less effort has been devoted to the interannual vari-

ability of tropospheric temperature over East Asia. There

have been many studies on the interannual variations of

tropical tropospheric temperature (TT) associated with

ENSO. As a dynamical response to equatorial diabatic

heating, the warm phase of ENSO is characterized by an

overall warming of tropical TT superimposed upon a dis-

tinctive equatorially symmetric dumbbell-shaped pattern

straddling the equator (Yulaeva and Wallace 1994). Re-

cent studies have identified differentmodes associatedwith

two flavors of El Nino in terms of the three-dimensional

structure of tropical atmospheric temperature. The first is

a deep-warm mode that features a coherent zonal mean

warming throughout the troposphere from 308N to 308Swith cooling aloft. The second is a shallow-warm mode

that features strong wave signatures in the troposphere

with warmth (coolness) over the central Pacific (western

Pacific) (Trenberth and Smith 2006, 2009). Both the deep-

warm mode and the shallow-warm mode can be rea-

sonably reproduced by atmospheric general circulation

models forced by historical sea surface temperature (SST),

demonstrating that the two modes are driven by tropical

SST variability (Zhou and Zhang 2011). There are also

studies on the extratropical TT changes that highlight the

eddy-driven meridional circulation changes (Seager et al.

2003). However, up to now no special attention has been

paid to the East Asian domain. The present study aims

to answer the following questions. 1) Are there any

distinct interannual variability modes of summer upper-

tropospheric temperature (UTT) overEastAsia? 2)What

are the thermal and dynamic processes that contribute to

the interannual variations of summer UTT over East

Asia? 3) Whether and how does ENSO affect the in-

terannual variability modes of East Asian summer UTT?

The remainder of the paper is organized as follows:

Section 2 describes the data and analyses method. In

section 3, principal interannual variability modes of the

summer UTT over East Asia are presented. The thermal

and dynamic processes associated with the first two

leading modes are examined in section 4. The relation-

ship betweenENSO and summerUTT changes over East

Asia and the potential mechanisms are discussed in sec-

tion 5. Summary and discussion are provided in section 6.

2. Data and method description

a. Data description

The datasets used in the present study include

1) National Centers for Environmental Prediction–

National Center for Atmospheric Research

(NCEP–NCAR) reanalysis data from 1951 to 2005

(Kalnay et al. 1996),

2) the 40-yrEuropeanCentre forMedium-RangeWeather

Forecasts Re-Analysis (ERA-40) dataset from 1958 to

2002 (Uppala et al. 2005),

3) Japanese 25-yr Reanalysis (JRA-25) conducted by

the Japan Meteorological Agency from 1979 to 2005

(Onogi et al. 2007),

4) monthly global SST for the period of 1950–2005 taken

from the Hadley Centre Sea Ice and Sea Surface

Temperature dataset version1 (HadISST1), provided

by the Met Office Hadley Centre for Climate Pre-

diction and Research (Rayner et al. 2003), and

5) the precipitation reconstruction data compiled by the

Climate Prediction Center at the National Centers

for Environmental Prediction (PREC) from 1948 to

2005 (Chen et al. 2002).

In addition, themonthlymeanNino-3.4 index, defined

as SST anomalies averaged within the region 58S–58N,

1208–1708W is also used.

In addition to monthly mean fields, 6-hourly hori-

zontal and vertical velocity and temperature data from

the reanalysis datasets are also used to calculate the

diabatic heating and transient eddy heating.

b. Methodology

To assess the relative roles of the thermodynamic

processes, we diagnose the temperature heat budget and

the vorticity equation of the East Asian summer upper

troposphere. Following Yanai and Li (1994), the tem-

perature equation is written as

›T

›t52u

›T

›x2 y

›T

›y2 (p/p0)

R/Cpv

�›u

›p

�

2 (p/p0)R/Cp

�$ �V0u01

›(v0u0)

›p

�1Q1 . (1)

The vorticity equation can be written as

›z

›t52V � $z2by2 (f 1 z) � $V

1

�2

�u0›z0

›x1 y0

›z0

›y

�1 z0 � $V0

�1 res , (2)

where u, y, v, T, z, and u are three-dimensional (3D)

velocities, temperature, relative vorticity, and potential

temperature;R andCp are the gas constant and the specific

heat at constant pressure of dry air; p05 1000 hPa; f is the

Coriolis parameter; b 5 df/dy is a constant; and Q1 is the

diabatic heating. The overbar denotes the monthly aver-

age and the double prime represents the departure of 6-h

reanalysis data from the monthly average.

6540 JOURNAL OF CL IMATE VOLUME 25

The rhs terms in Eq. (1) are usually termed as zonal

temperature advection, meridional temperature advec-

tion, adiabatic heating, transient eddy heating, and di-

abatic heating, respectively (Yanai and Li 1994; Seager

et al. 2003; Tamura et al. 2010). The adiabatic heating

includes two physical processes: one is the vertical ad-

vection associated with vertical motion and another is

that the air expands and cools as it rises or contracts and

grows warmer as it descends. The rhs terms in Eq. (2) are

the horizontal vorticity tendency derived from hori-

zontal vorticity advection, beta term, divergence, tran-

sient eddy vorticity flux, and residual term (res). The res

term contains dissipation, external forcing, vertical ad-

vection, and the twisting terms.

The 3D diabatic heating Q1 is diagnosed as a residual

in the thermodynamic equation (e.g., Hoskins et al.

1989; Nigam 1994):

Q15›T

›t1V � $T1 (p/p0)

R/Cpv›u

›p

1 (p/p0)R/Cp

�$ �V0u01

›(v0u0)

›p

�. (3)

To focus on year-by-year variations, the long-term

trend and decadal variations with a period longer than

9 yr are removed by using Fourier harmonic analysis of

the seasonal mean anomalies. We have done analyses by

using both NCEP–NCAR and ERA-40 reanalyses. The

large-scale interannual variation modes derived from

the two reanalysis datasets are highly consistent for their

common period from 1958 to 2002, adding fidelity to the

robustness of the results. For brevity, we only present

the results derived from NCEP–NCAR reanalysis, which

has a longer time period.

3. Two major interannual variability modes ofsummer UTT over East Asia

Owing to the large topography over East Asia, the so-

called surface air temperature actually measures the

temperature at different altitudes. In comparisonwith the

surface air temperature, the upper-tropospheric (200–

500-hPa mean) temperature can well indicate the com-

bined effect of zonal thermal contrasts and meridional

thermal contrasts between East Asia and the ambient

ocean on East Asian summer monsoon changes. Thus, it

is generally to measure the tropospheric mean tempera-

ture by using 200–500-hPa average (Zhao et al. 2007;

Zhou and Zou 2010). In our analysis we focus on the 200–

500-hPa mean temperature. The standard deviations of

summer (June–August) mean UTT derived from three

reanalysis datasets (i.e., NCEP, ERA-40, and JRA-25)

are shown in Figs. 1a–c. Variability is most prominent

over the extratropics with two centers located in west-

central Asia (308–508N, 308–808E) and East Asia (208–608N, 908–1508E). Results derived from the three

reanalysis datasets are highly consistent. We focus on the

variability over East Asia in our following analysis.

To reveal the dominant interannual variability modes

of summerUTT over EastAsia, an empirical orthogonal

function (EOF) analysis of the temporal correlation

matrix is performed on summer mean UTT over East

Asia (208–608N, 908–1508E). The first two leading modes

and corresponding principal components are shown in

Fig. 2. The first EOF mode in its positive sign features

a monopole pattern centered near 308–508N, 1008–1408E.The second EOF mode in the positive sign features

a meridional dipole mode, with the positive (negative)

center located south (north) of 358N.

FIG. 1. Standard deviation of summer (JJA) upper-tropospheric

temperature (K) (500–200-hPa mean) for the period from 1958

to 2002 for (a) NCEP–NCAR reanalysis, (b) ERA-40, and (c)

JRA-25 for the period from 1979 to 2002. Contour intervals are 0.1;

shaded areas are greater than 0.5 K.


The percentage variance accounted for by the first six

eigenvalues of EOF analysis is shown in Fig. 3a. The unit

standard deviation of the sampling errors associated

with each percentage eigenvalue is also shown in Fig. 3a.

The EOF1 (EOF2) accounts for 40.9% (19.7%) of total

variance. According to the rule of North et al. (1982),

the first two leading modes are well distinguished from

each other in terms of the sampling error bars and hence

are statistically significant. Note that the two leading

modes explain a large part (i.e., 60.6%) of total variance.

To reveal the dominant time periods of the two leading

modes, power spectral analysis is done on the corre-

sponding principal component (PC) time series shown

in Figs. 2c,d. The power spectrum density distributions

and corresponding red noise of the first and second PC

time series are shown in Figs. 3b,c. The first mode has

a major spectral peak at 3.5 yr. The second mode has

a low-frequency spectral peak around 3–5 yr and a sec-

ondary peak around 2–3 yr. The dominant time periods

indicate that the strong interannual variability modes

may be related to ENSO: this is further confirmed by the

evidences to be presented in the following section.

To reveal the vertical structure of temperature anoma-

lies associated with these two leading modes, the latitude–

height cross sections of regression coefficients between the

PCs and June–August (JJA)mean temperature anomalies

averaged between 908 and 1508E are shown in Fig. 4. For

the first leading mode significant negative regression co-

efficients are evident over the whole of East Asia (Fig. 4a)

with the minimum center at 458N near 300–250 hPa. For

the second leading mode (Fig. 4b) significant positive

(negative) regression coefficients exist south (north) of

328N centered at 258N (458N) near 250 hPa. Signals of the

two leadingEOFmodes are thus vertically robust beneath

all of the upper troposphere.

To verify reliability of the modes we have also per-

formed a regression analysis as Dommenget and Latif

(2002).After removing the signal of EOF1, the regression

analysis reveals an out-of-phase pattern between South-

east Asia and northeast Asia as for EOF2 shown above.

We also changed the domain from 908–1508E to 08–1808.The temperature anomalies over East Asia associated

with the first two leading modes remain unchanged (fig-

ures not shown), demonstrating that the leadingmodes of

the upper-tropospheric temperature over East Asia are

independent of the boundaries.

4. Thermal and dynamic processes associated withthe first two leading modes

a. The anomalous circulation associated with the twoleading modes

The composite winds at 200 hPa, vertical motion at

500 hPa, and precipitation anomalies are shown in Fig. 5.

FIG. 2. The first two leading modes and corresponding normalized principal components of an EOF analysis

applied to the temporal correlation matrix of summer UTT anomalies: (a) EOF1, (b) EOF2; (c) PC1, and (d) PC2.

Contour intervals in (a) and (b) are 0.08; shaded areas are statistically significant at 10% level using a Student’s t test.


The threshold for composite analysis is done based on

the normalized PC time series, which are larger (less)

than 0.8 (20.8), as shown in Table 1. The climatological

mean circulation shows that in boreal summer the upper

troposphere over the Asian continent is dominated by a

planetary-scale warm air mass centered on the southern

Tibetan Plateau (Figs. 5a,b), which is the well-known

South Asian high and is accompanied with anticyclonic

circulation (Zhou and Li 2002). A jet stream is zonally

oriented over East Asia (Figs. 5c,d), which is the so-

called EastAsian subtropical westerly jet stream (Zhang

et al. 2006).

Associated with EOF1, deficient and excessive rain-

falls are seen over northern East Asia and from South

China to the south of Japan, respectively (Fig. 5a). At

200 hPa there is a cyclonic anomaly over East Asia,

associated with a northerly (southerly) anomaly on its

western (eastern) flank. It significantly suppresses the

vertical motion over northern East Asia and enhances

vertical motion over South China (Fig. 5c), inducing

rainfall anomalies as shown in Fig. 5a.

For EOF2, a meridional tripolar pattern along the

East Asian continent is seen in the rainfall, with de-

ficient precipitation in South China (208–258N) and

North China, and excessive precipitation along the

mei-yu–changma–baiyu rainband (Fig. 5b). Compared

with EOF1, the excessive rainfall anomalies shift

northward. The anomalous vertical motions are consis-

tent with precipitation anomalies. In the upper tropo-

sphere an anomalous zonal wind along 308–408N at

200 hPa dominates East Asia and accompanies a north-

erly wind along the East Asian continent. However,

a southerly wind in the eastern part of East Asia is not

obvious.

FIG. 3. (a) The total variance (%) explained by each EOF

for their respective interannual time series. Error bars are de-

termined by the formula of North et al. (1982). The power spec-

trum density (solid line) and red noise (dashed line) of (b) the

first and (c) the second EOF principal component; X axis in (b)

and (c) is the period (yr).

FIG. 4. Regression coefficient maps between the principal

components and zonal mean (908–1508E) summer temperature

anomalies (K/std) as latitude–pressure cross section: (a) PC1 and

(b) PC2. Contour intervals in (a) and (b) are 0.05; shading denotes

the 90% confidence level using a Student’s t test.


b. Heat and vorticity budget analysis

To understand how the temperature anomalies sus-

tain and reveal the corresponding thermal and dynam-

ical structures, the heat and vorticity balance associated

with the first two leading EOF modes are quantified

by performing a composite analysis. According to the

temperature anomalies shown in Fig. 2a and Fig. 2b, the

terms of the temperature equation averaged between

200 and 500 hPa are shown in Fig. 6.

For EOF1 significant cold (warm) meridional advection

is seen over northeast China (Japan and Korea) with a

center value of 20.7 K day21 (0.2 K day21) (Fig. 5b),

which cause the northerly (southerly) anomaly on the

western (eastern) flank of the anomalous cyclone as shown

in Fig. 5c. The zonal temperature advection shows an op-

posite pattern to the meridional advection (Fig. 6a). The

adiabatic heating and diabatic heating terms overall cor-

respond to the anomalous vertical motion and precipita-

tion, respectively. Because of their anomalies shown in

Fig. 5a, significant diabatic cooling (adiabatic warming) is

seen over most areas of East Asia except for South China.

A transient eddy heat flux anomaly (Fig. 6d) is negative

over the East Asian continent and positive over north-

eastern East Asia, but the amplitude is far weaker than for

the other four heating terms. Thus the transient eddy heat

flux contributes less to the temperature balance over East

Asia and is nearly negligible.

FIG. 5. The composite patterns for (a),(b) precipitation (shaded, mm day21). (c),(d) As in (a),(b) but for wind at

200 hPa (vector, m s21) and omega at 500 hPa (shaded, Pa s21) for (left) EOF1 and (right) EOF2. The dots in (a) and (b)

and in (c) and (d) denote the precipitation and omega at 500 hPa are statistically significant at 10% level by using two-

tailed Student’s t test, respectively.Contours in (a) and (b) and (c) and (d) represent the climatological summer-meanUTT

and zonal wind at 200 hPa, respectively. The wind at 200 hPa, statistically significant at 10% level, is shown in (c) and (d).


The heat budget analysis for EOF2 is shown in Figs.

6f–j. The horizontal temperature advection (Fig. 6f,g)

shows similar patterns as for EOF1, but the magnitude

of meridional advection anomalies over Japan and

Korea is weaker than that of EOF1 because of the local

weaker southerly wind anomaly shown in Fig. 5d. As-

sociated with the tripolar pattern of anomalous vertical

motion, the adiabatic heating anomalies exhibit a me-

ridional ‘‘positive–negative–positive’’ tripolar pattern

along the East Asian continent (Fig. 6h), which is bal-

anced by diabatic heating anomalies (Fig. 6j). The am-

plitude of the transient eddy heat flux anomaly is still the

smallest term.

The temperature advection anomalies, (2u›T/›x)9 or(2y›T/›y)9, can be divided into two parts: the anom-

alies induced by anomalouswind, 2u9›T/›x or 2y9›T/›y,and that by the anomalous temperature gradient,2u›T9/›xor 2y›T9/›y). The relative role of each term was cal-

culated (figures not shown). It shows that the posi-

tive zonal advection over southern East Asia is a

result due to the westerly winds of the anomalous cy-

clone, which advect warm temperature from the Tibetan

Plateau (2u9›T/›x. 0). The zonal advection over

northern East Asia is mainly induced by the zonal

gradient of anomalous temperature, that is, 2u›T9/›x).Note that the meridional advection is dominated by

2y9›T/›y. The compensation between the zonal and

meridional advection indicates the importance of the

baroclinic jet.

Individual terms of the vorticity equation averaged

between 200 and 500 hPa for the first two leading modes

are analyzed and shown in Fig. 7. Associated with the

monopole cooling of EOF1, the magnitude of anoma-

lous zonal vorticity advection (Fig. 7a) is the largest,

with a negative center over northeast China and a posi-

tive center over the North Pacific. Both the beta and

divergence terms show opposite anomalies to the zonal

vorticity advection, but with weaker strength. The

transient eddy flux is a key factor for the vorticity bal-

ance, especially over the North Pacific (Fig. 7d). The

residual term also plays an important role in the vorticity

balance (Fig. 7e).

The vorticity budget of EOF1 is related to the

anomalous circulation shown in Figs. 5a,c. The mecha-

nism can be summarized as follows. The westerly wind

of the cyclonic anomaly advects negative vorticity from

the Tibetan Plateau to East Asia [2(u›z/›x)9, 0] and

advects positive vorticity from East Asia to the North

Pacific Ocean [2(u›z/›x)9. 0] (Fig. 7a). The anoma-

lous meridional wind generates positive (negative)

vorticity; that is, 2by9 . 0 (2by9 , 0) over China

(North Pacific) (Fig. 7b). In the meantime, the con-

vergence in the upper troposphere creates positive

vorticity centered in North China (Fig. 7c). The

anomalous circulation alters the basic state in which

transient eddies propagate, and finally changes the

pattern of transient eddy vorticity flux convergence and

divergence (Fig. 7d).

For the dipole pattern of EOF2 (Figs. 7f–j), it is still

the horizontal vorticity advection that plays the most

important role in maintaining the anomalous circula-

tion. However, the anomalous zonal vorticity advection

is astride each side of the climatological summer-mean

jet core (408N) (Fig. 7f). The beta effect, divergence, and

transient eddy flux terms show an opposite pattern to the

vorticity advection term and, thus, tend to balance the

vorticity advection term. The associated mechanism is

similar to that of EOF1.

The above budget analyses demonstrate that distinct

circulation anomalies of the leading EOF modes induce

different heating and momentum anomalies and sustain

the East Asian summer UTT anomalies. To reveal the

forcing factors for the unique circulation anomalies as-

sociated with the two EOF modes, we further examine

the corresponding SST anomalies in the following sec-

tion. The dominant time periods of the principal com-

ponents of the first two leading modes indicate that the

strong interannual variability modes may be related to

ENSO. The connection between first two leading modes

and ENSO are discussed in the coming section.

5. Relationship of the first two leading modeswith ENSO

To reveal the relationship between the East Asian

summer UTT change with ENSO, the lead–lag corre-

lation coefficients between the Nino-3.4 index time se-

ries and the first two PCs are shown in Table 2. Here

year (21) [year (1)] denotes that the Nino-3.4 index

leads (lags) in the PC time series by one year. The lead–

lag correlation between PC1 and the Nino-3.4 index

shows a significant simultaneous positive correlation

(60.23, which is statistically significant at the 10% level)

but no significant correlation with the previous winter,

December–February [D(21)JF(0)], Nino-3.4 index. A

TABLE 1. The years for positive and negative leading modes. The

threshold for identifying positive (negative) modes is based on the

normalized PC time series that are greater (less) than 0.8 (20.8).

Type Positive Negative

PC1 1956, 1965, 1968, 1976,

1989, 1992, 1997

1953, 1961, 1967, 1970, 1978,

1988, 1991, 1994, 1998

PC2 1952, 1953, 1957, 1967, 1972,

1977, 1983, 1987, 1993,

1995, 1998, 2003

1955, 1956, 1966, 1971, 1978,

1984, 1989, 1992, 1994,

2000, 2004


FIG. 6. Composite patterns of 200–500-hPa mean summer (a),(f) zonal advection (2u›T/›x)9; (b),(g) meridi-

onal advection (2y›T/›y)9; (c),(h) adiabatic heating [2(p/p0)R/Cpv(›u/›p)]9; (d),(i) transient eddy heating

f2(p/p0)R/Cp[$ �V0u0 1 ›(v0u0)/›p]g9; and (e),( j) diabatic heating anomalies (Q19) (K day21) for (left) EOF1 and

(right) EOF2. Contour intervals are 0.05 K day21; shading denotes the 90% confidence level using a two-tailed

Student’s t test.


FIG. 7. As in Fig. 6 but for the composite patterns of (a),(f) vorticity advection (2V � $z)9; (b),(g) beta term (2by9);(c),(h) divergence term f2[( f 1 z) � $V]9g; (d),(i) transient eddy vorticity flux [2(u0›z0/›x1 y0›z0/›y) 1 z0 � $V0]9;and (e),(j) residual term (res9) (10210 s22). Contour intervals are 0.05 3 10210 s22.


correlation coefficient statistically significant at the 10%

level is evident since May–July and persists through the

following winter, indicating that the first leading mode

may be associated with the El Nino developing summer.

The PC2 is significantly correlated with the Nino-3.4 index

in the previous winter, D(21)JF(0), with a correlation

coefficient exceeding 0.4. The significant correlation per-

sists until the following fall and then becomes statistically

insignificant at the 10% level, indicating that the second

mode is associated with the El Nino decaying summer.

To further confirm the relationship between upper-

tropospheric temperature changes and ENSO, the

composite SST anomalies between positive and negative

years of the first two leading modes are calculated. As

shown in Fig. 8a, during the previous winter associated

with the EOF1, strong cold SST anomalies are evident in

the central and eastern Pacific Ocean and tropical In-

dian Ocean. During the summer, the eastern equatorial

Pacific Ocean witnesses significant warm anomalies

(Fig. 8b). The warm anomalies expand across the whole

tropical Pacific in the following winter (Fig. 8c). Time

evolution of the SST anomaly pattern demonstrates that

the EOF1 of summerUTT over East Asia tends to occur

during the El Nino developing summer.

For SST anomalies associated with EOF2 of the East

Asian summer UTT, the eastern equatorial Pacific

Ocean is dominated by significant warm anomalies in

the previous winter (Fig. 8d). In the following summer,

the amplitude of warm anomalies becomes weaker

in the central and eastern tropical Pacific, but stronger in

the tropical Indian Ocean (Fig. 8e). In the following

winter, the warm anomalies in the tropical Pacific and

Indian Ocean are still evident but weak in amplitude

(Fig. 8f). The seasonal evolution of SST anomalies

strongly indicates that the second EOF mode of East

Asian summer UTT is associated with the decaying

phase of El Nino.

Based on the Nino-3.4 index, we classify the entire 55

summers from 1951 to 2005 into ENSO developing

(DV) and ENSO decaying (DC) summers (see Table 3).

The definition of ENSO DV and DC is the same as in

Chou et al. (2003) except that the ENSO persisting years

are regarded as ENSO DC years in this study. There-

fore, some ENSO DC years are not followed by ENSO

DV years. Out of the 55 summers, 37 are associated with

ENSO events. Among them, 17 summers are classified

as ENSO DV year summer and 20 are classified as

ENSO DC year summer.

The composite JJA UTT anomalies over East Asia

during ENSO developing and decaying summers are

shown in Fig. 9. In El Nino developing summers (Fig. 9a),

a cooling anomaly is seen over the East Asia continent

centered at 358N, 1108E. In contrast, in La Nina devel-

oping years (Fig. 9b), the warm air center shifts north-

eastward, centered at 458N, 1208E over northeast Asia.

The difference between Fig. 9a and Fig. 9b indicates the

asymmetry of the East Asian summer UTT anomalies

between El Nino and La Nina events. Because EOF

analysis only depicts the symmetric component of circu-

lation anomalies, the symmetric component of UTT dur-

ing ENSO developing summer (the composite field

between Fig. 9a and Fig. 9b) is shown in Fig. 9c. It can be

seen that the upper troposphere over East Asia is domi-

nated by negative anomalies with a cold center located at

458N, 1208E that resembles the spatial pattern of EOF1

shown in Fig. 2a. In ENSO decaying summer (Figs. 9d–f),

a dipole pattern is seen and is evident in La Nina decaying

summers (Fig. 9e). The symmetric component of ENSO

decaying summers (Fig. 9f) shows that the East Asian

UTT is dominated by a dipolelike anomaly with a cold

(warm) center in the north (south). This pattern resembles

that of EOF2 shown in Fig. 2b.

The spatial correlation coefficient between tempera-

ture anomalies of EOF1 (EOF2) and that of ENSO

developing (decaying) summers over the East Asian

region (208–608N, 908–1508E) is 0.67 (0.87). Composite

maps over the global scale of ENSO developing (de-

caying) summers also resemble that of EOF1 (EOF2)

(figures not shown). The corresponding spatial correla-

tion coefficient over 608S–608N, 08 east to 3608 is 0.71

(0.83). The significant spatial correlation coefficients

add reliability to the relationship between ENSO and

the first two leading modes, confirming that the first and

second leading modes of the summer UTT over East

Asia are representative features of ENSO developing

and decaying summers, respectively.

To reveal the relevant atmospheric circulation changes,

the composite 850-hPa and 200-hPa wind, vertical motion

TABLE 2. Lead–lag correlation coefficients between Nino-3.4

SST index and principal components of the first and second EOF

modes of the summer UTT anomalies (as 3-month means stepped

one month at a time). Boldface numbers indicate correlations

statistically significant at the 10% level.

PC1 PC2

D(21)JF(0) 20.18 0.41JFM(0) 20.12 0.40

FMA(0) 20.03 0.46

MAM(0) 0.07 0.51

AMJ(0) 0.16 0.49MJJ(0) 0.25 0.39

JJA(0) 0.27 0.30

JAS(0) 0.31 0.25

ASO(0) 0.31 0.25SON(0) 0.28 0.23

OND(0) 0.25 0.20

ND(0)J(1) 0.23 0.19


at 500 hPa, and precipitation anomalies during ENSO de-

veloping and decaying summers are shown in Fig. 10.

During the ENSO developing summer, the eastern Pacific

SST anomaly has attained a certain magnitude (Fig. 8b)

that induces a suppressed Indian summer monsoon

(Fig. 10a) (Wang et al. 2003). The reduced rainfall in the

Indian monsoon region may force a wave train extending

from northwestern India to East Asia in the upper tro-

posphere (Fig. 10c), which is the so-called Silk Road tele-

connection pattern, featuring a cyclonic anomaly over

East Asia. At a lower level (850 hPa, Fig. 10a), a cyclonic

anomaly over thewesternNorth Pacific is seen, created by

the anomalous heating over the equatorial central Pacific,

that induced local evaporation and transport of dry air

into eastern China (Chou et al. 2003; Lau et al. 2000;

Wu et al. 2003, 2009a). The cyclonic circulation at the

upper level and northerly wind at low level leads to sup-

pressed vertical motion and less rainfall over North China

(Fig. 10a). The cyclonic anomaly over East Asia, decreased

precipitation, and suppressed vertical motion provide the

circulation condition for the heat and vorticity balance

shown in Figs. 6–7 and maintains the UTT anomalies.

FIG. 8. Composite patterns of SST anomalies (8C) between positive and negative PC time series for (a)–(c) EOF1

and (d)–(f) EOF2. The SST anomalies are for the (top) preceding winter (DJF), (middle) summer (JJA), and

(bottom) following winter. Contour intervals are 0.18C; shading denotes the 90% confidence level using a two-tailed

Student’s t test.

TABLE 3. Classification of the 1951–2005 summers into ENSO developing (DV) and decaying (DC) summers.

Type El Nino La Nina

DV 1957, 1965, 1968, 1972, 1982, 1986, 1991, 1994, 1997, 2002, 2004 1954, 1970, 1973, 1984, 1988, 1998

DC 1958, 1966, 1969, 1983, 1987, 1992, 1995, 1998, 2003, 2005 1955, 1956, 1971, 1974, 1975,1976, 1985, 1989, 1999, 2000


During ENSO decaying summer, by the combined

effects of local SST anomaly (SSTA) forcing in the

northwestern Pacific and remote SSTA forcing from the

Indian Ocean basin mode (IOBM) (Fig. 8e), the pre-

cipitation over the western Pacific, especially near the

Philippines, decreased (Chang and Li 2000; Yang et al.

2007, 2010; Li et al. 2008; Xie et al. 2009; Wu and Zhou

2008; Wu et al. 2009a,b, 2010). The related anomalous

tropical heating forced a meridional Rossby wave train

as shown in Fig. 10b. This is referred to as the Pacific–

Japan (PJ) or East Asian–Pacific teleconnection pattern

(Nitta 1987; Huang and Sun 1992). In the upper level

(Fig. 10d), the atmosphere is significantly warmed in the

whole tropical region and increases the tropospheric

meridional temperature gradient in the subtropics, then

leads to an enhancement of westerly flow to the south of

the westerly jet stream axis by the thermal balance and

thereby a cyclonic anomaly over northeast Asia (Fig. 10d).

The tripolar precipitation and vertical motion anomalies

and the zonal extended cyclonic anomaly at the upper

troposphere provide the heating and momentum flux bal-

ance andmaintain the temperature anomaly pattern during

ENSO decaying summer.

6. Summary and discussion

a. Summary

Two dominant interannual variability modes of sum-

mer upper-tropospheric (500–200 hPa) temperature over

East Asia are revealed by using 55-yr NCEP–NCAR re-

analysis data. The thermodynamic processes dominating

themaintenanceof the two leadingmodes are analyzed by

doing budget and composite analysis. The atmospheric

FIG. 9. Composite patterns of UTT anomalies (K) for the (a) El Nino developing summer, (b) La Nina developing

summer, and (c) difference between (a) and (b); (d),(f) as in (a)–(c) but for ENSO decaying summers. Contour

intervals are 0.1 K; shading denotes the 90% confidence level using a two-tailed Student’s t test.


circulation changes controlling the heating and vorticity

budget are studied.

The interannual variability of upper-tropospheric (500–

200-hPa mean) temperature over East Asia is dominated

by two leading modes, which account for about 60.0% of

total variance. The first leading EOF mode in its positive

sign features a monopole cooling temperature anomaly

over all of East Asia with its center located near 458N at

250–300 hPa. The second EOF mode in its positive sign

features a meridional dipole mode with the positive

(negative) center located south (north) of 358N.

Budgets and correlation analyses show that the tem-

perature anomalies of the two interannual variability

modes are both related to ENSO. The first mode is as-

sociated with ENSO developing summers, while the

second mode coincides with ENSO decaying summers.

They are the result of dynamical teleconnections re-

motely induced by ENSO and local moist processes.

In the positive sign of EOF1, the uniformUTT cooling

is sustained due to the heating and vorticity balance

induced by the anomalous cyclonic circulation at the

upper troposphere, associated vertical motion, and de-

creased (excessive) precipitation over northern East

Asia (South China). They are induced by teleconnections

with the ENSO developing summer. During El Nino de-

veloping summer, the Indian summer monsoon precipita-

tion decreases and may force the Silk Road teleconnection

pattern at 200 hPa, which features an anomalous cyclone

center over the East Asian continent. Coupled with the

anomalous northerly wind in eastern China at 850 hPa, the

FIG. 10. Composite patterns of wind at 850 hPa (vector, m s21) and precipitation (shaded, mm day21) for (a) ENSO

developing summers and (b) ENSO decaying summers. (c),(d) As in (a) and (b) but for streamfunction at 200 hPa

(contours, 1026 m2 s21) and omega at 500 hPa (shaded, Pa s21). The contour denotes precipitation, and omega is sta-

tistically significant at the 10% level. The black dot indicates that stream function is statistically significant at 10% level

using a two-tailed Student’s t test. The wind at 850 hPa that is statistically significant at 10% level is shown in (a) and (b).


anomalous circulation over East Asia favors suppressed

(excessive) rainfall over North China (South China).

In the positive sign of EOF2, the heating and vorticity

balance for maintaining the upper-tropospheric tempera-

ture warming (cooling) over East Asia south (north) of

358N is caused by a zonally extended anomalous cyclone in

the upper troposphere and a meridional tripolar pre-

cipitation (vertical motion) anomaly along the East Asian

continent. This is related with the remote forcing of ENSO

during its decaying phase. In the El Nino decaying sum-

mer, driven by the combined effects of a local SST

anomaly over the western PacificOcean and remote warm

SST anomaly forcing from the Indian Ocean basin mode,

precipitation is reduced over the western Pacific Ocean.

Less latent heat is released and forces the Pacific–Japan

teleconnection pattern along East Asia, which induces

tripolar heating anomalies over East Asia. The East Asian

subtropical westerly jet at 200 hPa is enhanced (weak-

ened) south (north) of the climatological jet core, inducing

a cyclonic anomaly over north East Asia.

b. Discussion

The identification of ENSO-related modes for the

East Asian summer UTT is expected to be useful in East

Asian summer monsoon prediction. This study provides

an explanation on how the two distinct UTT anomalies

are sustained. How the UTT anomalies grow deserves

further study. In addition, although the first two leading

modes of East Asian summer UTT resemble the UTT

anomalies associated with ENSO, there are still some

differences between Figs. 2a,b and Figs. 9c,f. Thus, in

addition to ENSO forcing, other factors that influence

East Asian summer UTT anomaly should exist. These

additional forcing factors also deserve further study.

There are other opinions about the formation of Silk

Road pattern and Pacific–Japan pattern (Kosaka et al.

2009; Yasui andWatanabe 2010; Kosaka and Nakamura

2006). This work indicates that the decline of Indian

monsoon precipitation during the ENSO developing

summer and reduced precipitation over western Pacific

Ocean during the ENSO decaying summer may be the

main forcing factor for them, respectively. This result is

consistent with previous studies (Lau et al. 2000;Wu and

Wang 2002; Wu et al. 2003; Hu et al. 2005). This study

indicates that the Silk Road pattern tends to occur in the

ENSO developing summers, which may provide some

insight into the hypothesis that the optimal heating

anomaly pattern for the Silk Road teleconnection

may have a lagged relationship with ENSO (Yasui and

Watanabe 2010). However, the relative roles of diabatic

heating anomalies in different regions during ENSO

developing summers cannot be fully identified based

purely on data diagnosis and, thus, needs to be further

investigated using numerical models. Numerical study is

also needed for the ENSO decaying summers.

Acknowledgments. This work was founded by National

Program on Key Basic Research Project (Grant

2010CB951904) and National Natural Science Founda-

tion of China under Grants 41125017 and 40890054.

This work is also supported by the ‘‘Strategic Priority

Research Program-Climate Change: Carbon Budget

and Related Issues’’ of the Chinese Academy of Sci-

ences under Grant XDA05110301, the Program of

Excellent State Key Laboratory (Grant 41023002), and

the National High-Tech Research and Development

Plan of China (Grant 2010AA012302). Helpful review

comments from four anonymous reviewers, and the editor

Dr. Michael Alexander, are gratefully acknowledged.

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