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: [email protected]
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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.
REFERENCES
Adler, R. F., and Coauthors, 2003: The version-2 Global Pre-
cipitation Climatology Project (GPCP) Monthly Precipitation
Analysis (1979–present). J. Hydrometeor., 4, 1147–1167,
doi:10.1175/1525-7541(2003)004,1147:TVGPCP.2.0.CO;2.
6472 JOURNAL OF CL IMATE VOLUME 28
Unauthenticated | Downloaded 05/15/22 06:31 PM UTC
An, S.-I., and B. Wang, 2000: Interdecadal change of the structure
of the ENSO mode and its impact on the ENSO frequency.
J. Climate, 13, 2044–2055, doi:10.1175/1520-0442(2000)013,2044:
ICOTSO.2.0.CO;2.
Cai, W., P. V. Rensch, T. Cowan, and H. H. Hendon, 2011: Tele-
connection pathways of ENSO and the IOD and the mecha-
nisms for impacts on Australian rainfall. J. Climate, 24, 3910–
3923, doi:10.1175/2011JCLI4129.1.
Chen, C.-S., and K. E. Trenberth, 1988: Forced planetary waves in
the Northern Hemisphere winter: Wave-coupled orographic
and thermal forcings. J. Atmos. Sci., 45, 682–704, doi:10.1175/
1520-0469(1988)045,0682:FPWITN.2.0.CO;2.
Chen, M., P. Xie, J. E. Janowiak, and P. A. Arkin, 2002: Global
land precipitation: A 50-yr monthly analysis based on gauge
observations. J. Hydrometeor., 3, 249–266, doi:10.1175/
1525-7541(2002)003,0249:GLPAYM.2.0.CO;2.
Chen, Z., Z. Wen, R. Wu, P. Zhao, and J. Cao, 2014: Influence of
two types of El Niños on the East Asian climate during boreal
summer: A numerical study. Climate Dyn., 43, 469–481,
doi:10.1007/s00382-013-1943-1.
Chou, C., J.-Y. Tu, and J.-Y. Yu, 2003: Interannual variability of
the western North Pacific summer monsoon: Differences be-
tween ENSO and non-ENSO years. J. Climate, 16, 2275–2287,doi:10.1175/2761.1.
——,L.-F.Huang, J.-Y. Tu, L. Tseng, andY.-C.Hsueh, 2009: ElNiñoimpacts on precipitation in thewesternNorth Pacific–EastAsian
sector. J. Climate, 22, 2039–2057, doi:10.1175/2008JCLI2649.1.
Eliassen, A., and E. Palm, 1961: On the transfer of energy in sta-
tionary mountain waves. Geofys. Publ., 22 (3), 1–23.
Gill, A. E., 1980: Some simple solutions for heat-induced tropical
circulations. Quart. J. Roy. Meteor. Soc., 106, 447–462,
doi:10.1002/qj.49710644905.
Gu, D., and S. G. H. Philander, 1997: Interdecadal climate fluctua-
tions that depend on exchanges between the tropics and extra-
tropics. Science, 275, 805–807, doi:10.1126/science.275.5301.805.
Hall, N. M. J., 2000: A simple GCM based on dry dynamics and
constant forcing. J. Atmos. Sci., 57, 1557–1572, doi:10.1175/1520-0469(2000)057,1557:ASGBOD.2.0.CO;2.
Hoerling, M., A. Kumar, and M. Zhong, 1997: El Niño, La Niña, andthenonlinearity of their teleconnections. J.Climate, 10, 1769–1786,
doi:10.1175/1520-0442(1997)010,1769:ENOLNA.2.0.CO;2.
Horel, J. D., and J. M. Wallace, 1981: Planetary-scale atmospheric
phenomena associated with the Southern Oscillation. Mon.
Wea. Rev., 109, 813–829, doi:10.1175/1520-0493(1981)109,0813:
PSAPAW.2.0.CO;2.
Hoskins, B. J., and D. J. Karoly, 1981: The steady linear re-
sponse of a spherical atmosphere to the thermal and oro-
graphic forcing. J. Atmos. Sci., 38, 1179–1196, doi:10.1175/
1520-0469(1981)038,1179:TSLROA.2.0.CO;2.
——, and T. Ambrizzi, 1993: Rossby wave propagation on a realistic
longitudinally varying flow. J. Atmos. Sci., 50, 1661–1671,
doi:10.1175/1520-0469(1993)050,1661:RWPOAR.2.0.CO;2.
Huang, R., and L. Lu, 1989: Numerical simulation of the re-
lationship between the anomaly of subtropical high over East
Asia and the convective activities in the western tropical Pa-
cific. Adv. Atmos. Sci., 6, 202–214, doi:10.1007/BF02658016.Jia, X., S. Wang, H. Lin, and Q. Bao, 2015: A connection between
the tropical Pacific Ocean and the winter climate in the Asian-
Pacific region. J. Geophys. Res. Atmos., 120, 430–448,
doi:10.1002/2014JD022324.
Jin, F., and B. J. Hoskins, 1995: The direct response to tropical
heating in a baroclinic atmosphere. J. Atmos. Sci., 52, 307–319,
doi:10.1175/1520-0469(1995)052,0307:TDRTTH.2.0.CO;2.
Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Re-
analysis Project. Bull. Amer. Meteor. Soc., 77, 437–471,
doi:10.1175/1520-0477(1996)077,0437:TNYRP.2.0.CO;2.
Kanamitsu, M., W. Ebisuzaki, J. Woollen, S.-K. Yang, J. J. Hnilo,
M. Fiorino, and G. L. Potter, 2002: NCEP–DOE AMIP-II
Reanalysis (R-2). Bull. Amer. Meteor. Soc., 83, 1631–1643,
doi:10.1175/BAMS-83-11-1631.
Kummerow,C., andCoauthors, 2000:The status of theTropicalRainfall
Measuring Mission (TRMM) after two years in orbit. J. Appl.
Meteor., 39, 1965–1982, doi:10.1175/1520-0450(2001)040,1965:
TSOTTR.2.0.CO;2.
Kwon, M., J.-G. Jhun, and K.-J. Ha, 2007: Decadal change in East
Asian summer monsoon circulation in the mid-1990s. Geo-
phys. Res. Lett., 34, L21706, doi:10.1029/2007GL031977.
Lau, K. M., 1992: East Asia summer monsoon rainfall variability
and climate teleconnection. J.Meteor. Soc. Japan, 70, 211–242.
——, and P. H. Chan, 1983a: Short-term climate variability and at-
mospheric teleconnections from satellite-observed outgoing
longwave radiation. Part I: Simultaneous relationships. J.Atmos.
Sci., 40, 2735–2750, doi:10.1175/1520-0469(1983)040,2735:
STCVAA.2.0.CO;2.
——, and——, 1983b: Short-term climate variability and atmospheric
teleconnections from satellite-observed outgoing longwave radi-
ation. Part II: Lagged correlations. J. Atmos. Sci., 40, 2751–2767,
doi:10.1175/1520-0469(1983)040,2751:STCVAA.2.0.CO;2.
——, and J. S. Boyle, 1987: Tropical and extratropical forcing of
the large-scale circulation: A diagnostic study. Mon. Wea.
Rev., 115, 400–428, doi:10.1175/1520-0493(1987)115,0400:
TAEFOT.2.0.CO;2.
——, H.-T. Wu, and S. Bony, 1997: The role of large-scale atmo-
spheric circulation in the relationship between tropical con-
vection and sea surface temperature. J. Climate, 10, 381–392,
doi:10.1175/1520-0442(1997)010,0381:TROLSA.2.0.CO;2.
——, K. M. Kim, and S. Yang, 2000: Dynamical and boundary
forcing characteristics of regional components of the Asian
summer monsoon. J. Climate, 13, 2461–2482, doi:10.1175/
1520-0442(2000)013,2461:DABFCO.2.0.CO;2.
Lee, S., T. Gong, N. Johnson, S. Feldstein, and D. Pollard, 2011a: On
the possible link between tropical convection and the Northern
Hemisphere Arctic surface air temperature change between 1958
and 2001. J. Climate, 24, 4350–4367, doi:10.1175/2011JCLI4003.1.——, S. Feldstein, D. Pollard, and T. White, 2011b: Do planetary
wave dynamics contribute to equable climates? J. Climate, 24,
2391–2404, doi:10.1175/2011JCLI3825.1.
Li, L., andY. Zhang, 2014: Effects of different configurations of the
East Asian subtropical and polar front jets on precipitation
during the mei-yu season. J. Climate, 27, 6660–6672,
doi:10.1175/JCLI-D-14-00021.1.
Liao, Q., S. Tao, and H. Wang, 2004: Interannual variation of
summer subtropical westerly jet in East Asia and its impacts
on the climate anomalies of EastAsia summermonsoon.Chin.
J. Geophys., 47, 12–21, doi:10.1002/cjg2.449.Lin, H., and J. Derome, 1997: On the modification of the high- and
low-frequency eddies associated with the PNA anomaly: An
observational study. Tellus, 49A, 87–99, doi:10.1034/
j.1600-0870.1997.00006.x.
——, and ——, 2004: Nonlinearity of the extratropical response
to tropical forcing. J. Climate, 17, 2597–2608, doi:10.1175/
1520-0442(2004)017,2597:NOTERT.2.0.CO;2.
Lu, R., 2001: Interannual variability of the summertime North
Pacific subtropical high and its relation to atmospheric con-
vection over thewarmpool. J.Meteor. Soc. Japan, 79, 771–783,
doi:10.2151/jmsj.79.771.
15 AUGUST 2015 GUO ET AL . 6473
Unauthenticated | Downloaded 05/15/22 06:31 PM UTC
——, and Z. Lin, 2009: Role of subtropical precipitation anomalies
in maintaining the summertime meridional teleconnection
over the western North Pacific and East Asia. J. Climate, 22,
2058–2072, doi:10.1175/2008JCLI2444.1.
McBride, J. L.,M.R.Haylock, andN. Nicholls, 2003: Relationships
between theMaritimeContinent heat source and the El Niño–Southern Oscillation phenomenon. J. Climate, 16, 2905–2914,
doi:10.1175/1520-0442(2003)016,2905:RBTMCH.2.0.CO;2.
Nitta, T., 1987: Convective activities in the tropical western Pacific
and their impact on the northern hemisphere summer circu-
lation. J. Meteor. Soc. Japan, 65, 373–390.
North, G. R., T. L. Bell, R. F. Cahalan, and F. J. Moeng, 1982:
Sampling errors in the estimation of empirical orthogonal
functions. Mon. Wea. Rev., 110, 699–706, doi:10.1175/
1520-0493(1982)110,0699:SEITEO.2.0.CO;2.
Park, J. H., and S. I. An, 2014: The impact of tropical western
Pacific convection on the North Pacific atmospheric circula-
tion during the boreal winter. Climate Dyn., 43, 2227–2238,
doi:10.1007/s00382-013-2047-7.
Sabin, T. P., C. A. Babu, and P. V. Joseph, 2013: SST–convection
relation over tropical oceans. Int. J. Climatol., 33, 1424–1435,
doi:10.1002/joc.3522.
Schneider, D., C. Deser, and Y. Okumura, 2012: An assessment
and interpretation of the observed warming of West Antarc-
tica in the austral spring. Climate Dyn., 38, 323–347,
doi:10.1007/s00382-010-0985-x.
Takaya, K., and H. Nakamura, 2001: A formulation of a phase-
independent wave-activity flux for stationary and migratory
quasigeostrophic eddies on a zonally varying basic flow.
J.Atmos. Sci., 58, 608–627, doi:10.1175/1520-0469(2001)058,0608:
AFOAPI.2.0.CO;2.
Ting, M., and I. M. Held, 1990: The stationary wave response to a
tropical SSTanomaly in an idealizedGCM. J.Atmos. Sci.,47,2546–
2566, doi:10.1175/1520-0469(1990)047,2546:TSWRTA.2.0.CO;2.
Uppala, S.M., andCoauthors, 2005: TheERA-40Re-Analysis.Quart.
J. Roy. Meteor. Soc., 131, 2961–3012, doi:10.1256/qj.04.176.
Wallace, J. M., and D. S. Gutzler, 1981: Teleconnections in the
geopotential height field during the Northern Hemisphere
winter. Mon. Wea. Rev., 109, 784–812, doi:10.1175/
1520-0493(1981)109,0784:TITGHF.2.0.CO;2.
Wang, B., and Q. Zhang, 2002: Pacific–East Asian teleconnection.
Part II: How the Philippine Sea anomalous anticyclone is es-
tablished duringElNiño development. J. Climate, 15, 3252–3265,
doi:10.1175/1520-0442(2002)015,3252:PEATPI.2.0.CO;2.
——, R. Wu, and X. Fu, 2000: Pacific–East Asian tele-
connection: How does ENSO affect East Asian climate?
J. Climate, 13, 1517–1536, doi:10.1175/1520-0442(2000)013,1517:
PEATHD.2.0.CO;2.
——, J. Yang, T. Zhou, andB.Wang, 2008: Interdecadal changes in
the major modes of Asian–Australian monsoon variability:
Strengthening relationship with ENSO since the late 1970s.
J. Climate, 21, 1771–1789, doi:10.1175/2007JCLI1981.1.
Wang, C., 2002: Atmospheric circulation cells associated with the
El Niño–Southern Oscillation. J. Climate, 15, 399–419,
doi:10.1175/1520-0442(2002)015,0399:ACCAWT.2.0.CO;2.
Watanabe, M., and M. Kimoto, 2000: Atmosphere–ocean thermal
coupling in the North Atlantic: A positive feedback. Quart.
J. Roy. Meteor. Soc., 126, 3343–3369, doi:10.1002/
qj.49712657017.
Webster, P. J., 1982: Seasonality in the local and remote atmo-
spheric response to sea surface temperature anomalies.
J. Atmos. Sci., 39, 41–52, doi:10.1175/1520-0469(1982)039,0041:
SITLAR.2.0.CO;2.
——, and S. Yang, 1992: Monsoon and ENSO: Selectively in-
teractive systems. Quart. J. Roy. Meteor. Soc., 118, 877–926,
doi:10.1002/qj.49711850705.
——, and H. R. Chang, 1998: Atmospheric wave propagation in
heterogeneous flow:Basic flow controls on tropical–extratropical
interaction and equatorial wave modification. Dyn. Atmos.
Oceans, 27, 91–134, doi:10.1016/S0377-0265(97)00003-1.
Wu, B., T. Zhou, and T. Li, 2009: Seasonally evolving dominant
interannual variability modes of East Asian climate.
J. Climate, 22, 2992–3005, doi:10.1175/2008JCLI2710.1.
——, ——, and ——, 2012: Two distinct modes of tropical Indian
Ocean precipitation in boreal winter and their impacts on the
equatorial western Pacific. J. Climate, 25, 921–938,
doi:10.1175/JCLI-D-11-00065.1.
Wu, R., and B. P. Kirtman, 2007: Regimes of local air–sea interactions
and implications for performance of forced simulations. Climate
Dyn., 29, 393–410, doi:10.1007/s00382-007-0246-9.
Xie, P., and P. A. Arkin, 1997: Global precipitation: A 17-year
monthly analysis based on gauge observations, satellite esti-
mates, and numerical model outputs. Bull. Amer. Meteor.
Soc., 78, 2539–2588, doi:10.1175/1520-0477(1997)078,2539:
GPAYMA.2.0.CO;2.
Yang, S., and P. J. Webster, 1990: The effect of summer tropical
heating on the location and intensity of the extratropical
westerly jet stream. J. Geophys. Res., 95, 18 705–18 721,
doi:10.1029/JD095iD11p18705.
Yoo, C., S. Feldstein, and S. Lee, 2011: The impact of theMadden–
Julian Oscillation trend on the Arctic amplification of surface
air temperature during the 1979–2008 boreal winter.Geophys.
Res. Lett., 38, L24804, doi:10.1029/2011GL049881.
——, S. Lee, and S. B. Feldstein, 2012: Mechanisms of Arctic
surface air temperature change in response to the Madden–
Julian oscillation. J. Climate, 25, 5777–5790, doi:10.1175/
JCLI-D-11-00566.1.
Zhang, R., A. Sumi, andM.Kimoto, 1996: Impact of El Niño on theEast Asian monsoon: A diagnostic study of the ‘86/87 and ‘91/
92 events. J. Meteor. Soc. Japan, 74, 49–62.
Zheng, J., Q. Liu, C.Wang, andX.-T. Zheng, 2013: Impact of heating
anomalies associated with rainfall variations over the Indo-
western Pacific on Asian atmospheric circulation in winter.
Climate Dyn., 40, 2023–2033, doi:10.1007/s00382-012-1478-x.
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