Impact of Tropical Pacific Precipitation Anomaly on the East AsianUpper-Tropospheric Westerly Jet during the Boreal Winter

YUANYUAN GUO

Center for Monsoon and Environment Research, and Department of Atmospheric Sciences, Sun Yat-Sen

University, Guangzhou, China

ZHIPING WEN

Center for Monsoon and Environment Research, and Department of Atmospheric Sciences, Sun Yat-Sen University,

Guangzhou, and State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, and

Jiangsu Collaborative Innovation Center for Climate Change, Nanjing, China

RENGUANG WU

Center for Monsoon System Research, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

RIYU LU

State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics,

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

ZESHENG CHEN

Center for Monsoon and Environment Research, and Department of Atmospheric Sciences, Sun Yat-Sen

University, Guangzhou, China

(Manuscript received 8 October 2014, in final form 4 May 2015)

ABSTRACT

The leading mode of boreal winter precipitation variability over the tropical Pacific for the period 1980–

2010 shows a west–east dipole pattern with one center over the western North Pacific (WNP) and Maritime

Continent and the other center over the equatorial central Pacific (CP). Observational evidence shows that

the variability of the East Asian upper-tropospheric subtropical westerly jet (EAJ) has a significant corre-

lationwith precipitation anomalies over theWNPandCP and that tropical precipitation anomalies overWNP

and CP have a distinct influence on the variation of the EAJ. A series of numerical experiments based on a

linear baroclinic model are performed to confirm the influence of the heating anomalies associated with

precipitation perturbations over theWNP and CP on the EAJ. The results of numerical experiments indicate

that a heat source over the WNP can excite a northward-propagating Rossby wave train in the upper tro-

posphere over East Asia and facilitate a poleward eddy momentum flux. It results in the acceleration of the

westerlies between 308 and 458N, which favors a northward displacement of the EAJ. The response induced

by a heat sink over theCP features a zonal easterly band between 258 and 408N, suggesting that the response to

heat sink associated with negative precipitation anomalies over the CPmay weaken the EAJ. A strengthened

relationship was found between tropical Pacific precipitation and the EAJ since 1979. The modeling results

suggest that the shift of mean states might be responsible for the strengthened EAJ–rainfall relationship

after 1979.

1. Introduction

Based on observational and theoretical analyses over

the past decades, it has become clear that the diabatic

heating anomaly associated with tropical convection has

Corresponding author address: Zhiping Wen, Department of

Atmospheric Sciences, Sun Yat-sen University, Guangzhou

510275, China.

E-mail: eeswzp@mail.sysu.edu.cn

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pronounced influences not only on the tropics but also

on the midlatitudes (Hoskins and Karoly 1981; Jin and

Hoskins 1995; Kummerow et al. 2000; McBride et al.

2003; Wu et al.2012). Many investigations have been

undertaken in order to understand the role of tropical

heating with regard to the variability of midlatitude

circulations (e.g., Wallace and Gutzler 1981; Nitta 1987;

Zheng et al. 2013). Observational studies have found

that anomalous tropical precipitation has a significant

impact on extratropical atmospheric circulation changes

through Rossby wave propagation (e.g., Hoskins and

Ambrizzi 1993; Webster and Chang 1998; Yoo et al.

2012), which has been documented as an important

linkage between the circulations at midlatitude and

tropical convection. Some efforts have been made to

understand the relationship between the circulation

change at the middle-to-high latitudes and anomalous

convection over the tropical Indian and Pacific Oceans

(Zheng et al. 2013), the Maritime Continent (Lau and

Chan 1983a,b), the western North Pacific (WNP; Lu

2001; Wang 2002; Zheng et al. 2013; Park and An 2014;

Jia et al. 2015), central Pacific (CP; Wallace and Gutzler

1981), and subtropical East Asia (Kwon et al. 2007; Lu

and Lin 2009). In particular, convection over the WNP

has been claimed to hold the key for the variability of

atmospheric circulation over East Asia in some previous

studies. For instance, Park and An (2014) found that the

tropical WNP convection significantly influences the

North Pacific atmospheric circulation in boreal winter

by modulating the jet stream.

Numerical models have also been applied to un-

derstand whether anomalous precipitation can signifi-

cantly influence the midlatitude circulations in various

time scales. On the intraseasonal time scale, Huang and

Lu (1989) demonstrated that the enhanced convective

activities over the WNP could lead to the intensified

variation of subtropical high over East Asia by using an

atmospheric general circulation model (GCM). On the

interannual time scale, Lu and Lin (2009) indicated that

tropical precipitation anomalies, compared with sub-

tropical precipitation anomalies, may be relatively trivial

for the variation of large-scalemeridional teleconnection

over East Asia via a simple linear model. On the decadal

time scale, Zheng et al. (2013) examined the main mode

of rainfall variations over the Indo-western Pacific. They

suggested that the stronger rainfall mode after the cli-

mate regime shift of the 1970s may lead to the decadal

shift of atmospheric circulation over East Asia based on

the results obtained from a simple two-level model.

Extensive investigations have been undertaken in

order to figure out the interaction between anomalous

precipitation and upper-tropospheric circulations over

the midlatitudes. According to current knowledge,

anomalous precipitation can induce poleward-

propagating Rossby waves (Hoskins and Karoly 1981;

Chen and Trenberth 1988; Lee et al. 2011a,b). Lau et al.

(2000) suggested that the westerly jet over East Asia

plays a quite important role in linking tropical heating to

extratropical circulation systems. Thus, it is requisite to

devote attention to the interaction between tropical

precipitation and the East Asian upper-tropospheric

subtropical westerly jet (EAJ). Many studies have fo-

cus on the interaction between the EAJ and pre-

cipitation over tropical and subtropical regions (e.g.,

Yang and Webster 1990; Li and Zhang 2014). On the

one hand, the change of EAJ has a pronounced impact

on the variation of precipitation. The variations of the

EAJ could significantly contribute to rainfall anomalies

over East Asia via influencing the intensity of the South

Asian high in summer on interannual time scales (Liao

et al. 2004). On the other hand, some studies have in-

vestigated the impact of precipitation variation on the

westerly jet. For instance, Yang and Webster (1990)

noticed that the heating field in the adjacent hemisphere

is important in determining the location and magnitude

of the EAJ in boreal summer. On decadal time scales,

Kwon et al. (2007) showed that the decadal decrease of

the zonal wind speed over East Asia could be un-

derstood as a barotropic response to the heating asso-

ciated with increased precipitation in the southeastern

part of China after the mid-1990s in summer.

Although some efforts have been made to understand

the influence of precipitation variation over the tropics

on the westerly jet over the midlatitudes, relatively few

studies have focused on the impact of tropical pre-

cipitation change on the EAJ in boreal winter, as well as

the relative contributions of tropical rainfall over dif-

ferent regions to the EAJ variation. Thus, our focus here

is on how anomalous heating associated with tropical

precipitation anomalies over different regions affects

the EAJ during boreal winter. We also devote our at-

tention to the investigation of the relative contribution

of tropical precipitation over the WNP and CP to the

midlatitude zonal circulation change at upper levels.

The possible mechanism responsible for the influences

of winter precipitation variability over different tropical

regions on the variation of the EAJ is one of the ques-

tions we attempt to address here. It has long been known

that tropical rainfall exhibits a very strong interannual

variation associated with El Niño–Southern Oscillation

(ENSO) (e.g., Chou et al. 2003; Chou et al. 2009).

Considerable attention has been focused on extra-

tropical responses to the forcing of sea surface temper-

ature (SST) rather than the associated convection (e.g.,

Webster 1982; Hoerling et al. 1997; Wu et al. 2009; Chen

et al. 2014). However, the SST–convection relation

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varies with the seasons and regions (Lau et al. 1997; Wu

and Kirtman 2007; Sabin et al. 2013). Consequently, it is

necessary to directly focus on the effects of precipitation

rather than SST on the circulations in the midlatitudes.

It is not easy to diagnose the causality between pre-

cipitation and circulation changes through observational

analyses. Numerical model simulation may be a helpful

tool to study the role of tropical precipitation anomaly in

the variation of extratropical circulations over East Asia

and effectively isolate the impact of tropical pre-

cipitation anomalies over different regions. Thus, a dry

numerical model is used in this study to examine

whether precipitation variability can significantly affect

atmospheric circulations.

The rest of the paper is organized as follows. Datasets

and model are described in section 2. In section 3, we

present the spatial and temporal characteristics of pre-

cipitation variation over the tropical Pacific during bo-

real winter as background information. The relationship

between the variation of the EAJ and precipitation

anomalies over the WNP and CP is investigated here.

And we also discuss the individual impact of tropical

precipitation changes over the WNP and CP on the

EAJ. Section 4 describes the results of the linear baro-

clinicmodel experiments. Section 5 gives a summary and

discussion.

2. Datasets and model

The datasets used in the present study consist of

1) monthly variables derived from the National Centers

for Environmental Prediction (NCEP)–U.S. Department

of Energy (DOE) Reanalysis II with a horizontal reso-

lution of 2.58 in both zonal and meridional directions and

17 vertical levels (Kanamitsu et al. 2002); 2) monthly

precipitation data from the Global Precipitation Cli-

matology Project, version 2.2 (GPCPv2.2; Adler et al.

2003), which combines observations and satellite pre-

cipitation data onto 2.58 3 2.58 global grids; 3) the

monthly NOAA Precipitation Reconstruction (PREC)

dataset for 1948–2010 (Chen et al. 2002); 4) monthly

NCEP–National Center for Atmospheric Research

(NCAR) reanalysis data from 1948 to 2010 (Kalnay

et al. 1996); and 5) monthly data from the 40-yr Euro-

pean Centre for Medium-RangeWeather Forecasts Re-

Analysis (ERA-40) for 1957 through mid-2002 (Uppala

et al. 2005). The datasets of PREC, ERA-40, and

NCEP–NCAR reanalysis are only used to discuss the

interdecadal changes of the relationship between the

EAJ and tropical precipitation variations in section 4.

Before performing our analysis, the climatological mean

for the period 1980–2010 was removed, and the winter-

mean was calculated as average of December to the

following February [December–February (DJF)]. The

winter of 1980 refers to the 1979/80 winter.

A simple dry model named linear baroclinic model

(LBM) was used in our present study to examine the in-

fluence of tropical rainfall anomaly on the subtropical

westerly jet. It consists of the primitive equations linearized

about the observed winter climatology obtained from the

NCEP–DOE reanalysis dataset for the period 1980–2010.

This model was constructed based on a linearized atmo-

spheric general circulation model (AGCM) developed at

the Center for Climate System Research (CCSR), Uni-

versity of Tokyo, Japan, and the National Institute for

Environmental Studies (NIES), Japan (Watanabe and

Kimoto 2000). The model adopts a horizontal resolution

using a T42 spectral truncation, roughly equivalent to 2.88latitude 3 2.88 longitude, with 20 vertical levels using a

sigma coordinate system, and includes a horizontal (verti-

cal) diffusion, Rayleigh friction, and Newtonian damping.

The damping time scale was set at 0.5 day for the three

lowest levels and two topmost levels, and 30 days for the

other levels. We use a time integration method and in-

tegration is continued up to 30 days. The results of the 10-

day average from day 20 to day 30 are shown as the steady

response to a prescribed diabatic heating.

3. Relationship between tropical rainfall and EAJ

a. Leading mode of precipitation variability in winter

For the purpose of obtaining the spatiotemporal

precipitation variability in tropics in boreal winter, the

empirical orthogonal function (EOF) analysis is applied

to GPCP winter precipitation anomalies. As shown in

Fig. 1, the leading mode, which is distinguished from the

other modes based on the method proposed by North

et al. (1982), explains about 38.6% of the total variance

for the period 1980–2010. A well-organized zonal dipole

structure with a negative center over the WNP and

Maritime Continent and a positive center over the CP is

seen over the tropical Pacific. Based on the power

spectrum analysis, the prevailing period of the corre-

sponding principal component (PC1) is 2.5 yr. This

suggests that the leading mode of tropical precipitation

variability in winter is governed by interannual varia-

tions. As shown in the PC1 time series, the extreme

events of the first mode usually appear in ENSO years,

such as 1983 and 1998. The correlation coefficient be-

tween PC1 and winter-mean Niño-3.4 (58S–58N, 1708–1208W) index is 0.92, which is statistically significant at

the 99% confidence level according to the Student’s

t test. The linkage between the dipole pattern of tropical

Pacific rainfall variability and ENSO has been pre-

viously recognized (e.g., Lau 1992; Webster and Yang

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1992; Zhang et al. 1996; Xie and Arkin 1997; Cai et al.

2011). These previous studies found the precipitation

anomalies exhibit a striking dipole pattern with a dry

polarity over the WNP and Maritime Continent and a

wet polarity over the equatorial eastern-central Pacific

when El Niño matures in boreal winter. These studies

also point out that ENSO exerts tremendous impacts on

the dipole pattern of precipitation variations and circu-

lation anomalies in the equatorial Pacific Ocean (Wang

et al. 2000; Wang and Zhang 2002). However, the

present study, as described above, will focus on the in-

fluence of tropical precipitation anomalies on atmo-

spheric circulations in the midlatitudes, rather than the

factors being responsible for the rainfall pattern.

According to the leading mode of winter precipitation

anomalies, it is clear that precipitation over both the CP

andWNP varies tremendously. Hence, investigating the

variability of precipitation over these two regions is of

central importance for understanding the precipitation

fluctuation over the tropical Pacific during boreal win-

ter. Consequently, we calculate the area-averaged

rainfall anomaly over the WNP (58–158N, 1058–1358E)and CP (7.58S–2.58N, 1658E–1658W) as the western Pa-

cific precipitation anomaly index (WPPA) and central

Pacific precipitation anomaly index (CPPA), re-

spectfully, for a better understanding of the impact of

precipitation change in the two key regions. Figure 1c

displays the time series of normalizedWPPA andCPPA

for the period 1980–2010. The correlation coefficient

between CPPA and WPPA is 20.87, significant at the

99% confidence level. It suggests that the variation in

precipitation over the WNP is tightly coupled with that

over the CP. Figure 2 shows the correlation coefficients

between precipitation anomalies and theWPPA/CPPA.

Both the WPPA and the CPPA are significantly asso-

ciated with a dipole pattern of precipitation anomalies in

FIG. 1. (a) Spatial pattern of the leading EOF mode and (b) corresponding principal com-

ponent (PC1) of the precipitation anomalies over the tropical oceans in boreal winter for the

period 1980–2010. The central Pacific precipitation anomaly index (CPPA) and the western

Pacific precipitation anomaly index (WPPA) are defined as the area-averaged rainfall anomaly

over the westernNorth Pacific and central Pacific [black boxes in (a)]. (c) Relationship between

normalized WPPA and CPPA for the period 1980–2010. The red (blue) line denotes WPPA

(CPPA) and the gray dashed lines denote the61.0 standard deviation, which is used to define

the strong and weak rainfall years.

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the tropical Pacific, with the opposite polarity because

of the inverse relationship between the WPPA and

the CPPA.

b. Relationship between tropical rainfall and EAJ

To detect the relationship between tropical pre-

cipitation anomalies and EAJ, we perform a composite

analysis of atmospheric circulation anomalies of the

winter mean in this subsection. The years during which

the normalized WPPA and CPPA values are greater

than 1.0 or less than 21.0 standard deviation are se-

lected to make the composite. According to the criteria,

six strong rainfall years (1983, 1987, 1992, 1998, 2003,

and 2010) and eight weak rainfall years (1984, 1989,

1996, 1999, 2000, 2001, 2008, and 2009) are identified for

the CPPA for the period 1980–2010 (Fig. 1c). For the

case of WPPA, six strong rainfall years (1984, 1996,

1999, 2000, 2006, and 2009) and six weak rainfall years

(1983, 1987, 1990, 1991, 1992, and 1998) are selected.

Compared with the strong and weak convection years

used in the composite in Park and An (2014; see their

Table 1), the selected years are very different. Only two

strong WNP rainfall years (1999 and 2000) one weak

WNP rainfall year (1998) are the same.

During the strong rainfall years in the WNP, positive

rainfall anomalies are located to the east of the Philip-

pines with a maximum of 5–6mmday21 (Fig. 3a). Dur-

ing the strong rainfall years in the CP, a zonal band of

positive rainfall anomalies appears over the equatorial

eastern-central Pacific (Fig. 3b) with themaximumvalue

of about 7–8mmday21. As shown in Figs. 3c and 3e, the

composite zonal wind and streamfunction anomalies at

200 hPa feature an anomalous anticyclone over East

Asia during the strong WNP rainfall years. There are

westerly anomalies to the north of climatological loca-

tion of the EAJ and easterly anomalies to its south.With

the enhanced precipitation over the CP, the pattern of

wind anomalies is similar to that corresponding to en-

hanced precipitation over the WNP, but with opposite

signs (Figs. 3d and 3f). The pattern correlation of

200-hPa zonal wind between Figs. 3c and 3d is20.77 and

that of 200-hPa streamfunction between Figs. 3e and 3f

is 20.92. It is inferred that the EAJ migrates north-

ward (southward) with increased precipitation over the

WNP (CP). Our results are consistent with those of Lau

and Boyle (1987), who pointed out that the changes in

heating over the western and central Pacific were ac-

companied by significant changes in the EAJ.

To determine whether the composite pattern is the

primary mode of upper atmosphere circulation, we

perform an EOF analysis of 200-hPa zonal wind anom-

alies in the domain of 08–508N, 908–1408E during boreal

winter. The leading EOF (EOF1) accounts for 42% of

the total variance. The first mode features a pattern of

alternating positive and negative centers over East Asia

(not shown), which represents the north–south migra-

tion of the EAJ. It resembles the pattern shown in

Figs. 3c and 3d, suggesting that themeridional pattern of

zonal wind anomalies associated with positive (nega-

tive) rainfall anomalies over the WNP (CP) represents

the dominant mode of the variability of the East Asian

upper-tropospheric circulation during winter.

To further unravel the connection between precipitation

change in the key regions and the upper-tropospheric

FIG. 2. Spatial distribution of the correlation between the (a) WPPA and (b) CPPA and the

boreal winter precipitation. The dotting indicates significance at the 95% level.

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circulation, we built two indices to depict the variations

of the westerly jet, including its location and intensity.

Based on the pattern with opposite sign anomalies

of 200-hPa zonal wind in the meridional direction

(Figs. 3c and 3d) and the leading mode of 200-hPa zonal

wind anomalies in the domain of 08–508N, 908–1408Eduring boreal winter (figure not shown), a winter EAJ

index (WEAJI) measuring its meridional displacement

is defined by using the difference between the 200-hPa

zonal winds averaged over 22.58–32.58N, 1008–1308Eand

32.58–42.58N, 1008–1308E. A positive (negative) WEAJI

indicates a southward (northward) displacement of the

EAJ. In addition, an index measuring the intensity of

the EAJ (WEAJS) is defined using the area-averaged

200-hPa zonal winds over 27.58–37.58N, 1008–1308E.Table 1 depicts the correlation coefficients between

precipitation and circulation indices. The correlation

coefficient between the WPPA (CPPA) and WEAJI

is 20.75 (0.67). The correlation coefficient between

PC1 andWEAJI is 0.79. As expected, the EAJmigrates

northward (southward) corresponding to increased

precipitation over the WNP (CP). The precipitation

anomalies in the key areas are also significantly cor-

related with the intensity of the EAJ. The correlation

coefficient between the WPPA (CPPA) andWEAJS is

0.69 (20.61). The correlation coefficient between PC1

and WEAJS is 20.65. This result shows that the

westerly jet tends to be stronger (weaker) corre-

sponding to positive precipitation anomalies over

the WNP (CP).

As mentioned above, the precipitation anomalies

over the WNP and CP are strongly coupled with each

other. It is also noted that the negative precipitation

anomaly over the CP is apparently dominant during the

strong rainfall years of WNP (Fig. 3a). Thus, the com-

posite horizontal wind anomalies based on the WPPA

(Fig. 3c) may contain the impact of anomalous pre-

cipitation over the CP. The individual impact of anom-

alous convection over the WNP and CP cannot be seen

in the preceding composite analyses. Subsequently, we

FIG. 3. Composite map of (a),(b) precipitation anomalies (shaded; mm day21), (c),(d) 200-hPa wind anomalies (vector; m s21), and

(e),(f) 200-hPa streamfunction anomalies (contour;m2 s21) based on the (left) WNP and (right) CP strong rainfall years. In (a),(b),(e),(f),

the dotting indicates significance at the 95% level. In (c),(d), the 200-hPa zonal wind anomalies are shaded.Only the zonal winds significant at

the 90% confidence level are shown. The dashed red lines represent the climatological DJF mean zonal wind, and only the contour lines of

40 and 60m s21 are shown.

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should attempt to distinguish the influences of anoma-

lous precipitation over the WNP and CP by partial

correlation and numerical experiments.

c. Individual impact of anomalous precipitation onthe EAJ

To isolate impacts of precipitation anomalies over

the WNP and CP, partial correlation is applied. An

anomalous circulation pattern associated with the

WPPA (CPPA) after removing the effects of the

CPPA (WPPA) is described by the partial correlation

coefficients, denoted as WPjCP (CPjWP). The signifi-

cance of the partial correlation is estimated based on

the Student’s t test. The circulation anomalies asso-

ciated with rainfall variability over the WNP and CP

are assessed with correlation coefficient map of

streamfunction and horizontal winds at 200 hPa onto

the WPjCP and CPjWP. Some striking discrepancies

have been identified, which will be discussed

subsequently.

Figure 4 shows the partial correlations of upper-

tropospheric streamfunction and horizontal wind

anomalies with WPPA and CPPA for the period 1980–

2010. After intentionally excluding the impact of pre-

cipitation variations over the CP, positive correlations

appear over the subtropical Pacific and East Asia. It

indicates that a significant anticyclonic anomaly appears

over there with enhanced precipitation over the WNP

and eastern China (Fig. 4a). At the upper levels, the

pattern of wind anomalies is characterized by westerly

anomalies to the north of 308N and southeasterly

anomalies to the south (Fig. 4c). After removing the

influence of precipitation in the WNP, the stream-

function correlations show a pair of anticyclones strad-

dling the equator and a cyclone over the North Pacific

with positive precipitation anomalies over the CP

(Fig. 4b). This pattern over the North Pacific is similar to

the Pacific–North American (PNA) teleconnection

(Wallace and Gutzler 1981). There is a wide zonal band

of westerly anomalies between 1358E and 1208Wlocated

to the north of the subtropical anticyclone over the CP

(Fig. 4d). Comparison with Fig. 4c reveals that the

westerly jet over the south of Japan and the North Pa-

cific tends to intensify because of significant enhance-

ment of westerlies associated with positive precipitation

anomalies over theCP, while the range of the EastAsian

subtropical jet might expand northward due to the

anomalous anticyclone over eastern China associated

with positive precipitation anomalies over the WNP.

According to the results of partial correlation, we

speculate that the impact of precipitation anomalies

over the WNP and CP on the variability of the sub-

tropical westerly jet might be different. The result

obtained from the partial correlation also implies that

the meridional displacement of the EAJ between 908and 1308E is relevant to precipitation anomalies over the

WNP, while precipitation anomalies over the CP may

affect the intensity of the subtropical jet over the North

Pacific between 1358E and 1208W. Note that the pre-

cipitation variation over the CP seemingly does not

strongly link to the variation of the EAJ between 1008and 1308E (Fig. 4d). The relationship between pre-

cipitation anomalies over the CP and zonal wind

anomalies over the EAJ’s entrance region will be fur-

ther discussed in section 4.

We have noted that the variation of precipitation in

the two key regions is significantly related with the

upper-tropospheric circulation and the results of partial

correlations may give a hint of the individual impact of

precipitation anomalies over theWNP and CP. Hence, a

subsequent question is how anomalous precipitation

over the tropical Pacific influences the atmospheric cir-

culation, especially the EAJ during boreal winter. This

part is investigated by virtue of partial correlations of

wave activity flux defined and formulated by Takaya and

Nakamura (2001) with the WPjCP and CPjWP. The def-

inition of this wave activity flux (hereinafter T-N flux)

can be written as

FTN5p cosu2jUj

U

a2 cos2u

"�›c0

›l

�2

2c0›2c0

›l2

#1

V

a2 cosu

�›c0

›l

›c0

›u2c0 ›2c0

›l›u

�

U

a2 cosu

�›c0

›l

›c0

›u2c0 ›2c0

›l›u

�1

V

a2

"�›c0

›u

�2

2c0›2c0

›u2

#

f 20N2

�U

a cosu

�›c0

›l

›c0

›z2c0 ›2c0

›l›z

�1

V

a

�›c0

›u›c0

›z2c0 ›2c0

›u›z

��

8>>>>>>>>>>><>>>>>>>>>>>:

9>>>>>>>>>>>=>>>>>>>>>>>;

, (1)

TABLE 1. Correlation coefficients between the WPPA, CPPA,

and PC1 andWEAJI andWEAJS during the period of 1980–2010.

An asterisk indicates that the value is significant at the 99% level.

PC1 WPPA CPPA

WEAJI 0.79* 20.75* 0.67*

WEAJS 20.65* 0.69* 20.61*

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where f0 5 2V sinu is the Coriolis parameter and u and

l are latitude and longitude, respectively; z 5 2H lnp,

where H is a constant scale height and p is the pressure

divided by 1000hPa; and N2 5 (RapkH21)(›u/›z) is the

buoyancy frequency squared where u denotes potential

temperature, Ra is the gas constant of dry air, and k is

defined as Ra normalized by the specific heat of air at

constant pressure. Also, U and V denote a steady basic

flow in the zonal andmeridional directions, respectively;

c is the streamfunction; and a is Earth’s radius. The

three-dimensional perturbations deviating from the

time mean are denoted by primes. The T-N flux

considers a steady zonally inhomogeneous basic flow

and could be applicable to small-amplitude quasigeo-

strophic disturbances, either stationary or migratory. It

is independent of wave phase and parallel to the local

group velocity of Rossby waves. So, it is a powerful di-

agnostic tool to identify the energy propagation of the

observed perturbation (Takaya and Nakamura 2001;

Zheng et al. 2013).

The primary impact of precipitation variations over the

WNP on the EAJ is confirmed by a careful examination

of the partial correlation between the T-N flux and

WPPA (CPPA) when the effects of precipitation varia-

tions over CP (WP) are removed. When our attention is

devoted to the relationship between the acceleration/

deceleration of the zonal-mean zonal wind and anoma-

lous perturbations, a simplifiedT-Nfluxwas formed here.

In light of our focus on variations of the zonal westerly jet

during boreal winter where the meridional flow is rela-

tively trivial, we assume that the basic states are dominant

by zonal flows (V 5 0). Then, the T-N flux becomes the

traditional EP flux (Eliassen and Palm 1961) after the

zonal average is taken in Eq. (1). The divergence of

simplified T-N flux is closely correlated with the vari-

ability of zonal-mean flow. It indicates that the original

T-N flux could help us explore the horizontal propagation

of Rossby wave energy excited by the heating in tropics

(shown in Fig. 5), while the simplified T-N flux could

favor a better understanding of the impact of heat-

induced distributions on the EAJ (shown in Fig. 6).

At the upper levels, correlations of the T-N flux with

WPPA are mostly poleward to the north of 208N over

East Asia (Fig. 5a). Consistent with previous studies, the

poleward-propagating Rossby wave trains excited by

tropical convection can reach middle-to-high latitudes,

modulate extratropical circulations, and even affect

polar amplification (Hoskins and Karoly 1981;

Schneider et al. 2012; Yoo et al. 2011; Yoo et al. 2012). In

Fig. 6a, we note that there are positive correlations at the

upper levels between 208 and 408N, indicating a flux

divergence to the north of 208N. Thus, there is increased

wave absorption in the vicinity of 308N. The poleward

and upward propagation of simplified T-N flux is dom-

inant in the midlatitudes. The horizontal component of

simplified T-N flux is proportional to the product of eddy

momentum flux and cosu. Thus, it suggests that there is

eddy momentum flux from the tropics to the mid-

latitudes associated with anomalous precipitation over

the WNP. The enhanced poleward propagation of

Rossby wave activity associated with precipitation var-

iations over the WNP results in an increased eddy

FIG. 4. Partial correlations of (a),(b) 200-hPa streamfunction (contour;m2 s21) and (c),(d) horizontal winds (vector; m s21) with (left)

WPjCP and (right) CPjWP during the boreal winter. The dotting in (a),(b) indicates significance at the 95% level. In (c),(d) the shading

indicates that the partial correlation coefficient of 200-hPa zonal wind is significant at the 95% confidence level.

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momentum flux divergence in the midlatitudes, leading

to an acceleration of zonal winds at the upper levels.

When the effects of precipitation variations over the

WNP are removed (Fig. 5b), the poleward propagation

of T-N flux over East Asia apparently weakens and

significant correlations appear over the subtropical Pa-

cific. By the vicinity of the date line, the correlations are

mostly equatorward. For the vertical component of

simplified T-N flux (Fig. 6b), the anomalous center of

the flux divergence at upper levels is located between 258and 558N. It implies an acceleration of zonal winds at the

upper levels corresponding to enhanced precipitation

over the CP.

Based on Fig. 6, it seems that the variation of zonal

mean flow ismore relevant to the variability of anomalous

precipitation over CP than that over WNP. However,

from Fig. 5b, the associated T-N flux varies largely in the

zonal direction. When the zonal average is done, signifi-

cant signals over the North Pacific account for a large

proportion of the variations shown in Fig. 6b. Focusing on

East Asia, the precipitation change over WNP plays a

relatively more important role in the variability of the

zonal flow (Fig. 5a), comparedwith that over CP (Fig. 5b).

Overall, the results based on the partial correlations sug-

gest that the poleward-propagating Rossby waves excited

by tropical thermal forcing are of considerable impor-

tance to the variation of the basic flow over East Asia.

We note that significant signals of T-N flux appear

over the central Pacific around 208N between 1708 and1208W after the effects of CPPA are removed (Fig. 5a).

Actually, the precipitation and horizontal wind anom-

alies regressed onto the WPjCP were also examined

(figures not shown). When the effects of WPPA are re-

moved, significant negative precipitation anomalies still

appear over the central South Pacific and North Pacific.

An anticyclone was located over the central Pacific be-

tween 08 and 308N in the middle and upper troposphere

(not shown). The anticyclone anomaly over the central

North Pacific could also be seen in Fig. 4c. Hence, we

speculate that the negative precipitation anomalies over

the CP may result in significant signals there (Fig. 5a)

even if we removed the effects of CPPA. Thus, the im-

pacts of precipitation change over the WNP and CP

cannot be completely isolated through simple statistical

methods because the precipitation variations over the

WNP is highly correlated with that over the CP

(r 5 20.87). The results obtained from the partial cor-

relations may be partly credible and the further nu-

merical investigations are required. Accordingly, a

series of model experiments with a dry model named

LBM are conducted to investigate the relative role of

tropical precipitation anomalies over the WNP and CP

in atmospheric circulation change over the midlatitudes

in the next section.

FIG. 5. Partial correlations of 200-hPa T-N flux (vector) with (a)WPjCP and (b) CPjWP during

the boreal winter. The shading indicates their corresponding divergence of T-N flux. The

dotting indicates that the partial correlation coefficient of divergence of the T-N flux is sig-

nificant at the 90% confidence level.

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4. Numerical results by LBM

To determine whether the variation in precipitation

over the WNP and CP results in the variation of the

EAJ, several numerical experiments based on LBM

were carried out. As the LBM experiments in this study

include only the linear response, the model results to be

presented may be understood qualitatively, not

quantitatively.

Figure 7 shows the heating over the tropical Pacific

prescribed in the model in three numerical experiments.

In the first experiment, to investigate the impact of

precipitation variation over the WNP, an elliptic heat

source placed onto the WNP (centered at 108N, 1188E)was sustained for the integration period (Fig. 7a). Its

maximum at 400hPa (s 5 0.45) is 3.7Kday21 (Fig. 7b).

It is approximately equivalent to heating associated

with a precipitation of 4–5mmday21 and consistent with

the composite results (Fig. 3a). In the second experi-

ment, the heat perturbation is centered at 58S, 1708Wand takes an elliptical form in the horizontal (Fig. 7c).

The vertical profile of the heat sink peaks at 400 hPa

with a vertically averaged heating rate at the center

of24.5Kday21 (Fig. 7d). This absolute value of heating

rate is roughly equivalent to 6–7mmday21 of anoma-

lous precipitation (Fig. 3b). With the aim of in-

vestigating the joint impact of the anomalous heating

over the WNP and CP in the third experiment, the heat

source over the WNP and the heat sink over the CP are

identical to the heating imposed in the first two experi-

ments, respectively (Figs. 7e,f). Since the model we used

in the present study is linear, the responses of the joint

impact experiment are actually the results adding the

first two numerical experiments together. However, to

further explore the relatively contribution to the varia-

tion of the EAJ clearly, the results of the joint impact

experiment are still shown in the present study.

a. Impact of heat source over the WNP

We now consider the impact of anomalous heating

associated with precipitation anomalies over the WNP

on the westerly jet at the upper levels. To emphasize the

steady response to a prescribed diabatic heating, the

results of 10-day average response from day 20 to day 30

are shown in Fig. 8.

In the lower troposphere (Fig. 8a), an anomalous cy-

clone response appears over the South China Sea and

Philippines. An anomalous westerly is observed to the

west of the heat source placed onto the WNP with a

relatively weak easterly to the east. At the upper levels

(Fig. 8b), the heat source induces a wave train pattern

with a northeast–southwest tilt structure, which features

a cyclone anomaly over the North Pacific and an anti-

cyclonic anomaly over eastern China and South Asia.

Thus, there are westerly anomalies to the north of the

East Asian anticyclone and easterly anomalies to its

south (Fig. 8c). It represents a northward displacement

of the EAJ. Note that the pattern of streamfunction in

Fig. 8c is, to some extent, similar to that of the partial

correlations with the WPjCP (Fig. 4c). Consistent with

the preceding results in section 3c, enhanced pre-

cipitation over theWNP significantly favors a northward

displacement of the EAJ by inciting a Rossby wave

FIG. 6. Partial correlations of vertical component of simplified T-N flux (vector) with (a) WPjCP and (b) CPjWP

during the boreal winter. The shading indicates their associated divergence. The dotting indicates that the partial

correlation coefficient of simplified T-N flux divergence is significant at the 90% confidence level.

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train, which features a distribution of alternating anti-

cyclone and cyclone over East Asia and the North Pa-

cific. Many previous studies with numerical models have

used forcing over the tropical western Pacific and the

results show that the heat anomaly over the WNP gen-

erally excites a northward-propagating Rossby wave

train (Hoskins and Ambrizzi. 1993; Lu and Lin 2009;

Zheng et al. 2013), which significantlymodulates the EAJ.

To further confirm that this pattern is a Rossby wave

train excited by the tropical forcing, the associated T-N

flux is shown inFig. 8d. Consistentwith the teleconnection

pattern from East Asia to the North Pacific, the T-N flux

emits from around 208N toward Japan and is mostly

poleward. Overall, the result indicates that the poleward

Rossby wave propagation is related to the wave train

pattern. A heat-induced Rossby wave train extending

fromEast Asia to the North Pacific enhances the westerly

to the north of climatological location of westerly jet.

b. Impact of heat sink over the CP

In the second experiment, we devote our attention to

the individual effect of the heat sink related with

negative precipitation anomalies over the CP. Figure 9

illustrates the model responses of streamfunction and

horizontal winds in the case of CP heat sink. The re-

sponse at the lower level features westerlies over the

equatorial central-eastern Pacific and easterlies over the

equatorial western Pacific (Fig. 9a). At the upper levels,

Fig. 9b shows a pattern of alternating cyclone and anti-

cyclone centers from the CP to the North Pacific. The

wave train of streamfunction anomalies is similar to a

teleconnection pattern known as the PNA proposed by

Wallace and Gutzler (1981). As demonstrated by

Hoskins and Karoly (1981) and Horel and Wallace

(1981), a heating anomaly associated with El Niño can

excite a stationary barotropic Rossby wave train, whose

spatial pattern is somewhat similar to the PNA.

Figure 9b also shows a cyclone to the northwest of the

heat sink at upper levels, which is associated with the

equatorial Rossby wave response. At the same time, to

the east of the heat sink the Kelvin wave response with

equatorial easterlies extends eastward along the equator

to 908E (Fig. 9a). The equatorial response is consistent

with the typical Gill (1980) pattern.

FIG. 7. (left) Horizontal distribution (contour, interval: 1 K day21, at the sigma level of 0.45) and (right) vertical

profile at sigma levels of the specific heating (K day21) in the cases of (a),(b) heat source over the WNP, (c),(d) heat

sink over the CP, and (e),(f) both. In (a),(c),(e) negative contours are dashed.

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Acyclone develops over the subtropical Pacific and an

anticyclone over the North Pacific (Fig. 9b). Thus, there

is a strong easterly anomaly band around 308N (Fig. 9c).

It implies that the heat sink over the CPmay weaken the

westerlies over East Asia and the North Pacific. The

associated T-N flux does not exhibit any meaningful

information about the propagation of Rossby wave

(figure not shown).

Observational results in preceding section show a

pattern with anomalous easterlies around 308N between

1358E and 1208W, suggesting that the variability of the

EAJ intensity is possibly relevant to anomalous pre-

cipitation over the CP (Fig. 4d). However, the variations

in precipitation in these two regions are tightly corre-

lated with each other (r 5 20.87). The observational

results with respect to the individual effect of rainfall

anomalies over the WNP and CP might be partially in-

accurate. In this subsection, the numerical experiments

demonstrate that the heat sink over the equatorial CP

possibly result in the weakening of the maximum west-

erlies over East Asia and the North Pacific, which is

different from the observations (Figs. 3d and 4d).

c. Joint impact of the heat source over the WNP andheat sink over the CP

In this subsection, emphasis is placed on the joint

impact of the precipitation anomalies over theWNP and

CP. When precipitation over the WNP is enhanced and

that over the CP is suppressed, a cyclone develops over

the South China Sea at 850hPa and a weak anticyclone

appears over the North Pacific (Fig. 10a). At 200hPa

(Fig. 10b), anticyclonic anomalies over eastern China

and South Asia build up and a PNA-like wave train

extends from the equatorial CP to the North Pacific.

Based on the response of streamfunction induced by the

WNP and CP heating anomalies, there are northeasterly

anomalies to the south of this anticyclone over eastern

China and westerly anomalies to the north. The anticy-

clone over East Asia in Fig. 10b resembles that in

Fig. 8b. It suggests that the heating anomaly over the

FIG. 8. Response of (a) 850-hPa horizontal winds (vector; m s21),

(b) 200-hPa streamfunction (contour; 1026 m2 s21), and (c) 200-hPa

horizontal winds (vector; m s21) to the heat source over the WNP

showing the 10-day average from day 20 to day 30. (d) The asso-

ciated T-N flux. Only the fluxes great than 0.3m2 s22 are shown.

Contours with negative values are dashed in (b). In (c) and (d), the

dashed lines represent the climatological location of the EAJ.

FIG. 9. As in Figs. 8a–c, but for the response to the heat sink over

the CP.

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WNP, compared to the heat sink over the CP, may be a

dominant factor responsible for the meridional move-

ment of the EAJ between 1108 and 1508E. Note that the

cyclonic anomalies over CP induced by anomalous

heating of both the WNP and CP (Fig. 10) are similar to

that induced by the heat sink over the CP (Fig. 9) and the

easterly response to the north of this cyclone is dominant

over theNorth Pacific around 308N.Hence, the heat sink

over the CPmay be one of the possible factors that affect

the strength of the subtropical jet over the North Pacific.

Apart from the anomalous heating over theWNP and

CP and the mean flow, other factors may also affect the

location and intensity of the EAJ. In Fig. 10b, the anti-

cyclonic anomaly appears over eastern China and South

Asia, but differs appreciably from the diagnosed one

(Fig. 3). A major difference between the simulated and

diagnosed zonal wind anomalies is that the simulated

centers of streamfunction anomaly are located to the

north of Indo-China and the Indian subcontinent. A

possible reason might be that the model used in this

study is a dry version, and thus the heat source or heat

sink imposed on the model cannot result in any anom-

alous tropical precipitation as in reality. For instance,

the observational result shows that there are positive

precipitation anomalies over the WNP, South Asia, and

Maritime Continent (Fig. 3a). The anomalous heating

with an elliptical form in the horizontal (Fig. 7a) in the

numerical experiments, however, is quite different from

the observations (Fig. 3a). This might be one of the

factors influencing the pattern of the heat-induced

Rossby wave train. Overall, the modeling results, to

some extent, capture the major characteristics in spite of

some appreciable discrepancies.

d. Nonlinear response to tropical heat forcing

The model used in the present study focuses on the

linear response to the steady forcing and rules out the

nonlinear interactions between forcing and basic

states. The present model is linearized about the basic

states in integration period. However, the direct re-

sponse to tropical forcing can change the extratropical

flow, which modifies the wave energy propagation

from the tropics (Lin and Derome 2004). It was noted

that the small variations in the basic flow can make

important differences in the response (Ting and Held

1990; Hall 2000). Nevertheless, the numerical experi-

ments by Hoerling et al. (1997) show that the extra-

tropical response to the tropical Pacific SST anomalies

does not depend on the zonally varying flow of El Niñoor La Niña states. Hence, in this subsection, to in-

vestigate the dependence of the upper-tropospheric

zonal wind response to the tropical forcing in the two

cases of WNP and CP, two numerical experiments are

designed.

In the present results, the response of linear in-

tegration when using the model climatology as a basic

state and heat-induced Rossby wave train is clear

(Figs. 8 and 9). Lin and Derome (2004) discussed the

nonlinearity of extratropical response to El Niño/LaNiña based on some linear numerical experiments.

Following Lin and Derome (2004), as the heat-induced

direct linear response, the anomalies of day 15 to the

heating anomalies placed onto the WNP or CP are su-

perposed on the model primary climatology, denoted as

WNP or CP basic states, respectively. To assess

whether a changed basic state can significantly alter the

forced solution, theWNP andCP basic states are used as

the basic states of the linear baroclinicmodel. To test the

sensitivity of anomalies imposed into the model primary

basic states, we have also performed the numerical at-

mospheric response with day 10 and day 20. It is found

that the spatial patterns of the upper-tropospheric cir-

culations are rather insensitive to the chosen anomalies.

Figures 11a and 11c illustrate the response to the heat

source over the WNP with the WNP basic states.

Comparison with Fig. 8 shows that the anticyclone over

East Asia does not change a lot and the cyclone over

the North Pacific significantly intensifies. The result

FIG. 10. As in Figs. 8a–c, but for the response to the heating

anomalies over both the WNP and CP.

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indicates that nonlinear interactions between forcing

and basic states might be trivial for the anticyclone re-

sponse at the westerly jet over East Asia. As shown in

Figs. 11b and 11d, the pattern of streamfunction induced

by the heat sink over the CP resembles that in Fig. 9. The

heat-induced cyclone over the subtropical CP inten-

sifies, resulting in decelerating the maximum westerlies

near the date line. The interactions of heat sink over

the CP and basic flow may further enhance the heat-

induced cyclone and tend to slow down the zonal

westerly wind.

e. Sensitivity to the basic state

We have performed some other numerical experi-

ments by imposing heating anomalies with different

basic background flow, and found that the responses of

circulations are sensitive to these differences. Before the

sensitive experiments, we noted an interdecadal change

in the relationship between tropical Pacific convection

and the EAJ. We examined the changes in tropical Pa-

cific precipitation and found some differences between

the two periods before and after 1979. To avoid the in-

fluence of plausible unrealistic interdecadal variability

in this study, the precipitation anomalies obtained from

the NOAA PREC dataset and the zonal wind obtained

from the NCEP–NCAR reanalysis and ERA-40 are

all subjected to a harmonic analysis to obtain periods

shorter than 8yr. The leading EOFmode of winter-mean

precipitation anomalies (obtained from the NOAA

PREC dataset) over the tropical Pacific Ocean for the

period 1949–2010 (figure not shown) is very similar to that

for 1979–2010 (Fig. 1a). Consistent with the increased

magnitude of ENSO (Wang et al. 2008), the PC1 ampli-

tude has increased after the late 1970s. Based on the cor-

relation analysis between the zonal wind anomalies

(obtained fromNCEP–NCAR reanalysis andERA-40) in

winter for 1949–2010 (ERA-40 for 1958–2002) and PC1

(Fig. 12), the relationship between precipitation variations

and the EAJ is apparently different between the two pe-

riods. The results obtain from theERA-40 are very similar

to that from theNCEP–NCAR reanalysis, and the pattern

correlation between Figs. 12a and 12c (Figs. 12b and 12d)

is 0.88 (0.95).

Themean state of theupper-tropospheric circulationwas

examined and the results show that the background flow

experiences an interdecadal change since the late 1970s

(figure not shown). Hence, it is hypothesized that the

shift of the mean state before and after 1979 may give

rise to the interdecadal change in the enhanced re-

lationship between tropical Pacific convection and the

EAJ. Subsequently, some numerical experiments are

performed to examine the sensitivity to the background

flow. The basic states imposed into the model in this

subsection are the observed winter climatology obtained

from the NCEP–NCAR reanalysis dataset for the period

1949–79. The heat source over the WNP and heat sink

over the CP are identical to the heating imposed in sec-

tions 4a and 4b, respectively. Figures 12e and 12f show

FIG. 11. Linear response of (a),(b) 850-hPa horizontal winds (vector; m s21) and (c),(d) 200-hPa streamfunction (contour; 1026 m2 s21)

to (left) the heat source over theWNP with the WNP basic states and (right) the heat sink over the CP with the CP basic states. Negative

contours are dashed.

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the responses of 200-hPa streamfunction and winds to the

heating after the basic flow has been altered. Compared

with Figs. 8b and 8c, the anticyclone and the anomalous

easterly to the south of 308N in Fig. 12e apparently

weaken. Furthermore, the response of the anomalous

easterly to the heat sink over theCPwith the basic states of

1949–79 also exhibits relatively weak signals over East

Asia (Fig. 12f), compared with Fig. 9c. The feeble anom-

alouswesterly and easterly in the vicinity of theEAJmight

be evidence for the subdued correlation between the Pa-

cific precipitation and the EAJ before 1979.

5. Summary and discussion

The impact of the precipitation anomalies over the

tropical Pacific on the upper-tropospheric subtropical

westerly jet over East Asia during boreal winter has been

investigated by diagnosis and numerical experiments. In

boreal winter, the leading mode of precipitation anoma-

lies over the tropical oceans for the period 1980–2010 is

characterized by a dipole pattern with a positive anom-

alous center over the CP and a negative one over the

WNP. The precipitation variations over the CP andWNP

FIG. 12. Patterns of correlation coefficients between the winter-mean 200-hPa zonal wind anomalies from NCEP–NCAR reanalysis

(NCEP-1) data and the principal component (PC1) both (a) before and (b) after 1980. The PC1 is obtained from the EOF analysis of the

precipitation anomalies over the tropical Pacific during the period 1949–2010. (c),(d) As in (a),(b), but the zonal wind data are obtained

from ERA-40 data for 1958–2002. The PC1 is also obtained from the EOF but for the period 1958–2002. The linear response of 200-hPa

horizontal winds (vector; m s21) and 200-hPa streamfunction (contour; 1026 m2 s21) to the heating over the (e) WNP and (f) CP with the

basic states obtained from NCEP–NCAR reanalysis dataset for 1949–79. Only the winds with absolute values greater than 1.0m s21 are

shown. The contour interval is 3 3 1026 m2 s21, and negative contours are dashed.

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are significantly correlated with the meridional displace-

ment of the EAJ. It is found that when precipitation over

the WNP (CP) is enhanced, EAJ tends to migrate

northward (southward). To investigate the individual

impact of anomalous precipitation over theWNPandCP,

partial correlation is performed. The partial correlations

of streamfunction and wind anomalies withWPjCP showsan anticyclone centered around 308N over East Asia with

anomalous westerlies to its north and easterlies to its

south. The partial correlations with CPjWP show a strong

westerly band over the central North Pacific. There is a

hint of different impact of precipitation anomalies over

the WNP and CP on the upper-tropospheric westerly jet.

To examine the relative contribution of precipitation

variability over the CP and WNP to the variation of the

EAJ, a series of numerical experiments based on a simple

dry baroclinicmodel have been performed. A heat source

over the WNP, which is equivalent to positive pre-

cipitation anomalies over the western Pacific, excites a

northward-propagating Rossby wave train in the upper

troposphere over East Asia. A strong westerly anomaly

appears to the north of the anticyclone over easternChina

and an easterly anomaly to its south, which enhances

the westerly flow and favors a northward shift of the

EAJ. The response induced by a heat sink over the CP is

quite different from that induced by the WNP heat

source. A zonal easterly band is built up to the north of

the heat-induced cyclonic circulation over the subtropical

central Pacific, suggesting that the precipitation change

over the CP may influence the intensity of the EAJ, in-

stead of its location. The response of atmospheric circu-

lations in the midlatitudes is similar to the diagnostic

results when the joint impact of precipitation anomalies

over these two regions is taken into account. Therefore, it

could be concluded that positive precipitation anomalies

as heat source anomalies over the WNP play a relatively

dominant role in the meridional displacement of the EAJ

and negative precipitation anomalies over the CP may

be a possible factor that changes the intensity of the

EAJ. In addition, many studies have suggested that the

transient eddy feedback reinforces the extratropical re-

sponses to the thermal forcing (Lin and Derome 1997,

2004). The lack of eddy forcing in this linear model may

lead to the differences of circulations between the ob-

servations and numerical experiments in themidlatitudes.

Furthermore, the interactions of tropical forcing and

basic flow are discussed in the present study. The numer-

ical results show that the direct response to the heat sink

over the CP changes the atmospheric circulations. Then,

the changed background flow intensifies the response cir-

culations. The response to heat source over theWNP does

not seem to depend on the nonlinear interactions between

forcing and basic states over the westerly jet region.

Many studies demonstrated that the ENSO amplitude

has increased and its periodicity has become longer after

the late 1970s (e.g., Gu and Philander 1997; An andWang

2000; Wang et al. 2008). Hence, we investigated the in-

terdecadal enhanced relationship between the tropical

Pacific precipitation anomalies and the EAJ since 1979.

The sensitive experiments to the background flow in

section 4e give a hint of the considerable importance of

the changed mean states before and after 1979 in the

interdecadal strengthened relationship between the pre-

cipitation anomalies and the EAJ. However, more diag-

nosis and numerical experiments need to be undertaken to

further investigate the role of the mean flow and what

authentically gives rise to interdecadal changes in the en-

hanced relationship between tropical Pacific convection

and the EAJ since the late 1970s.

It is worth mentioning that the obtained relationship

between the precipitation anomalies over the WNP and

the meridional displacement of the EAJ is in agreement

with Park and An (2014). However, our work compared

precipitation changes over WNP and CP and their rela-

tive contributions to the variability of the EAJ. The re-

sults show that the individual effects of tropical

precipitation changes over the WNP and CP on the EAJ

are quite different, which was not discussed in Park and

An (2014). Another major difference between our work

and Park and An (2014) is that the role of mean flow in

the interdecadal enhancement of the relationship be-

tween the precipitation anomalies over theWNP and CP

and the EAJ was discussed in our paper. In addition, the

possiblemechanism responsible for the impact of tropical

precipitation variation on the EAJ, proposed in our work

from the perspective of momentum transportation, en-

hances the understanding of the impact of tropical heat-

ing on the midlatitude circulation variability.

Acknowledgments. The authors thank Prof.

M. Watanabe for his permission to use the LBM.

This research was jointly supported by National Key

Basic Research and Development Projects of China

(2014CB953901), National Natural Science Foundation

of China (41175076 and 412111046), and State Key

Laboratory of Severe Weather opening project. RW

acknowledges the support of a National Natural Science

Foundation of China grant (41275081). YG acknowl-

edges the support of the high-performance grid com-

puting platform of Sun Yat-sen University.

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