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Biennial and Lower-Frequency Variability Observed in the Early Summer Climate in
the Western North Pacific
TOMOHIKO TOMITA
Department of Environmental Sciences, Kumamoto University, Kumamoto, and Frontier Research Center for Global Change, Japan
Agency for Marine-Earth Science and Technology, Yokohama, JapanTAKAO YOSHIKANE
Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, JapanTETSUZO YASUNARI
Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, and Hydrospheric
Atmospheric Research Center, Nagoya University, Nagoya, Japan(Manuscript received 4 April 2003, in final form 16 April 2004)
ABSTRACT
Early summer climate in the western North Pacific is largely represented by the baiu phenomenon. The
meridional fluctuations of the baiu front on interannual time scales and the associated large-scale circulations
are examined using the empirical orthogonal function (EOF) analysis and composite or correlation analyses
based on the EOF time coefficients.
The first EOF mode indicates a 5- or 6-yr low-frequency fluctuation (LF mode) appearing south of 358N. The
development is concurrent with horseshoe sea surface temperature anomalies (SSTAs) in the entire tropical
Pacific that are associated with the El Niño–Southern Oscillation (ENSO). SSTAs in the western North Pacificcontrol the anomalous southward expansion of the baiu front through a modification of the convection at around
208–358N. The LF mode is negatively correlated with the south-southeast Asian summer monsoon.
The second EOF mode is characterized by a meridional seesawlike fluctuation with a node at around 288N
and a time scale of biennial oscillation (BO mode). The horseshoe SSTAs again control the anomalous meridional
circulations, but with a different spatial phase through a convection off the Philippines. The spatial phasedifference between the two horseshoe patterns is about 908 in both the zonal and meridional directions. The BO
mode is negatively correlated with the tropical western North Pacific monsoon.
SSTAs associated with the BO mode tend to be confined to the tropical western Pacific, while the signals of
the LF mode extend rather broadly in the tropical Pacific–Indian Ocean sector, suggesting that the tropical BO
is an aborted ENSO in the tropical central–western Pacific. The spatial phase of horseshoe SSTAs adjusts theinterannual variability of the meridional fluctuation of the baiu front in the western North Pacific.

1. Introduction
Early summer climate in the western North Pacific ischaracterized by a locally specified monsoon off thePhilippines and by the ‘‘baiu’’ (in Japanese, ‘‘mei-yu’’in Chinese, ‘‘changma’’ in Korean) phenomenon to thenorth (Murakami and Matsumoto 1994). Precipitationin the baiu season is, in fact, a prerequisite for summer
water resources in East Asia. This work examines theinterannual variability of the baiu system in the western
North Pacific.
The baiu front, which appears from early June
Corresponding author address: Dr. Tomohiko Tomita, Departmentof Environmental Sciences, Faculty of Science, Kumamoto University, Kumamoto 860-8555, Japan.
E-mail: tomita@sci.kumamoto-u.ac.jp

q 2004 American Meteorological Society

through late July near Japan, climatologically extends
along the northwestern periphery of the North Pacificsubtropical high (the Pacific high) as part of the Asiansummer monsoon (Fig. 1, Murakami and Matsumoto
1994; Wang and LinHo 2002). A large meridional temperature gradient in the midlatitudes, which implies alarge baroclinicity and the resultant westerly thermalwind jet in the upper troposphere, confines the latitudinal location of the baiu front to midlatitudes in thefirst order. In addition, superimposed variations of the
Pacific high and the Okhotsk high to the north modify
the characteristics of the baiu front in the western North
Pacific (Ninomiya and Mizuno 1987). The developmentof a large-scale heat low over the Asian continent, which
is synchronized with the annual cycle of the sun andseasonally directs moisture from the Tropics to the mid-

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latitudes, is a significant factor in determining the seasonal march of the baiu front (Kawamura and Murakami
1998). Multiscale features in and around the baiu front
are well summarized in Ninomiya and Akiyama (1992),
and the climatological seasonal march of baiu is comprehensively described by Yoshino (1965, 1966). Largescale circulations control both spatial and temporal developments of the baiu phenomenon.
The unique nature of the baiu phenomenon, as well
as society’s deep concern over its impact on water resources, has attracted the attention of a large number of
researchers. Weather forecasting, of course, is concernedwith baiu precipitation, and the latest regional forecast
models provide practical predictions on a synoptic scale.
However, general circulation models (GCMs) still failto simulate a realistic baiu front (Kawatani and Takahashi 2003). To extend the prediction and understandthe process of interannual variability, it is necessary to
broaden our field of vision to a larger scale and recognize the baiu phenomenon as part of the global climatic system. In particular, it is critical to understandthe physical linkage with the main part of the Asiansummer monsoon and with the El Niño–Southern Oscillation (ENSO), which is the most outstanding interannual variation in the world.
The baiu activity demonstrates a significant interannual variation, and the baiu rainfall shows a significant
correlation with the ENSO cycle that is indicated by thesea surface temperature anomalies (SSTAs) in Niño-3(48S–48N, 1508–908W; Tanaka 1997). In other words,
the baiu precipitation tends to be large in the early summers after the withdrawal of El Niño events, concurrent
with an anomalously weak tropical western North Pacific (TWNP) monsoon. Wang et al. (2000) and Wang
and Zhang (2002) proposed a physical mechanism
whereby the ENSO affects the following summer climate in East Asia through a modification of the TWNP
monsoon; in their proposal, they emphasized a specificair–sea interaction with the horseshoe SSTAs extending
through the entire tropical Pacific. Diagnosing the output from GCM experiments, Lau and Nath (2000) suggested another physical mechanism based on a Rossbytype atmospheric response to the SSTAs in the tropicalwestern Pacific.
Using observations for almost 100 yr, Krishnan andSugi (2001) statistically confirmed a year-to-year outof-phase relationship with Indian rainfall, that is, theIndian summer monsoon. To give a physical explanation
to the negative correlation, they proposed two dominantroutes of teleconnection: one is the southern oceanicroute from the Indian subcontinent to the western Pacificand the other is the midlatitude continental route overthe Eurasian continent. Yet, further verification is needed to understand the robust physical mechanisms whereby the baiu phenomenon is related to the ENSO and to
the interannual variations of the Asian summer monsoon. In particular, the physical linkage to the biennial
oscillation (BO) dominating the Asian–Australian mon-4255

soon (Meehl 1987; Tomita and Yasunari 1996; Li and
Zhang 2002) is still unclear.
This study first reveals the interannual variations of
the baiu front, particularly, focusing on the meridional
fluctuation in the western North Pacific. Then, the influence of large-scale circulations on the interannual
variability is examined with specific emphasis on theeffects from the Tropics. The physical mechanisms are
discussed with the interannual ENSO and with the tropical BO of the Asian–Australian monsoon. Since theamplitude tends to reach its maximum in boreal winterof both the BO and ENSO, the physical processes are
examined from northern winter to early summer. The
findings derived from this study will contribute to raising the predictability of the baiu phenomenon and enhance the physical understanding of interannual variability prevailing in the early summer climate in thewestern North Pacific.
In the remaining sections of this paper, section 2 introduces the employed datasets and analysis procedures.
Section 3 exhibits the dominant meridional fluctuationsof the baiu front on interannual time scales. The concurrent large-scale circulations and the time development from the preceding winter to early summer are
examined in section 4. Section 5 gives a summary andfurther discussion. Note that seasons in the text correspond to those in the Northern Hemisphere unless otherwise noted.
2. Data and analysis procedures
In this work, the following three datasets were used:
1) global precipitation estimated in the Climate Prediction Center (CPC) Merged Analysis of Precipitation(CMAP; Xie and Arkin 1996, 1997); 2) atmosphericparameters assimilated in the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis (Kalnay et al.
1996); and 3) observational global ice and SST (GISST)
compiled at the Met Office (Rayner et al. 1996). The
SST data without sufficient observations were reconstructed by a linear sum of covariance empirical orthogonal functions (EOFs) weighted by the correspondingly adjusted time coefficients (Parker et al. 1995). The
three datasets had no missing data, and the data were
all arrayed on grids covering the entire globe at 1.08 or
2.58 intervals. Since our strategy was to exhibit significant variations on interannual time scales, time resolution in months was employed for the periods of 1979–
2000 (22 yr) for CMAP and 1979–99 (21 yr) for GISST
and NCEP–NCAR reanalysis. In order to examine thephase relationship with the interannual ENSO and Indian summer monsoon variations, we further employed
the monthly Southern Oscillation index (SOI) definedby the sea level pressure difference between Tahiti andDarwin (Tahiti 2 Darwin) and the monthly all-India
rainfall updated by the Indian Institute of Tropical Meteorology (Parthasarathy et al. 1995).

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FIG. 1. Long-term (1979–2000) Jun–Jul precipitation mean. The contour interval is 3 mm day 21 , andregions with values larger than 3 (6) mm day 21 are lightly (darkly) shaded.

In order to extract dominant spatial patterns that may
be involved in a spatiotemporal dataset, we applied EOF
analysis for the spatial covariance matrix estimated in
the time direction. In addition, the periodicity of the
EOF time coefficients was examined using power spectral analysis. The composite and lag-correlation techniques based on the EOF time coefficients are usable
to demonstrate the spatial patterns reflecting each of the
EOF modes on any anomaly fields. The significance of
the anomalies and correlations was statistically estimated by the two-tailed Student’s t test.
3. Meridional variation of the baiu front oninterannual time scales

FIG. 2. Meridional section at 1308–1408E of the Jun–Jul precipitation mean: (a) the long-term (1979–2000) mean and (b) the realinterannual variation for 1979–2000. The contour interval is 1 mm
day 21 in (b), and regions with values larger than 6 (8) mm day 21 arelightly (darkly) shaded.

The baiu precipitation near Japan is distinguishedfrom the main body of the Asian monsoon at a glance(Fig. 1). The humidity advection from the south encounters the cold advection from the north in the midlatitudes near Japan and results in large baroclinicity
there, that is, the baiu front (Ninomiya 2000). It is,therefore, conceivable that the meridional fluctuation of
the baiu front sensitively reflects the large-scale meridional circulations in the western Pacific. As representative longitudes we set 1308–1408E.
The interannual variation of the baiu front is identified
not only in strength but also in spatial location. Figure
2a illustrates the long-term (1979–2000) mean of themeridional rainfall distribution averaged during the baiu
season (June and July) in the 1308–1408E longitude
strip. The baiu front is climatologically located at 308–
358N near Japan, as indicated by the local maximum(Figs. 1 and 2a). The interannual fluctuation is, however,large in both amplitude and meridional distribution, as
shown in Fig. 2b. For instance, during the last two decades, the baiu has experienced its largest and smallest
precipitation in 1993 and 1994, respectively, within ayear. Moreover, the baiu front extended far south in 1982
and 1983. It is ascertained from Fig. 2b that the fluc-1 NOVEMBER 2004

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FIG. 3. (a) First EOF interannual fluctuation of Jun–Jul precipitation
mean averaged in the 1308–1408E longitude strip (Fig. 2b) and (b)time coefficients. The time coefficients are normalized, while thestandard deviation is imposed on the EOF. (c) Power spectral density(PSD) of the normalized time coefficients (solid line) with 95% significance (dotted line) based on the red noise spectrum estimatedfrom a first-order autoregression.

tuations occurred quasi biennially, which further suggests that the variability is synchronized with the biennial modulation of the Asian–Australian monsoon (cf.
Meehl 1987; Tomita and Yasunari 1996; Li and Zhang
2002). At latitudes south of 308N, the biennial fluctuation seems to be less dominant than the interannual
variation with a longer period of 5 or 6 yr.
In order to extract the dominant interannual variations
that are indicated in Fig. 2b, we applied the EOF analysis to a time–latitude dataset. Figures 3 and 4 exhibit
the first and second EOFs, respectively, and the corresponding normalized time coefficients, where the standard deviations are imposed on the respective EOFs.
The contribution rates to the total variance of the firsttwo EOF modes are 37.8% and 32.5%, respectively, andtotal over 70%, while that of the third EOF mode is
17.8%. Since a propagating feature is usually explainedby two EOF modes with almost equal contribution rates
but with a quadrature phase difference in time, we additionally inspected the periodicity of the time coefficients (Figs. 3c and 4c). A power spectrum analysis
evidently demonstrates that the predominant periods are

FIG. 4. As in Fig. 3 but for the second EOF.

different in the two EOF modes, indicating that not only
simultaneous but also lagged correlations are low between the two time coefficients. This assures that thetwo EOF modes are not a pair expressing a propagating
variation. The almost equal contribution rates and thedifferent periodicity suggest that there are two distinct
controlling mechanisms for the interannual meridionalvariation of the baiu front. The present work sheds lighton the mechanisms associated with the two leading EOF
modes.
The first EOF mode (Fig. 3) manifests a standing
oscillation to the south of 358N with a period of 5 or 6
yr. In the spatial pattern (Fig. 3a), a positive maximum
appears in the 258–328N latitudinal band to the southof 358N. The temporal variation (Fig. 3b) indicates thedominance of a 5- or 6-yr period (Fig. 3c), and the yearsof positive maximum are well synchronized with warm
ENSO events, that is, 1982–83, 1987–88, 1992–93, and
1997–98. In fact, the simultaneous correlation of timecoefficients with the SOI is 20.47, significant at the 5%
level (0.37). In other words, the baiu precipitation tends
to be large in early summers after the warm ENSO
events, as Zhang et al. (1996) and Tanaka (1997) havepointed out. Contrary to the findings of Krishnan andSugi (2001), however, the concurrent correlation withthe all-India rainfall is 20.19, insignificant at the 5%
level.

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JOURNAL OF CLIMATETABLE 1. List of composite years (1900 is subtracted).

LF (1)
LF (2)BO (1)BO (2)

82
79
80
79

83
80
82
81

87
81
83
84

88
84
85
86

89
85
88
87

93
86
91
89

97
90
93
90

98
91
95
92

99
92 94 95 96
97 98
94 96 99

The second EOF mode (Fig. 4) specifies a seesawlike
variation with a node at around 288N fluctuating quasi
biennially. The period is definitely shorter than the 5or 6-yr period of the first EOF mode (cf. Fig. 4c with
Fig. 3c). The quasi-biennial fluctuation represents a variation at 308–358N, shown in Fig. 2b. Hereafter, the first
EOF mode is referred to as the lower-frequency (LF)
mode, while the second EOF mode is referred to as thebiennial oscillation (BO) mode. The original time variation is well explained by the spatiotemporal phase of
the LF and BO modes, as identified in Fig. 2b. For
instance, the largest baiu event of 1993 corresponded
to the largest positive maximum of the BO mode (Fig.
4b) and to the moderate positive maximum of the LF
mode (Fig. 3b). In contrast, the phase was opposite in1999, when the largest positive maximum appeared in
the LF mode (Figs. 3b and 4b). In that year, the baiu
front largely expanded southward without a node around288N (Fig. 2b). Unlike those of the LF mode, the timecoefficients of the BO mode are not significantly correlated with either the SOI (20.09) or the all-India rainfall (0.13). Correlations with the ENSO or with the Indian summer monsoon will be considered in greater
detail in the following sections, which examine the accompanying spatial patterns in anomaly fields.

VOLUME 17

4. Large-scale circulations associated with the LF
and BO modes
Spatial anomaly distributions of atmospheric parameters were examined to determine significant differences
between the interannual LF and BO modes. This section
particularly focuses on the large-scale circulations surrounding the baiu front, ENSO, and the Asian summer
monsoon. To exhibit the differences clearly, we employed composite and lag-correlation techniques basedon the EOF time coefficients for each mode (Figs. 3band 4b). The composite years are chosen by signs listed
in Table 1. The anomalies are defined by subtracting themean of the negative years from that of the positive
ones, and the significance of the anomaly is tested byapplying the two-tailed Student’s t test to the differenceof two sample means. The lag correlations are also tested
by a similar t statistic.
a. Circulation fields in the baiu season
Precipitation in the Tropics, which is also a proxy
indicator of latent heating, can expose an anomalous
convection. Figure 5 shows the anomaly fields when thebaiu rainfall is large near Japan, as associated with the
LF and BO modes. In both the LF and BO fields, distinct
positive anomalies extend from the East China Sea tothe east zonally. Parallel positive and negative anomalies appearing in the tropical central–eastern North Pacific are also common in the two fields, although theanomalies are somewhat smaller in the BO field (Fig.
5b). The parallel pattern reflects the intertropical convergence zone that anomalously shifts equatorward,

FIG. 5. Composite precipitation anomalies based on the first (LF) and second (BO) EOF time coefficients (Table 1) (a) for the LF modeand (b) for the BO mode. The contour interval is 0.75 mm day 21 . Dotted lines indicate negative contours, while zero lines are omitted forclarity. Regions being positively (negatively) significant at 5% are darkly (lightly) shaded.

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FIG. 6. Anomaly vectors of vertically integrated water vapor flux based on the (a) LF and (b) BO EOF time coefficients (Table 1). The
reference vector of the two is exhibited in common near the bottom of (b). Contours indicate the size of the vector at an interval of 25 kg
m 21 s 21 ; regions larger than 50 kg m 21 s 21 are shaded.

which is typical when an ENSO event appears (e.g.,
Wallace et al. 1998).
In the western North Pacific, the Pacific–Japan (PJ)like pattern (Nitta 1987, 1989) is discernible in the two
fields, that is, an anomalously weak convection aroundthe Philippines and a strong one to the north that resultsin larger-than-normal baiu precipitation near Japan. In
the BO mode (Fig. 5b), the meridional contrast in anomaly well reflects the seesawlike fluctuation of the baiu
front (Fig. 4a), whereas the anomalous southward expansion of the baiu front (Fig. 3a) is certainly notable
near Japan in the LF mode (Fig. 5a). In addition, we
can identify distinct spatial differences in the large-scale
zonal distribution of negative anomalies around the Philippines. The negative anomalies extend westward fromthe South China Sea (SCS) to the Indian subcontinent
in the LF field (Fig. 5a) but eastward from the SCS to
the TWNP in the BO field (Fig. 5b). The zonal differences of the anomalous convection along 208N, that is,east or west of 1208E, correspond to the predominantmode. Therefore, the BO mode is negatively correlatedwith the TWNP monsoon, the LF mode, and with thesouth-southeast Asian (SSEA) monsoon, although there
are some areas with insignificant negative anomalies in
the Indian subcontinent. Such differences favorably adjust the anomalous distribution of the baiu front throughthe modification of meridional circulations in the western North Pacific, which will be further examined in alater subsection. Similar results can also be derived byexamining the outgoing longwave radiation instead of
the precipitation (not shown).

The vertically integrated water vapor flux in the troposphere is a useful parameter to trace the moisture
following large-scale circulations. In order to determine
the relative differences between the LF and BO modes,
we examined the vectors defined as follows:
Q[

1
g

E

1000 hPavq dp,

300 hPa

where g is the acceleration of gravity, v is a horizontal
wind vector, and q is specific humidity that is quite small
and negligible above 300 hPa.
Figure 6 illustrates the distributions of anomalous Q
when the baiu precipitation is large near Japan, as associated with the LF and BO modes. When the LF modeis dominant (Fig. 6a), the region with large eastward Q( | Q | . 50 kg m 21 s 21 ) extends from the East China
Sea to the central North Pacific zonally. A large-scale
cyclonic circulation with its center located in the southof Japan is superimposed on the western part of theregion. The cyclonic circulation with a southward meander corresponds to the anomalous southward expansion of the baiu front that characterizes the LF mode
(Figs. 3a and 5a). Additionally, two other large Q regions are observed in the Indian Ocean and the equatorial central Pacific. The latter, with an easterly direction, supports the idea that the LF mode is closely related
to ENSO, while the former, with a westerly direction,
indicates that the SSEA monsoon is weak when the LF
mode is prominent.
The region with large eastward Q is more zonal near

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FIG. 7. Composite anomalies of the velocity potential at 200 hPa based on the (a) LF and (b) BO EOF time coefficients (Table 1). The
positive anomalies are set to anomalous convergence. The contour interval is 3 3 10 5 m 2 s 21 , and the dotted lines indicate negative values.
Significance is tested using the Student’s t test, and regions with dark (light) shading are positively (negatively) significant at 5%.

Japan when the BO mode is predominant, which again
extends from the eastern part of the Asian continent tothe central North Pacific (Fig. 6b). The anomalous baiu
precipitation characterizing the BO mode (Figs. 4a and5b) is attributed to the convergence of Q near Japanthat is estimated from vectors with opposite directions
straddling the 358N latitude. Furthermore, there is a region with large easterly Q in the TWNP south of 208N,
which implies stronger-than-normal trade winds. A
large-scale anticyclone is formed along 208N in the
TWNP between two large Q regions with opposite zonal
directions, where the precipitation anomalies are certainly negative (Fig. 5b). Besides the two large Q belts
aforementioned, regions with relatively large Q can be
identified in the equatorial central Pacific and to thenorth of Japan. The equatorial vectors are placed between 1608E and 1608W to the west of the region shownin the LF mode (Fig. 6a), about 308 longitude. The fourlarge Q belts extending northward in the western North
Pacific with alternate zonal directions are reminiscentof the PJ pattern found by Nitta (1987, 1989).
When the LF mode is dominant, large Q regions appear broadly in the Pacific–Indian Ocean sector (Fig.
6a), whereas four large Q belts are confined to narrower
longitudes in the central–western North Pacific whenthe BO mode prevails (Fig. 6b). The LF mode is characterized by a global structure consistent with the warm
ENSO event and with the weak SSEA monsoon, while
the structure of the BO mode is less global and is locallysynchronized with the TWNP monsoon.
In order to further identify the large-scale circulations
associated with each of the LF and BO modes, we examined the velocity potential at the 200-hPa level (w 200 )using similar composite analysis (Fig. 7). Note that thepositive w 200 corresponds to convergence, as defined by=w 5 (u, y), where u and y are the zonal and meridional
components of horizontal winds, respectively. As acommon feature, we identify a large positive/negative
difference between the Indonesian Maritime Continentand the tropical eastern Pacific, implying a weaker-thannormal Walker circulation. The positive anomalies
around the Maritime Continent correspond well to thenegative anomalies in precipitation (Fig. 5). In general,
the Asian summer monsoon is weak when either modeis largely positive.
Differences between the two modes have also been
identified. When the LF mode is dominant (Fig. 7a), thepositive anomalies extend broadly in the Asian monsoon
region where the maximum axis of the positive anomalies inclines from the northwest to the southeast. At
longitudes between 1208 and 1608E, the local maximum
and minimum appear around the equator and near Japan,
respectively, reflecting the weaker-than-normal Hadleycirculation in the western North Pacific. The locallyweak Pacific high allows the baiu front to expand southward and bring more precipitation near Japan (Figs. 3aand 5a).
On the other hand, when the BO mode is prominent(Fig. 7b), the region with positive anomalies is confinedto the western Pacific where the axis of the positive
maximum is meridional and the significant signals areto the east of 1208E. At longitudes of 1208–1608E, thefour local maxima/minima are identified at around 58S,
208N, and 358N near Japan and at 508N, where the positive maximum along 208N is the most noteworthy. The
structure of the BO mode, which differs from the larger

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FIG. 8. Latitude–height sections of vertical p-velocity anomalies (Pa s 21 ) averaged in the 1308–1408E longitude strip in
June and July associated with the (a) LF and (b) BO modes (Table 1). The contour interval is 0.003 Pa s 21 , and the dottedlines indicate negative (upward) values. Significance is tested using the Student’s t test, and regions with dark (light) shadingare positively (negatively) significant at 5%.

one-cell structure of the LF mode, consists of three cellsof anomalous meridional circulation in the western Pacific. The local minimum around Japan and the maximum along 208N correspond to a southward shift of the
Pacific high in the western North Pacific and reflect a
seesawlike fluctuation of the baiu precipitation in the
BO mode (Figs. 4a and 5b). The spatial scale of thecirculations is smaller in both zonal and meridional directions in the BO mode than in the LF mode.
In order to specify the meridional modulation of the

Hadley circulation and the anomalous zonal circulations
associated with the LF and BO modes, which reflect the
deformation of the subtropical Pacific high, we examined a latitude–height section of the anomalies of thevertical p velocity in the TWNP (1308–1408E) (Fig. 8).
Then, we investigated the longitude–height section
along 208N (158–258N) (Fig. 9) where zonal differences
are significant in a precipitation field (Fig. 5).
When the LF mode is dominant (Fig. 8a), anomalous
upward flows are significant in 208–358N where the baiu

FIG. 9. As in Fig. 8 but averaged in the 158–258N latitude strip.

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front expands southward anomalously (Figs. 3a and 5a).
This indicates a relatively weak Pacific high in the western part. An ascent is also detectable about 58S of theequator, while a compensative descent appears in theregion between the equator and 158N, implying that theHadley circulation is anomalously modified in the
TWNP. As suggested earlier, the LF mode is characterized by a large-scale anomalous meridional circulation
centered at around 208N between 58 and 358N. Thereis also a significant descent in the lower troposphere to
the north at the narrow 358–408N latitudes.
When the BO mode is prominent (Fig. 8b), an anomalous descent at around 208N and an ascent at 358N
complementarily adjust the baiu front northward (Figs.
4a and 5b). The Pacific high is strong (weak) in thesouthern (northern) part of the TWNP in conjunction
with the BO mode dominance. The latitudes of the ascent and descent correspond to those of the positive andnegative anomalies in precipitation, respectively (Fig.
5b), and reflect the seesawlike fluctuation of the baiu
front with a node at around 288N (Fig. 4a). The meridional circulation characterizing the BO mode, that is,
the anomalous ascent and descent between 158 and
358N, is located about 58 to the north of those in the
LF mode. Furthermore, the distance between the descending and ascending axes, that is, 208 and 358N (Fig.
8b), is narrower than that between the 108 and 308N of
the LF mode (Fig. 8a), indicating that the scale of anomalous meridional circulation is smaller in the BO mode
than in the LF mode, as shown in Fig. 7. The ascentnear 58N (Fig. 8b) does not seem to reflect the positive
precipitation anomalies (Fig. 5b) because of the weak
convection with a height lower than 600 hPa. Between
the equator and 408N, the branches of the anomalous
ascent and descent in the BO mode are placed in thequadrature phases of those of the LF mode. The differences in meridional circulations are essential to themeridional modulation of the baiu front.
In order to investigate the zonal differences along
208N shown in Figs. 5, 6, and 7, we examined the anomalous zonal circulations associated with each mode,
which further diagnosed details of the relationship with
the Asian summer monsoon (Fig. 9). As found in Fig.
8, significant differences are identified in the anomaliesof the vertical p velocity at 1308–1408E; that is, it is
upward in the LF mode (Fig. 9a) but downward in the
BO mode (Fig. 9b). Moreover, we observed a significant
anomalous descent around 1008E in the LF mode (Fig.
9a), indicating that the SSEA monsoon is weak when
the baiu activity is strong with this mode (see also Fig.
5a). The negative correlation between the baiu activity
and the SSEA monsoon is consistent with the findingsof Tanaka (1997). On the other hand, when the baiu
precipitation is large with the BO mode (Fig. 9b), a
significant descent appears to the east of 1208E in thewestern North Pacific, reflecting the weak TWNP monsoon (see also Fig. 5b). The broad expansion of thepositive anomalies to the east of 1208E corresponds well

VOLUME 17

to the negative anomalies in precipitation (Fig. 5b), indicating that the Pacific high is relatively strong in thesouthern or southwestern part. The correlation between
the Asian monsoon and the baiu precipitation near Japanis locally modified according to the dominant mode, thatis, the SSEA monsoon with the LF mode and the TWNP
monsoon with the BO mode.
b. Development of the LF and BO modes from winterTomita and Yasunari (1993) pointed out that the El
Niño events are classified into two categories according
to duration: a single winter BO event and a two-consecutive-year LF event. The significant differences of
the two appear in the low-level atmospheric circulation
off the Philippines, especially in the recessive stage fromwinter to spring. However, these researchers did not
examine the possibly large influence in the following
baiu season. Using the lag-correlation technique basedon the EOF time coefficients (Figs. 3b and 4b), thissubsection examines the physical process from winterto summer that causes the development of the two interannual modes of the baiu front. The significance ofthe lag-correlations is judged through the two-tailed Student’s t test.
Figure 10 shows the lag correlations in precipitation
within the 2-month mean fields aligned from winter toearly summer. In the winter preceding the LF modedevelopment (Fig. 10a), weak positive correlations,which approach Japan when strengthened by the following seasons (Figs. 10c and 10e), are placed to thesoutheast of Japan with an axis from the southwest tonortheast. Concurrently, a pair of regions with significant positive and negative correlations moves westward
in the tropical Pacific. The resulting spatial pattern isshown in Fig. 5a. The development of the LF mode
seems to be synchronized with the retreat of the ENSO
event, as can be demonstrated by two facts: First, thecorrelation pattern in the tropical Pacific, that is, thepositive correlations in the equatorial Pacific and thenegative ones to the north, is typical when the ENSO
events occur. Second, the pair of anomalies moves westward from winter to summer slowly (Lau and Sheu
1988; Tomita 2000). It should be noted that, in the progression, no significant correlations appear along 208N
in the TWNP.
On the other hand, in seasons proceeding to the BO
mode development (Figs. 10b,d,f ), regions with significant negative correlations tend to appear around the
TWNP through Southeast Asia, from where they move
westward along the 208N latitude during the seasons.
Since no significant correlations appear in the tropicaleastern Pacific, this clearly implies that the BO modeis less related to the ENSO than the LF mode. In atheoretical study using a simple five-box model, Changand Li (2000) pointed out that the BO of the Asian–Australian monsoon is driven by the anomalous convection around the Indonesian Maritime Continent and1 NOVEMBER 2004

TOMITA ET AL.

4263

FIG. 10. Lag-correlations of precipitation estimated with the EOF time coefficients for the (left) LF and (right) BO modes. The letters JF,
MA, MJ, and JJ indicate Jan–Feb, Mar–Apr, May–Jun, and Jun–Jul. The contour interval is 0.2, and the dotted lines indicate negative
correlations. Significance is tested using the two-tailed Student’s t test, and the darkly (lightly) shaded areas indicate positively (negatively)
significant correlations at the 5% level (60.37). For brevity, the zero lines are omitted.

that the signals are weak in the tropical eastern Pacific.
The findings of this study are consistent with their results. As shown in Figs. 5a and 5b, the meridional distribution of positive/negative correlations in the TWNP
is different between the two modes in early summer(Figs. 10e and 10f ). In other words, positive correlations extend farther south of Japan in the LF mode (Fig.
10e), while they are confined over Japan in the BO mode
(Fig. 10f ).
In order to determine the surface thermal conditions
associated with the two baiu modes, we examined a SST
field again using a similar lag-correlation technique(Fig. 11). As expected, a pronounced horseshoe pattern
was identified in the entire tropical Pacific prior to the
LF mode development (Figs. 11a,c,e), although the easternmost correlations expanded somewhat off the equator. The horseshoe pattern consists of positive correlations to the south of Japan and in the tropical eastern
Pacific and of negative ones in between. The positive
correlations to the south of Japan remain significant
throughout the seasons from winter to early summer(Figs. 11a,c,e). The negative correlations in the central
equatorial Pacific move westward about 108 longitude
from winter to spring (Figs. 11a and 11c) as the positive
ones expand westward in the equatorial eastern Pacific,
consistent with the westward movement identified in the
precipitation field (Figs. 10a and 10c). Significant negative correlations can also be found in the eastern North

Pacific off the western coast of North America, which
may be considered as a response to the atmosphericPacific–North American pattern prevailing when the
ENSO event occurs (Wallace and Gutzler 1981). In general, the correlations extending over the entire Pacificbecome weak as the seasons progress.
It is of interest that a horseshoe pattern is also identified in the seasons prior to the BO mode development(Figs. 11b,d,f ). The pattern is, however, not well organized in the tropical eastern Pacific but advances tothe tropical central–western Pacific. In addition, the spatial phase is clearly different from that of the LF mode.
In other words, in the tropical Pacific, the negative andpositive correlations in the BO mode (Figs. 11b,d,f )extend in regions near the zero lines in the LF mode
(Figs. 11a,c,e), suggesting a quadrature phase differencein space. In the BO mode development (Figs. 11b,d,f ),the significant negative correlations move westward in
the TWNP off the Philippines without distinct positive
ones near Japan. Furthermore, the finding that there areno significant correlations in the equatorial eastern Pacific again supports the idea that the BO mode is lessrelated to the ENSO. The differences between the LF
and BO modes are attributed to the spatial phase ofhorseshoe SSTAs in the tropical Pacific.
Wang et al. (2000) suggested that the ENSO can affectthe following summer monsoon in East Asia through aspecific teleconnection with horseshoe SSTAs in the4264

JOURNAL OF CLIMATEVOLUME 17

FIG. 11. As in Fig. 10 but for lag correlations of SST with thick zero lines.

tropical Pacific. The present study further points out thatthe BO is also related to the Asian summer monsoon
through the horseshoe SSTAs but with a different spatial
phase of about 908. It is conceivable that the differencesin the spatial phase modify the anomalous meridional
circulations through the SSTAs in the TWNP, particularly off the Philippines, since the mean SST in the rangeof 288–298C is sensitive to the development of organization of the convection (Graham and Barnett 1987;
Lau et al. 1997). In fact, the negative and positive
SSTAs in the TWNP correspond well to the descent ataround 208N in the BO mode and the ascent at 208–
358N in the LF mode (Fig. 8). Modified meridional circulations in turn control the anomalous meridional expansion of the baiu front that characterizes the LF and
BO modes. The SSTAs off the Philippines may also
control the zonal contrast of Asian summer monsoons
through anomalous zonal circulation, that is, the correlations between the two baiu modes and the SSEA orTWNP monsoons, although, in the BO mode, the anomalous dipole straddling 1208E along the 208N latitude
is not significant in the vertical p-velocity field (Fig. 9).
5. Summary
The baiu (mei-yu or changma) phenomenon is one ofthe dominant features characterizing early summer climate in the western North Pacific. The baiu front, which
advances northeastward from the southeastern foot of
the Tibetan Plateau to the central North Pacific along
the western periphery of the subtropical Pacific high(Fig. 1), demonstrates distinct interannual variability
that significantly affects the water resources in East
Asia. Focusing on the meridional fluctuation in the western North Pacific (1308–1408E), this study has identified
the interannual variability and examined the relevant
large-scale circulations.
The datasets mainly used in this study are the following three: 1) the GISST compiled at the Met Office,2) precipitation estimated in the CMAP, and 3) atmospheric parameters assimilated in the NCEP–NCAR reanalysis. These are all gridded datasets covering theentire globe in monthly time resolution for the periodsof 1979–2000 (CMAP) or 1979–99 (GISST and NCEP–
NCAR reanalysis). In addition, the monthly time series
of SOI and all-India rainfall are employed to examine
the relationship with the ENSO and with the Indiansummer monsoon. The analysis techniques used include
the EOF, composite, lag correlation, and power spectral
analyses, the latter three being based on time coefficients
derived from the former EOF analysis. The significanceof the anomalies and the correlations is estimatedthrough the two-tailed Student’s t test.
The EOF analysis detects the spatiotemporal structureof the dominant interannual variability that is involvedin the time–latitude cross section of precipitation in thewestern North Pacific (1308–1408E) (Fig. 2). The first
EOF mode represents a standing variation to the southof 358N with a period of 5 or 6 yr (LF mode). The
periodicity implies that the LF mode is synchronized to
the ENSO cycle; that is, the baiu precipitation tends to1 NOVEMBER 2004

TOMITA ET AL.

FIG. 12. Schematic diagrams illustrating the anomaly fields in earlysummer associated with the (a) LF and (b) BO modes. Regions withpositive (negative) precipitation anomalies in the Indian Ocean–western Pacific sector are hatched (stippled) (Figs. 5 and 10), while vectors
designate the location and direction of relatively large water vapor
flux (Fig. 6). Circles with a dot (a cross) indicate the local centersof anomalous ascent (descent) (Figs. 8 and 9). SSTAs in the tropical
Pacific are divided by thick lines with 6 signs (Fig. 11).

be large after warm ENSO events. On the other hand,the second EOF mode demonstrates a seesawlike variation with a node at around 288N fluctuating quasi biennially (BO mode). Since the contribution rates of thetwo EOF modes total over 70%, the composite of twomodes can explain well the entire interannual variationin the meridional fluctuation of the baiu front in thewestern North Pacific. Although this study examines thelarge-scale circulations associated with the two EOF
modes separately, it is, of course, necessary to compile
the results to draw a more realistic picture of interannual
fluctuation.
The anomalous large-scale circulations in early summer associated with the LF and BO modes are schematically summarized in Fig. 12. When the LF modeis dominant (Fig. 12a), the positive precipitation anomalies appearing along the baiu front expand southward,
while the negative ones are evident in the SSEA monsoon region. The anomalies of the water vapor flux display a consistent zonal flow pattern from the East China
Sea to the central North Pacific, with a southward meander that is part of a large-scale eddy to the south ofJapan. The flux anomalies are also large in the equatorial
central Pacific and the Indian Ocean, indicating that the
LF mode is linked to the ENSO. The weaker-than-normal Walker circulation, which is estimated from the fieldof the velocity potential at the 200-hPa level (w 200 ) (Fig.

42657), also supports the linkage with the ENSO. The negative precipitation and positive w 200 anomalies around
Indochina reflect the weaker-than-normal SSEA monsoon. When the LF mode is prevailing, the correlationis negative between the baiu precipitation and the SSEA
monsoon. The meridional circulations characterizing the
LF mode comprise branches of descent at around 108N,
ascent at 208–308N, and descent to the north in the western North Pacific. The anomalous zonal circulation
along 208N, centered at 1208E, is also significant in theregion of the Asian summer monsoon (Fig. 9).
On the other hand, when the BO mode is dominant(Fig. 12b), the meridional contrast is significant in precipitation between the regions near Japan and off thePhilippines. The baiu precipitation is negatively correlated with the TWNP monsoon but not with the SSEA.
In the anomaly field of the water vapor flux, the BO
mode is distinguished by four banded layers from theequator to approximately 508N in the western North
Pacific (four vectors in Fig. 12b). Since the longitudes
are confined to the western North Pacific west of 1608W,
the tropical BO is more localized than the conventional
ENSO and may be recognized as an aborted ENSO in
the tropical central–western Pacific. The w 200 anomalies
associated with the BO mode (Fig. 7b) also indicate
four local maxima/minima in the western Pacific. The
branches of meridional circulations are arrayed in theorder of ascent at around 58N, descent at 208N, andascent at 358N from the south in the western North
Pacific and are in a quadrature phase relationship withthose in the LF mode. The anomalies of vertical p velocity clearly reflect the meridional seesawlike fluctuation of the baiu front that characterizes the BO mode.
Although Nitta (1987, 1989) pointed out that the PJ
pattern appeared in the western North Pacific in conjunction with the interannual variation of the baiu phenomenon, the present work further reveals that the zonal
and meridional circulations are definitely different between the LF and BO modes. The LF mode has significant correlation with the ENSO and with the SSEA
monsoon, and the BO mode with the TWNP monsoon.
From the preceding winter, the development of the LF
mode indicates the close linkage with the ENSO throughthe horseshoe SSTAs extending in the entire tropical
Pacific. The anomalous ascent (descent) corresponds
well to the positive (negative) SSTAs in the western
North Pacific, which leads to the meridional modification of the baiu front that characterizes the LF mode.
Although the development of the BO mode is also related to similar horseshoe SSTAs, the regions with significant SSTAs are confined to the tropical western Pacific and exhibit a quadrature phase difference in bothzonal and meridional directions. Accordingly, the meridional circulations in the western North Pacific are ina quadrature phase relationship with those of the LF
mode. In particular, there is a distinct difference along
208N latitude. The meridional distribution of the SSTAs
in the TWNP, which is attributed to the spatial phase of4266

JOURNAL OF CLIMATEthe anomalous horseshoe pattern in the tropical Pacific,
affects the distribution of the baiu front through anomalous meridional circulations. The spatial phase of
SSTAs may also have a zonal effect on the strength of
the Asian summer monsoon between the SSEA and the
TWNP. Further study is needed to examine the processesof the interannual variability of the Asian summer monsoon and verify the effects of the ENSO and the tropical
BO on the processes. Approaching how horseshoe
SSTAs modify the ENSO/monsoon system would be
crucial to this end.
Acknowledgments. The authors would like to thanktwo anonymous reviewers for their constructive comments, which were very helpful in improving the manuscript. They are also grateful to Drs. Bin Wang and
Tim Li for valuable information, comments, and suggestions that enhanced the discussion. This study was
supported by the Frontier Research Center for Global
Change, Japan Agency for Marine-Earth Science andTechnology, and by a Grant-in-Aid for Scientific Research (14740283) by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan.
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