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Why the Western Pacific Subtropical High Has Extended Westward since the Late 1970s
TIANJUN ZHOU,* RUCONG YU,* JIE ZHANG,*,1 HELGE DRANGE,# CHRISTOPHE CASSOU,@
CLARA DESER,& DANIEL L. R. HODSON,** EMILIA SANCHEZ-GOMEZ,@ JIAN LI,*,1
NOEL KEENLYSIDE,11 XIAOGE XIN,*,1 AND YUKO OKUMURA&*LASG, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
1Graduate School of the Chinese Academy of Sciences, Beijing, China#Nansen Environmental and Remote Sensing Center, and Bjerknes Centre for Climate Research, Bergen, Norway@CNRS-CERFACS, Toulouse, France&National Center for Atmospheric Research, Boulder, Colorado**Center for Atmospheric Modeling, University of Reading, Reading, United Kingdom
11Leibniz Institute of Marine Sciences, Kiel, Germany(Manuscript received 12 March 2008, in final form 1 October 2008)
ABSTRACTThe western Pacific subtropical high (WPSH) is closely related to Asian climate. Previous examination ofchanges in the WPSH found a westward extension since the late 1970s, which has contributed to the interdecadal transition of East Asian climate. The reason for the westward extension is unknown, however. The
present study suggests that this significant change of WPSH is partly due to the atmosphere’s response to theobserved Indian Ocean–western Pacific (IWP) warming. Coordinated by a European Union’s Sixth Framework Programme, Understanding the Dynamics of the Coupled Climate System (DYNAMITE), five AGCMs
were forced by identical idealized sea surface temperature patterns representative of the IWP warming andcooling. The results of these numerical experiments suggest that the negative heating in the central and eastern
tropical Pacific and increased convective heating in the equatorial Indian Ocean/Maritime Continent associated with IWP warming are in favor of the westward extension of WPSH. The SST changes in IWP influences
the Walker circulation, with a subsequent reduction of convections in the tropical central and eastern Pacific,
which then forces an ENSO/Gill-type response that modulates the WPSH. The monsoon diabatic heating
mechanism proposed by Rodwell and Hoskins plays a secondary reinforcing role in the westward extension of
WPSH. The low-level equatorial flank of WPSH is interpreted as a Kelvin response to monsoon condensational heating, while the intensified poleward flow along the western flank of WPSH is in accord with Sverdrup
vorticity balance. The IWP warming has led to an expansion of the South Asian high in the upper troposphere,
as seen in the reanalysis.

1. Introduction
The western Pacific subtropical high (WPSH), which
occupies about 20%–25% of the Northern Hemisphericsurface, plays a major role in the global circulation of
the atmosphere and oceans (Liu and Wu 2004). Greateffort of East Asian scientists has been devoted to the
WPSH studies, owing to its dominance on East Asian
climate. Residing over the western portion of the subtropical North Pacific basin throughout the year, the
WPSH tends to be stronger in summer than in winter.

Corresponding author address: Tianjun Zhou, P.O. Box 9804,
LASG, Institute of Atmospheric Physics, Chinese Academy of
Sciences, Beijing 100029, China.
E-mail: zhoutj@lasg.iap.ac.cn
DOI: 10.1175/2008JCLI2527.1
Ó 2009 American Meteorological Society

The WPSH has been identified as one important component of East Asian summer monsoon (EASM) system (Tao and Chen 1987; Ding 1994; Huang and Wu
1989; Lau and Li 1984). The seasonal variation of
WPSH is closely related to the onset and withdrawal of
EASM. The abrupt northward movement of WPSH
from winter to summer is accompanied with abruptchanges in the circulation patterns over the East Asianmonsoon area (Ye and Zhu 1958). The distribution ofsummer rainfall in China is associated with the seasonalvariations of WPSH in intensity, structure, and location(e.g., Tao and Xu 1962; Huang and Yu 1962; Huang
1963). The monsoon precipitation along the Yangtze
River valley is the product of the mei-yu front, whichis quasi stationary and is a characteristic feature of
the interaction between the warm, moist winds from the2200

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tropics and the cold, dry airflow originating from thenorth (Lau and Li 1984; Zhu et al. 1986; Tao and Chen
1987). The low-level jet along the northwestern edge of
WPSH transports a large amount of water vapor into
East Asia; any change in WPSH would influence themei-yu front by affecting the convergence of tropicalwater vapor with extratropical airflow (Ninomiya andKobayashi 1999; Zhou and Yu 2005). The WPSH also
plays a major role in modulating the weather and climate in Korea and Japan (e.g., Kurihara 1989).
The WPSH exhibits a remarkable seasonal cycle. The
mechanisms that underlie the development and maintenance of WPSH are still disputed. The proposed factors include the monsoon diabatic heating (Ting 1994;
Hoskins 1996; Rodwell and Hoskins 2001; Chen et al.
2001), land–sea heating (Wu and Liu 2003; Liu et al.
2001, 2004; Miyasaka and Nakamura 2005), diabatic
amplification related to the cloud-reduced radiative
cooling (Bergman and Hendon 2000), and air–sea interaction (Seager et al. 2003). In addition to seasonal
cycle, the WPSH also exhibits interannual changes (Nitta
1987; Kurihara and Tsuyuki 1987; Huang and Sun 1992;
Lu 2001; Lu and Dong 2001; Sui et al. 2007; Wu andZhou 2008). The interannual variability of large-scalequasi-stationary EASM frontal zones is dominated by
the position, shape, and strength of WPSH. When the
WPSH becomes stronger and shifts toward the equator,
the rainband is located along the Yangtze River valley(Zhou and Yu 2005).Variations of WPSH are crucial tothe understanding of interannual variations in the meridional structure of the monsoon–sea surface temperature (SST) relationship (Chang et al. 2000a,b).
East Asian climate has experienced an interdecadalscale transition since the late 1970s (Hu 1997; Wang 2001;
Hu et al. 2003; Yu et al. 2004; Li et al. 2005; Xin et al.
2006; Yu and Zhou 2007). This transition is also evident
in WPSH (Hu 1997; Gong and Ho 2002; He and Gong
2002). Since the late 1970s, the WPSH has extended
westward (Fig. 1), which has resulted in a rainband shift
over China, with excessive rainfall along the middle andlower reaches of the Yangtze River valley, and deficient
rainfall in north China (e.g., Hu et al. 2003; Yu and Zhou2007). The mechanism relevant to the westward extension of WPSH remains unresolved. Some statisticalanalyses have linked the decadal shift of WPSH withthe change of ocean temperature. Hu (1997) found thatthe interdecadal variability of summer precipitation andtemperature in East Asia is largely influenced by thechanges of SST and convective activity over the tropical Indian Ocean–western Pacific (IWP). Gong and Ho(2002) also suggests the IWP warming as a driving
mechanism. However, statistical analyses only provide a
hypothesis of relevant mechanisms. The role of ocean

VOLUME 22

forcing in producing the changes of WPSH remains to beclarified. The present study aims to explore the causes
for the westward extension of WPSH by addressing thequestion of whether the IWP SST change has beenmostly relevant to the decadal scale shift of WPSH. We
employ an idealized SST-forcing atmospheric generalvirculation model (AGCM) approach to gain insight on
this question and to quantify the robustness of IWP
impacts on the WPSH. Our results suggest that thewarming trend of IWP is in favor of the westward extension of WPSH since the late 1970s via the negative
heating in the central and eastern tropical Pacific andincreased monsoon condensational heating in the equatorial Indian Ocean/Maritime Continent.
The rest of the paper is organized as follows. We introduce the model experiment and analysis method in
section 2. We then present the analyses of model results
in section 3. A discussion is given in section 4. Section 5summarizes the study and its findings.

2. Experiments and analysis method
Since 1977 tropical SSTs have increased by approximately 0.4 K in the IWP relative to the period of 1950–
76 (Webster et al. 1999; Hoerling et al. 2001; Deser andPhillips 2006). The aim of our study is to investigate theimpacts on climate of the IWP basin-scale warming. To
do so, we carried out experiments in which AGCMs
were forced by idealized SST patterns representativeof the IWP warming. These coordinated experiments
were performed as part of the European Union’s (EU’s)
Sixth Framework Programme, Understanding the Dynamics of the Coupled Climate System (DYNAMITE;
information online at http://dynamite.nersc.no/). We
carried out two sets of experiments in which the IWP
SST anomaly was added to (IOP) or subtracted from(ION) the climatology. Note the SST climatology wasprescribed in the regions outside of the IWP. In addition
to the IOP and ION experiments, we have also performed a control run with prescribed climatological SST
throughout global oceans. It is worthwhile to note thatexamining the response to a single mode of SST variability implicitly assumes that the atmospheric response
to many SST modes that make up the total field can belinearly separated.
The SST forcing pattern for the IWP warming experiment was derived from the monthly mean trends
in SST between 1951 and 1999. The Hadley Centre sea
ice and sea surface temperature (HadISST) dataset isused (Rayner et al. 2003). The full forcing SST pattern,
IOP (ION) was generated by adding (subtracting) scaled
anomalies (IO9) to (from) the SST climatology over the
IWP region (roughly bounded by 358S–258N, 308–1608E):

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FIG. 1. Contour lines for 5870 gpm of 500-hPa geopotential height for each summer during (a), (c)1980–99 and (b), (d) 1958–79 from (a), (b) NCEP reanalysis and (c), (d) ERA-40. The thick contour line
shows the mean for each time period.

IOP 5 Clim 1 23.5IO9,(1)

ION 5 Clim

(2)

24.5IO9.

For IOP, the IO9 anomaly is scaled by the number ofyears between 1951 and the midpoint of the period used
to create the climatology (1961–90), hence, 23.5 yr. For
ION, using 1999, we obtain 24.5 yr. These two SST
patterns reflect the changes of SST in the IWP between
1951 and 1999. Figure 2 illustrates the SST anomalies in
the IWP domain. A prominent warming trend is evident
in the tropical Indian Ocean and far western Pacific.
The central intensity over the tropical Indian Ocean isabout 0.58C(23.5 yr)21.
The experiments were integrated for 40 yr of which
the first 10 yr were discarded as being the spinup stage.
The analysis is done for the boreal summer season. The
summertime mean was computed by averaging 3-month
periods [June–August (JJA)] in each year. Finally, summertime means were calculated by averaging each JJA
over the period of model experiment. Assuming thateach year is statistically independent, this is equivalent
for the anomaly experiments to an ensemble mean with30 realizations.
The models that have been examined in this study
include four AGCMs from the DYNAMITE project

participants, which are as follows: the European Center for Research and Advanced Training in Scientific
Computation (CERFACS) in France, the Max Planck
Institute for Meteorology (MPI) in Germany, StateKey Laboratory of Numerical Modeling for Atmospheric
Sciences and Geophysical Fluid Dynamics (LASG)/
Institute of Atmospheric Physics (IAP) in China, andthe Met Office’s Hadley Centre for Climate Prediction
and Research (UKMO) in the United Kingdom. In addition to the DYNAMITE partners, the climate variability working group of National Center for AtmosphericResearch (NCAR) in the United States also carried out
the same simulation using the NCAR Community Atmosphere Model, version 3 (CAM3). Thus, we have theoutputs of five AGCMs. Table 1 shows a brief summaryof each model. All AGCMs were forced by an identical
SST field described above.
The verification data used in the present study consist ofthat from 1) the National Centers for Environmental
Prediction (NCEP)–National Center for AtmosphericResearch (hereafter NCEP) reanalysis data from 1958 to1999 (Kalnay et al. 1996), and 2) the 40-yr European
Centre for Medium-Range Weather Forecasts (ECMWF)
Re-Analysis (ERA-40) dataset from 1958 to 1999 (Uppalaet al. 2005). The observational geopotential height andwind field are obtained from the NCEP reanalysis andERA-40.

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3. Results
a. Response of WPSH to the prescribed IWP
warming and cooling

FIG. 2. Monthly mean distribution of the SST trends [8C(23.5 yr)21] between 1951 and 1990 from the HadISST dataset:(a) June, (b) July, and (c) August. These SST anomalies wereused to drive the AGCMs in IWP warming runs.

How can the position of the WPSH be measured? Although the vast subtropical anticyclone dominates boththe low and middle troposphere of the North Pacific inboreal summer, it is termed as WPSH mainly in the middle
troposphere. In the low troposphere the center of the
anticyclone is located in the eastern Pacific, while inthe middle troposphere the center of the anticyclone islocated in the western Pacific and thus has a close connection to East Asian climate (Liu et al. 2001; Liu andWu 2004). The WPSH is conventionally measured by
the geopotential height at 500 hPa (Z500) (e.g., Tao andChen 1987; Ding 1994; Zhou and Li 2002; among manyothers). In our following discussions, we also use Z500
to describe the position of WPSH, along with part results
at the other lower levels. We examine the results of eachindividual model but focus on the response of multimodel ensemble (MME).

To gain information about the decadal-scale shift of
WPSH, we measure the WPSH by the contour line for5870 gpm of Z500. Figure 1 shows that there is a remarkable difference in the position and extent of WPSH
between 1958–79 and 1980–99. Since the late 1970s, the
WPSH has extended westward. Both the NCEP reanalysis and ERA-40 data show similar tendencies. During
1958–79, the mean position of the western edge of
WPSH is located at 133.58E (127.58E) in the NCEP
reanalysis (ERA-40). During 1980–99, the mean western edge of WPSH has shifted to 119.58E (117.58E) in
the NCEP reanalysis (ERA-40). There are some discrepancies between the two reanalysis datasets in revealing the westward extension. The decadal shift of thewestern edge is 148 in the NCEP reanalysis and 108 in
ERA-40. The interdecadal transition of the East Asian
climate has also been manifested in other physical variables, including rain gauge data (e.g., Wang 2001; Hu
et al. 2003; Yu et al. 2004; Yu and Zhou 2007), adding
fidelity that the interdecadal shift of WPSH in the reanalysis data is real. Since the East Asian summer
rainband is sensitive to the location and intensity of
WPSH, this shift gives rise to an anticyclonic circulation
anomaly over the region from the South China Sea tothe western Pacific, and it results in prevailing southerlies over southern China (Gong and Ho 2002).
To verify whether the westward extension of WPSH
resulted from the observed IWP warming, the responseof WPSH to a warming (cooling) IWP was investigatedin the coordinated experiments described in section 2.
Because the climate mean intensities of WPSH simulated by five models are different, we delineate the
WPSH by departures of Z500. The departure is from thezonal belt mean between 08 and 408N. The results of
the reanalysis and five AGCMs are shown in Fig. 3. The
values of zonal belt mean height between 08 and 408N
are marked at the right corner of each panel. The isolineof 10-gpm height, which corresponds to 5870-gpm isoline in the absolute geopotential height, arrives in 1308E
in the NCEP reanalysis. The result derived from ERA40 is highly consistent (figure not shown). For brevity,
we only present the results derived from NCEP reanalysis in the following discussions. In comparison withthe reanalysis, the position of the 10-gpm isoline extends westward in CAM3 and Grid Atmospheric Modelof IAP/LASG (GAMIL) but retreats eastward in thefifth generation of the ECHAM general circulation model(ECHAM5), the third Hadley Centre atmospheric model(HadAM3), and Action de Recherche Petite Echelle15 APRIL 2009

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ZHOU ET AL.
TABLE 1. Description of five AGCMs.

Institute

AGCM

Resolution

NCAR

CAM3

T85L26MPIECHAM5

T63 L31

UKMO

HadAM3

2.58 lat 3 3.758 lon,
L19

IAP

GAMIL

CERFACS

ARPEGE4.4

2.88 lat 3 2.88 lon,
L26
T63 L31

Convection scheme
Deep convection is parameterized following
Zhang and McFarlane (1995);
shallow and upper-level convectionuses Hack (1994)
Tiedtke (1989), with modifications fordeep convection according to Nordeng (1994)
Gregory and Rowntree (1990), with theaddition of convective downdrafts(Gregory and Allen 1991)
Zhang and McFarlane (1995)
Deep convection is represented by a mass flux
scheme with detrainment as proposed byBougeault (1985); stratiform and shallow
convection cloud formation is evaluated
via a statistical method described in
Ricard and Royer (1993)

Grande Echelle (ARPEGE). The spread among themodels in the climate mean states makes it difficult tomeasure the position of WPSH by using a same contour
line such as 10 gpm. To handle the problem and makedifferent model results comparable, we define the climatological isoline straddling the longitude of 1308E as
the characteristic WPSH isoline. As shown in Fig. 3,
there are differences in the strengths of the characteristic WPSH isoline among the models. For CAM3, theisoline used to delineate WPSH is 14 gpm at 500 hPa.
The characteristic WPSH isoline is zero gpm for
ECHAM5, 5 gpm for HadAM3, 20 gpm for GAMIL,
and 210 gpm for ARPEGE. The corresponding isoline
for the NCEP reanalysis is 10 gpm.
The response of WPSH to a warming (cooling) IWP is
depicted by the characteristic WPSH isoline. The resultsof five models are shown in Fig. 4. It is clear that the
IWP warming (cooling) has led to a consistent westward
extension (eastward retreat) of WPSH relative to its
climate mean position. Qualitative comparison reveals
that all AGCMs exhibit near-similar responses. Relative to the result of the ION experiment, the WPSH of
the IOP experiment has a westward extension of 228 in
CAM3, 148 in ECHAM5, 148 in HadAM3, 188 in GAMIL,
and 138 in ARPEGE. The response of CAM3 is thestrongest, while that of ARPEGE is the weakest. Only
the 30-yr mean condition is presented in Fig. 4. We havealso examined the internal variability of each model bycomparing the spread of characteristic WPSH isoline
positions among the 30 realizations. The results indicatethat both CAM3 and GAMIL models have a low level ofinternal variability compared to ARPEGE, HadAM3,
and ECHAM5 (figures not shown). Because the physical package of GAMIL model is derived from the Com-

Reference
Boville et al. (2006)

Roeckner et al. (2003)
Pope et al. (2000)

Li et al. (2007)
Gibelin and Déqué (2002)

munity Atmosphere Model, version 2 (CAM2) (Li et al.
2007), the similarity between these two models in termsof internal variability suggests an active role of model
physics in producing the internal variability.

b. The role of anomalous tropical convection locatedto the east of the date line
Multimodel intercomparison indicates that the westward extension of WPSH might be partly due to thewarming of IWP. What is the mechanism responsible
for this robust response? In Fig. 5, the anomalies at
100 hPa in the divergent wind component and velocity
potential field are shown. Regions of low-level convergence over the IWP warmer water are mirrored in theupper troposphere by regions of divergence, as seen by
the anomalous outflow at 100 hPa in Fig. 5. Note thatthe upper-tropospheric convergence is located to theeast of the date line, although a climatological SST wasprescribed over there. This feature indicates the impact
of IWP warming on the Walker circulation. Figure 5
indicates the above-normal convective activity centeredon the equator at 1208E over the warmer water, but withevidence of reduced convective activity to the east of
the date line.
In Fig. 6, the responses of JJA rainfall to IWP warming
are shown. The upper-tropospheric divergent/convergent
features in the velocity potential field have corresponding rainfall anomalies. The warming of IWP has caused
rainfall to increase over most of the local basin and the
Maritime Continent. Excessive rainfall develops over the
Arabian Sea, Indian peninsula, and the Bay of Bengal.
This is particularly the condition for the CAM3, GAMIL,
and ECHAM5 models. The center of excessive rainfall
is located to the south of Arabian Sea in HadAM3. In

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FIG. 3. Departures of JJA mean geopotential height at 500 hPa in the (a) reanalysis and (b)–(f) control
runs of AGCMs. The departure is calculated as the difference from the zonal mean between 08 and 408N.
The values of zonal mean between 08 and 408N are marked at the right corner (units: gpm). The name of
AGCM is marked at the left corner. IAP refers to the GAMIL model. Contour interval is 10 gpm. The
bold solid line corresponds to the isoline of zero.

addition to the positive centers located along the northern Indian Ocean, excessive rainfall also develops in the
Maritime Continent, including the South China Sea.
Note that the positive rainfall anomaly over the western
Pacific in the ARPEGE model is stronger than the others,
which is responsible for the slightly different subtropical
anticyclone response discussed later. A further comparison of ARPEGE with the observational estimates indicates that the mean state for precipitation in ARPEGE
over this area is not very realistic and might amplify the
response (figures not shown here). In addition to theexcessive rainfall anomalies associated with warmer
water over the IWP, deficient rainfall anomalies are evident in the central and eastern tropical Pacific.
The excessive (deficient) rainfall anomalies indicate
positive (negative) heating sources. The WPSH appears
as the maximum in streamfunction of boreal summertime low-level circulation. Lu and Dong (2001) suggested
that the WPSH at 850 hPa is also stable and robust. To
exhibit the relation between the WPSH variation and theanomalous heating, the horizontal streamfunction at 850

hPa for JJA based on the difference between the IOP
and ION experiments for five AGCMs are presented inFig. 7. In the vicinity of tropical heating implied by theprecipitation anomalies, the streamfunction anomalies
show a strong cyclonic couplet in both hemispheres
straddling the equator over the tropical Indian Ocean.
The close association between the largest precipitation
anomalies and the streamfunction anomalies suggeststhat the anomalous latent heat release is likely to be
forcing the circulation, which is consistent with the theory of Gill (1980). Inspection on JJA 850-hPa streamfunction (Fig. 7) reveals that nearly all models produce a
cyclonic anomaly centralizing in the Arabian Sea, except
for the ARPEGE model. The center of the cyclonicanomaly is located over the South China Sea rather thanthe Arabian Sea therein. The responses of streamfunction
are consistent with the positions of precipitation anomalies, as shown in Fig. 6.
In addition to the Gill-type local response, anotherprominent feature of Fig. 7 is the North Pacific subtropical anticyclone. The close structural similarities

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FIG. 4. (b)–(f) The positions of characteristic WPSH isoline at 500 hPa in IWP warming (red)and cooling (blue) experiments and control runs (long-dashed black line) in the five AGCMs.
The name of AGCM is marked at the left corner. (a) The model results are for a 30-yr mean.
IAP refers to the GAMIL model. The condition of the NCEP reanalysis is shown [1980–99 (redline), 1958–79 (blue line), and 1958–99 (long-dashed black line) mean].

among five AGCMs are very impressive. This anticyclonicresponse is consistent with the westward extension of
WPSH shown in Fig. 4. What mechanisms govern the
North Pacific response to IWP SST forcing? We suggestthat it is due to the negative heating source associatedwith the deficient rainfall anomalies located to the eastof the date line (cf. Fig. 6). The convection change in thecentral and eastern tropical Pacific result in the vigorous
twin anticyclone gyres in the Pacific. Because there is no
anomalous SST forcing in the Pacific, this should be aremote response to the extra heating in the regions ofenhanced precipitation over the IWP region. The SSTchanges in the Indian Ocean and far western Pacificinfluence the Walker circulation, as evidenced by Fig. 5,
with the subsequent changes in the tropical Pacificconvective anomalies forcing an ENSO/Gill-like response that modulates the WPSH.
Examination on the vertical structure found that theintensity of North Pacific anticyclone gyre gradually
decreases from the surface to the middle troposphere(figures not shown here). Given the strong response of

WPSH at 500 hPa, there should be a secondary reinforcing mechanism that favors the WPSH change. As
to be discussed below, the monsoon diabatic heating
mechanism proposed by Rodwell and Hoskins (1996)
may work here.

c. The role of monsoon diabatic heating
Rodwell and Hoskins (2001) showed that the diabatic
heating associated with the South Asian summer monsoon rainfall could affect the summer subtropical circulation over the North Pacific. To understand better
how this mechanism might operate in our IWP SSTforcing experiments, we show in Fig. 8 the change of
low-tropospheric flow associated with enhanced precipitation. The low over the Arabian Sea and Indian
peninsula is consistent with the Gill-type response of
streamfunction shown in Fig. 7. Over the North Pacificalong the eastern Asian continent, there is a poleward
anomalous flow between about 08–408N and 908–1358E.
Over the Pacific, this poleward flow forms the western
flank of the WPSH. Above analyses demonstrate that

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FIG. 5. 200-hPa velocity potential (106m2 s21) and divergent component of the wind vectors (m s21) for
JJA mean based on the difference between the IWP warming and cooling experiments. Shaded regions
are statistically significant at the 1% level according to the Student’s t test. Vectors are the divergent
components of the wind field: (a) CAM3, (b) ECHAM, (c) HadAM3, (d) IAP, (e) ARPEGE, and(f) MME.

the anticyclone gyre driven by negative heating source
over the central and eastern tropical Pacific is the primaryreason for this poleward flow. The secondary reinforcing
agent may come from the Sverdrup vorticity balance.
The poleward anomalous flow is associated with the
Sverdrup vorticity balance, which is expressed asby ’ f

›v
,
›p

(3)

where y is meridional wind, v is the vertical velocity, f is
the Coriolis parameter, and b is its meridional gradient(Rodwell and Hoskins 2001; Hoskins and Wang 2006).
The Sverdrup balance can also be expressed in otherforms to emphasize the importance of the vertical distribution of heating (Wu et al. 1999).
In agreement with Sverdrup vorticity balance of thesteady flow, strong poleward flow at the low level should
be seen below the maximum ascent. This relationship is

FIG. 6. Composite of JJA precipitation differences (mm day21) between the IWP warming and IWP coolingexperiments: (a) CAM3, (b) ECHAM, (c) HadAM3, (d) IAP, (e) ARPEGE, and (f) MME. IAP refers to the GAMIL
model. Values statistically significant at the 1% level according to the Student’s t test are encircled by black lines.

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FIG. 7. Horizontal streamfunction at 850 hPa for JJA based on
the difference between the IWP warming and cooling experiments(106 m2 s21). Shaded regions are statistically significant at the
1% level according to the Student’s t test. Vectors are the nondivergent components of the wind field (m s21). (a) CAM3,(b) ECHAM, (c) HadAM3, (d) IAP, and (e) ARPEGE. IAP
refers to the GAMIL model.

well captured by the model results. The MME rainfall
anomaly pattern, the longitude–height plots of verticalvelocity gradient ›v/›p, and the meridional wind for the
08–208N average, the latitude–height cross section of
meridional wind, are shown in Fig. 9. There is strong
upward motion above the heating region. A positive
gradient ›v/›p develops below the maximum upward
motion. The strongest vertical velocity gradient ›v/›p
is located around 600–700 hPa along 1208E (Fig. 9b).
Following the strong ascent gradient, poleward flows to
the east of the heating center are evident in the MME
responses. Corresponding to the three anomalous rainfall
centers located in the southern Arabian Sea, the Bay of

FIG. 8. Surface pressure (Pa) and 850-hPa horizontal wind anomalies(m s21) calculated as the difference between the IWP warming andcooling experiments. Positive contours are solid and negative contours
are dashed. (a) CAM3, (b) ECHAM, (c) HadAM3, (d) IAP,(e) ARPEGE. Shaded regions are statistically significant at the 5%
level according to the Student’s t test. IAP refers to the GAMIL model.

Bengal, and the South China Sea, respectively, strong
ascent and the associated vertical gradients are evident,
straddling 658, 808, and 1208E, respectively. The gradientof upward motions over 1208E is the strongest and2208

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FIG. 9. Anomalous JJA (a) precipitation (mm day21), (b) longitude–height cross section of vertical velocity
gradient ›v/›p (1026 s21), (c) longitude–height cross section of meridional wind (m s21), and (d) latitude–heightcross section of meridional wind (m s21). The anomalies were calculated as the difference between the IWP
warming and IWP cooling experiments. All results are for a multimodel ensemble. The anomalies are shown as
08–208N average in (b) and (c), and as a 1158–1308E average in (d). Values statistically significant at the 1% levelaccording to the Student’s t test are encircled by black lines.

penetrates upward to 400 hPa (Fig. 9b). Associated withthe positive vertical gradients of upward motion, poleward anomalous meridional winds prevail (Fig. 9c).
Along 1208E, where meridional wind anomalies develop, we show the latitude–height cross section of
meridional wind anomalies in Fig. 9d. The meridional
wind anomalies straddling 1208E extend northward up
to 308N. The core of the meridional wind anomaly is
located around 600 hPa, which matches that of thevertical velocity gradient shown in Fig. 9b, indicating agood correspondence between the maximum verticalvelocity gradient and the maximum meridional wind.
This situation strictly matches the Sverdrup vorticity
balance. Hence, the Sverdrup balance works well in themodel results. The poleward meridional wind straddling
1208E corresponds to the western flank of WPSH. Thechanges of meridional wind are beneficial to an intensified western flank of WPSH.
We have also performed a model-by-model comparison, although there are differences in the positions of
upward motion, the maximum vertical velocity gradient,and the associated poleward flows in the middle andlow troposphere; all five models’ responses support themechanism that the condensational heating induces arobust ascent, along with a positive vertical velocity
gradient below the maximum ascent, which further leads

to intensified poleward flows in the low troposphereresulting from the Sverdrup vorticity balance (figures notshown). Hence, the formation of western flank of theanomalous WPSH can be partly explained in terms of the
Sverdrup vorticity balance, as a secondary reinforcing
mechanism.
In addition to the poleward meridional flow, strengthened easterlies in the equatorial Pacific are evident inall models’ responses (see Figs. 7–8). This anomalous
easterly forms the southern flank of the WPSH. The
anticyclonic gyre generated by the negative heat source
over the central and eastern tropical Pacific dominates
the anomalous easterlies, as discussed in the last section.
The condensational monsoon heating may serve as asecondary mechanism. These wind anomalies show
close similarities to the analytical results of Gill (1980).
They are identifiable with the equatorial Kelvin wave
solution to South Asian monsoon heating; that is, theflow pattern to the east of the heating region is due tothe eastward propagation of the Kelvin waves, and thus,
the winds are easterlies toward the heat source and tend
to be parallel to the equator (Hoskins and Wang 2006).
Therefore, the intensified summertime low-level equatorial flank of the WPSH can also be partly interpreted
as a Kelvin wave response to the intensified South Asianmonsoon heat source. The stronger WPSH easterly is15 APRIL 2009ZHOU ET AL.

2209

FIG. 10. Spatial distributions of JJA mean geopotential height (gpm) at (from top to bottom) 500,
600, 700, and 850 hPa from the (left) reanalysis and (right) multimodel ensemble. The results of IWP
warming (cooling) experiment are shown in color (long-dashed black) in the right column. The resultsof NCEP reanalysis are shown in color (long-dashed black) for 1980–99 (1958–79) mean in the left
column.

primarily the southern part of the anticyclonic gyre
driven by the negative heating source over the central
and eastern tropical Pacific, and secondarily a responseto the increased convective heating associated with the
IWP warming.
In above discussions, we either describe the westward
extension of WPSH by using Z500 conventionally or
analyzing the model results at 850 hPa, as in previous
modeling studies. This does not mean that the changes of
WPSH at the other levels of the middle–low troposphere
are not significant. Figure 9 has already shown that themodels’ responses are robust from 400 hPa to the surface (cf. Figs. 9b–d). In Fig. 10, we present the changesof geopotential height from 500 to 850 hPa for both thereanalysis and the MME simulation. It is clear that in
the reanalysis, the interdecadal scale westward extension of WPSH is not a unique feature at 500 hPa, but has
occurred throughout the whole middle–low troposphere(Figs. 10a–d). In the MME numerical simulation, awarming IWP has led to a westward extension of WPSH
from 500 hPa to the surface (Figs. 10e–h). Above mechanism analyses concerning both the primary negative

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JOURNAL OF CLIMATEVOLUME 22

heating in the central and eastern tropical Pacific and thesecondary monsoon diabatic heating should also apply tothe other levels of middle–low troposphere.

d. The upper-tropospheric response
In addition to the WPSH in the middle and lower
troposphere, another persistent subtropical anticyclone
controlling the free atmosphere over Asia during boreal
summer is the South Asian anticyclone, termed the‘‘South Asian high’’ (‘‘SAH’’), in the upper troposphere
over the region to the north of the Bay of Bengal (Taoand Chen 1987). The climate impact of IWP warming
may also be manifested in the SAH. In Gill’s model, thevertical structure of the tropical atmosphere is described by a single baroclinic mode, such that circulation
anomalies in the upper troposphere are opposite tothose in the lower troposphere (Gill 1980; Sutton andHodson 2007). The 100-hPa streamfunction anomalies
between IOP and ION experiments are presented inFig. 11. The patterns of twin upper-level anticyclones
over the warming IWP region in the CMA3, ECHAM5,
HadAM3, and GAMIL models resemble that predicted
by Gill’s theory. The twin anticyclone simulated by
ARPEGE model is more significant at a level higher
than 100 hPa (figure not shown). In addition, the vigorous twin upper-level cyclones straddling the equatorin the Pacific primarily come from the negative heating
source forced by convection changes in the central andeastern tropical Pacific, as discussed in section 3b.
Previous studies demonstrate that the latent heating
released by the South Asian monsoon rainfall is beneficial to the formation of SAH (Liu et al. 2001; Yanget al. 2007). According to Eq. (3), above the maximum
ascent, the Sverdrup balance requires an equatorward
flow. This is evident in Figs. 11a–d. An examination on
the vertical cross section also demonstrates that the coreof maximum equatorial wind at upper tropospherematches that of vertical velocity gradient ›v/›p (figures
not shown). The meridional wind component of SAH
can be partly explained by the Sverdrup balance, as asecondary mechanism to the direct Gill-type response.
In observation, the SAH is usually measured by thecontour line of 16 800 gpm of the 100-hPa geopotential
height (Zhang et al. 2000). Similar as the condition of
WPSH at 500 hPa, the climate mean intensities of SAH
simulated by five models are different; we delineate theSAH by the departures of the 100-hPa geopotential
height. The departure is from the zonal belt mean between 08 and 608N. The results of the reanalysis and five
AGCMs are shown in Fig. 12. The simulated SAH differs from the reanalysis in both the intensity and coverage. The center of SAH shifts westward in CAM3 andnorthward in the GAMIL model. The central intensity

FIG. 11. Horizontal streamfunction at 100 hPa for JJA mean
based on the difference between the IWP warming and IWP
cooling experiments (106 m2 s21). Shaded regions are statisticallysignificant at the 1% level according to the Student’s t test. Vectors
are the nondivergent components of the wind field (m s21).
(a) CAM3, (b) ECHAM, (c) HadAM3, (d) IAP, and (e) ARPEGE.
IAP refers to the GAMIL model.

of SAH is weaker than the reanalysis in nearly allmodels except for CAM3. The coverage of the 200-gpm
isoline is larger than the reanalysis in all five models. In
the following discussion, we define the contour line of
200 gpm as the characteristic SAH isoline and makecomparisons between IWP warming and IWP coolingexperiments for each individual model.
Associated with the interdecadal scale transition of
the East Asian climate since the late 1970s, the SAH
has experienced an expansion in zonal coverage (Zhang
et al. 2000). This intensified SAH is evident in Fig. 13a.
To verify whether the SAH expansion is partly due tothe IWP warming, the response of SAH to a warming15 APRIL 2009ZHOU ET AL.

2211

FIG. 12. Departures of JJA mean geopotential height at 100 hPa in the (a) reanalysis and (b)–(f) control runs of
AGCMs. The departure is calculated as the difference from the zonal mean between 08 and 608N. The values of zonal
mean between 08 and 608N are marked at the right corner (gpm). The name of AGCM is marked at the left corner.
Contour interval is 50 gpm. The bold solid line corresponds to the isoline of zero. IAP refers to the GAMIL model.

(cooling) IWP is depicted by the characteristic SAH isoline. The results of five models are presented in Figs.
13b–f, respectively. The IWP warming (cooling) has led
to a consistent zonal expansion (shrinkage) of SAH relative to the climate mean position. Given the fact that all
AGCMs exhibit similar responses, the IWP warming iscontributive to the expansion of SAH since the late 1970s.

4. Discussion
In nature, there is an interaction between the Asianmonsoon and WPSH in the sense that either one would
be different without the other. This study only addresses
how the South Asian monsoon heating affects the summertime WPSH and not vice versa. The results reveal a
qualitative agreement of five AGCMs in producing the
westward extension of WPSH owing to the IWP warming. The results indicate that both the negative heating
source over the central and eastern tropical Pacific andthe direct condensational heat release accompanying theincreased South Asian monsoon rains are contributive tothe westward extension of WPSH. The warming of IWP

has caused an increasing of monsoon rains over mostparts of the local basin in all five models; further inspections on the spatial pattern and intensity of rainfall
anomalies simulated by five AGCMs still find some differences (Fig. 6). The local maximum of rainfall anomalies reflects the center of diabatic heating. Although all
AGCMs employed an identical SST forcing, theirrainfall responses are not exactly identical. The difference of diabatic heating that accompanies the increased
monsoon rains accounts for the discrepancies among
five models in producing some regional features of circulation responses (Figs. 7–8). Why does an identical
SST forcing lead to slightly different model responses?
This discrepancy is very likely to be at the origin ofdifferences between models. Although an identical SST
forcing was employed, there are spreads among the
precipitation responses of five AGCMs resulting fromdifferent model sensitivities.
It is important to note that the East Asian summer
climate is extremely sensitive to both the western boundary and ridge line of the WPSH. Any bias of the climate
models in simulating the climate mean position of WPSH

2212

JOURNAL OF CLIMATEVOLUME 22

FIG. 13. (b)–(f) Coverage of JJA mean characteristic SAH isoline at 100 hPa in IWP warming(red) and cooling (blue) experiments and control runs (long-dashed black line) in the five
AGCMs. The name of AGCM is marked at the left corner. IAP refers to the GAMIL
model. The model results are for the 30-yr mean. (a) The condition of NCEP reanalysis isshown [1980–99 (red line), 1958–79 (blue line), and 1958–99 (long-dashed black line) mean].

would lead to a bias in monsoon circulations and the associated mei-yu front. Unfortunately, most AGCMs analyzed here suffer from this kind of bias. For example, theclimatological mean positions of the ridge line for WPSH
simulated by the CAM3, ECHAM5, and GAMIL models
are about 58–68 north of the reanalysis (Figs. 3–4). The
western edges of climatological WPSH simulated by the
CAM3 and GAMIL models extend to the west of thereanalysis (Figs. 3–4). These biases make it difficult tocompare the simulated wind anomaly over the East Asian
continent with that of the reanalysis.
The limitations of our experimental setup should be
acknowledged. As shown in Fig. 2, the Philippine Sea
has been included as part of the IWP region. Previous
studies found a relationship between the zonal extent of
WPSH and the intensity of atmospheric convection overthe Philippine Sea. For example, stronger (weaker)
convection over the Philippine Sea is associated withmore eastward retreat (westward extension) of WPSH
(Lu 2001). Numerical model experiments also show thatthe suppressed convection caused by colder SST in thePhilippine Sea results in a westward extension of WPSH
(Lu and Dong 2001). Although this relationship is basedon studies of the interannual variability of WPSH, it
implies that the prescribed warming trend in the Philippine Sea might have a negative contribution to thewestward extension of WPSH. This hypothesis shouldbe verified by numerical model experiments that exclude the SST anomalies in the western Pacific warm
pool region shown in Fig. 2. A set of such coordinatedmultimodel experiments are being undertaken as partof the European Union’s Sixth Framework Programme(i.e., DYNAMITE).
Another potential limitation is the use of a prescribed
SST boundary forcing in the Asian–Australian monsoon
domain. Recent research suggests that the prediction/
simulation of an Asian–Australian summer monsoon
requires taking into account local monsoon–warm poolocean interactions (Wang et al. 2005; Wu and Kirtman
2005; Krishna Kumar et al. 2005; Zhou et al. 2008, 2009).
In our experiments, the atmosphere is forced to respondpassively to the specified SSTs, while in nature the SSTs

15 APRIL 2009

2213

ZHOU ET AL.

are also partly forced by the atmosphere (Wang et al.
2005), suggesting that the role of IWP SST forcing might
be exaggerated in our experiments.

5. Summary
The WPSH plays a major role in East Asian climate.
Previous observational analyses found that the WPSH
has extended westward since the late 1970s, contributing to the interdecadal transition of East Asian climate.
The cause for the westward extension of WPSH is addressed by employing an idealized SST-forcing AGCM
approach. Coordinated by a European Union’s Sixth
Framework Programme ‘‘Understanding the Dynamics
of the Coupled Climate System’’ (‘‘DYNAMITE’’), we
carried out experiments in which five AGCMs wereforced by an identical idealized SST pattern representative of the IWP warming. The results of numericalexperiments indicate that both the negative heating in
the central and eastern tropical Pacific and the increased
convective heating in the equatorial Indian Ocean/
Maritime Continent, associated with a warming of IWP,
have contributed to the westward extension of WPSH
since the late 1970s. The major results are summarized asfollows:
1) Responses of WPSH to IWP warming exhibit a westward extension in all five AGCMs. The magnitude of
the westward extension is model dependent, ranging
from 138 in ARPEGE to 228 in CAM3. The extension in the MME is 16.28, which is slightly stronger
than the average of the data of the two reanalysis(12.08).
2) The IWP warming has caused excessive rainfall overmost parts of the IWP basin in all AGCM responses.
Excessive rainfall develops over the Arabian Sea,
the Indian peninsula, the Bay of Bengal, and the
Maritime Continent, including the South China Sea,
indicating an intensified convective heating. The SSTchanges in the IWP influence the Walker circulation
and result in deficient rainfall in the central andeastern tropical Pacific, signifying a negative heating
source over there.
3) The negative heating source associated with the tropical Pacific convective anomalies forces an ENSO/
Gill-like response that modulates the WPSH. The
resulted anticyclonic gyre in the North Pacific is favorable to the westward extension of WPSH and isregarded as the primary forcing mechanism.
4) The monsoon diabatic heating mechanism proposedby Rodwell and Hoskins (1996) plays a secondary
reinforcing role in the westward extension of WPSH.
The low-level equatorial flank of WPSH is inter-

preted as a Kelvin response to the monsoon condensational heating, while the intensified poleward
flow along the western flank of WPSH is in accord
with Sverdrup vorticity balance.
5) The climate impact of IWP warming is also evident
in South Asian high in the upper troposphere. The
increased convective heating associated with IWP
warming results in an expansion of SAH, as seen in
the reanalysis. Both the primary Gill-type response
and the secondary Sverdrup balance are in favor of
the expansion.
Acknowledgments. This work was jointly supportedby the Major State Basic Research Development Program of China (973) under Grant 2006CB403603, theNational Natural Science Foundation of China underGrants 40625014, 40523001, and 40628006, and the EU’s
Sixth Framework Programme under Contract 003903GOCE (DYNAMITE).

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