April 1990

N. Hirasawa

Variation
Western

and T. Yasunariin theAtmosphericPacific

Associatedwith

IndianSummerof theBy Naohiko

129

Circulation

Hirasawal*

and

the

over
40-day

Asia

and

the

Oscillation

MonsoonTetsuzo

Yasunari

Institute of Geoscience, University of Tsukuba, Tsukuba 305, Japan(Manuscript received 23 June 1989, in revised form 15 December 1989)

Abstract

Through use of the outgoing longwave radiation (OLR) and 700 mb height fields for 1979, an
investigation was conducted of the variation of the atmospheric circulation over Asia and the western
Pacific associated with the 30-60 day variation of the Indian summer monsoon.
Results of the analysis of the OLR indicate that convection over the northwestern Pacific along
15*N is active slightly before the active phase of the Indian monsoon over central India, while there
is a decrease in convective activity prior to the break phase. This active convection (between 140°E
and 150*E) propagates southward with a period of 40 days from 15*N to the equatorial zone. The
cloudiness to the north of the Tibetan Plateau (around 55*N, 75*E) and over the subtropical highregion (southeast of Japan) reaches a minimum slightly before the active phase of the Indian monsoon.
Conversely,cloudiness reaches amaximum prior to the break phase of the Indian monsoon.
From the results of the analysis of the 700 mb geopotential heights, it was found that to the northof the Tibetan Plateau the maximum cloudiness corresponds to the stagnation of the trough while theminimum cloudiness is associated with the development of the ridge. The position of the subtropicalhigh shifts northward (along 25-30*N) slightly before the active phase of the Indian monsoon andshifts southward (along 10-20*N) slightly prior to the break phase. The phase of the 30-60 day
variation to the north of the Tibetan Plateau precedes that of the subtropical high region.
These results suggest a close association among the 30-60 day variation of the Indian monsoon, theconvection over the western Pacific, the westerly-wave movements over the Asian continent and thesubtropical high in the western Pacific.
These large scale interactions affect the weather regime around Japan during the Baiu season.
The meridional position of the Baiu front (east of 130*E) around Japan shifts from 40°N, when thesubtropical high shifts northward, to 30*N when the subtropical high shifts southward. This is due tothe variation of the meridional position of the subtropical high, which is associated with the 40 day
variation of the monsoon. The meridional position of the Mei-yu front (west of 130*E), in contrast,
does not exhibit a cyclical variation.

1. Introduction
Many studies have revealed that the 30-60 day oscillation is a global-scalephenomenon, the structureof which is an eastward propagating wave of zonal
wavenumber 1 (Madden and Julian, 1971, 1972;M.
Murakami,1984; Lorenc,1984; Knutson et al., 1986;
Krishnamurti et al., 1985 among others). Yasunari(1979, 1980, 1981) and Sikka and Gadgil (1980)
found that cloud bands propagated northward, overthe Indian monsoon region from the equatorial zone
to the foothills of Himalayas, with a 30-60 day period and that this propagation was associated with
11) Water Research Institute , Nagoya University, Chikusaku, Nagoya 464, Japan
1990, MeteorologicalcSociety of Japan

active and inactive cycles of the Indian monsoon. In
the present study the active phase of the monsoon isdefined as that which occurs when the phase of the
maximum cloudiness is over northern central India(about 20*N). Conversely,the break phase is defined
as when the phase of the minimum cloudiness is overthis region. These definitions approximately coincide with those of Krishnamurti and Sabrahmanyam(1982), who made use of the 850mb zonal wind velocity over the Arabian Sea.
Although this fluctuation appears mainlyyin thetropics, some studies were also conducted on the interaction between low and middle latitudes in the
monsoon activity. Raman et al. (1980), Raman andRao (1981) and Tanaka (1983) pointed out that

130

Journalof the Meteorologicalthe blocking ridge over the Eurasian continent appeared before the break in the monsoon. Moreover,
M. Murakami (1984, 1985) showed that the anomalous wind (the 30-60 day component) appeared distinctly in the middle and high latitudes. With respect to these facts, Yasunari (1986) stressed thatthe meridional movement of the heat source aroundthe Indian monsoon region, as suggested theoretically by Hoskins and Karoly (1981) and Webster(1981, 1982), induces the 30-60 day oscillation in
the middle latitudes of the Northern Hemisphere.
On the other hand, Japanese meteolorogistshavelong been interested in the relationship between the
Indian monsoon and the Baiu front. Suda andAsakura (1955) pointed out the parallelism in thestarting date of the Indian monsoon and the Baiu.
Recently, some aspects of the relationships between tropical convection and the atmospheric circulation around Japan were investigated. Chen etal. (1988) showed that the convergencecenter of thedivergent component field of water vapor transport
in the lower layer migrates cyclically between theeast coast of China and the area south of Japan.
Lau and Chan (1986) investigated the variation in
the outgoing longwaveradiation (OLR) during thenorthern summer. It was revealed that the Mei-yu
front intensified and the subtropical high pressurearea over the western Pacific shifted northwestward
when convection became active over central India,
being associated with a 40-day variation. It is wellknown that the weather around Japan during the
Baiu season has a close relationship with the variation of the subtropical high (Nakanishi, 1972; Taoand Ding, 1981;Kato,1989 and others). These facts
suggest that the variation of the local weather systems over east Asia is affected by the global 30-60
day oscillation.
As has been shownabove, there are probably close
relationships between tropical convection over the
India-western Pacific and the atmospheric circulation in the middle and high latitudes. This paper
will discuss the relationships between the intraseasonal 30-60 day oscillation of the Indian monsoon
and variations in the atmospheric circulation over
the Asian continent and the northwestern Pacific byuse of the outgoing longwaveradiation, the 700 mb
geopotential heights and synoptic weather charts forthe summer of 1979. Special attention is paid to thevariation of synoptic weather regimes around Japan(mainly the Baiu) in association with that of thesubtropical high.
2. Data and analysis method
Use is made of the twice daily OLR data at grid
points separated by 2.5° latitude and 2.5* longitude,derived from the NOAApolar orbiting satellite data,from May to September 1979. These data were averaged to form daily data over the domain of 60-180*E

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and 10*S-70*N.Missing data were linearly interpolated in time.
In the tropics, low values of OLR can be regarded
as masking the existence of deep convection withhigh cloud top. Care must be taken when examining
extratropical OLR data due to low surface temperatures. However, since the analysis period for thisstudy corresponds to the northern summer season,
the low values of OLR may be taken as a measureof cloudiness with higher cloud tops, although they
may not always be convective clouds.
Use is made of the 700 mb geopotential height
data, which were based on the FGGE3b data set
produced by the European Center for Medium
Range Weather Forecasts (ECMWF). The data
were rearranged at the Department of Meteolorogy,
Florida State University,to correspond to daily values at global grid points of 11.25° latitude by 11.25°
longitude. The region used for the present analysis
extends from 56.25-180°E to 11.25*S-67.5*Nandcovers the period from May 1 through September
30, 1979.
The synoptic charts, produced at the Japan Meteorological Agency (JMA), were also used for determining the positions of cyclones and fronts. The
Monthly Bulletin of the Seasonal Weather Forecast
and the Monthly Report of Meteorological Satellite Center, which have been published by the JMA,
were additionally used.
In order to examine the spatial structure of theintraseasonal oscillation with the 30-60 day periodthat occurs over the Indian monsoon region througheast Asia, a band-pass filter (M. Murakami, 1979)
was applied to the OLR and geopotential heightdata. Figure 1 shows the frequency response of thisfilter, with half power at 30 and 60 days while fullpower occurs at about 42 days.
3. Fluctuations in the OLR field
3.1 Average and standard deviation
Figure 2a and b show the spatial distributions of
the time average and standard deviation of OLR, respectively,during the period from May 1 to September 30, 1979. In Fig. 2a the shaded areas of lower
values (less than 230 W/m2) in the low latitudes exist over the eastern and southern areas of the Bayof Bengal, the peninsula of Indochina, Borneo, thePhilippines and the equatorial zone between 140160*E.These areas correspond to the active convective areas.
In the middle latitudes, areas of lower values arepresent over the eastern Tibetan Plateau and eastern Siberia. The lower values over the Tibetan
Plateau may be regarded as clouds since the surface temperature over this area is fairly high. The
higher values are distributed over the area between
120*Eand 180*Ealong 25*N, which correspond to
relatively little cloudiness associated with the sub-

April 1990

N. Hirasawa

and T. Yasunari131

viation is also apparent along 30*N, from 110*Eto
160*E, i,e., near Japan.
Figure 3 shows the spatial distribution of the ratioof the standard deviation of 30-60 day period component to the total standard deviation. This ratio,
defined as R (30-60), is calculated as follows:

Fig. 1. Response of the band-pass filter (forthe case of the 30-60 day oscillation).

Fig. 2. Spatial distributions of (a) average and(b) standard deviation of the OLR for May
1 to September 30 during 1979. (a) Shaded
areas are less than 230 W/m2. The contour interval is 20 W/m2. (b) Shaded areas are greater than 40 W/m2. The contour interval is 10W/m2.

tropical high.
In Fig. 2b the areas of large standard
deviation
tend to be located along the northern
rim of lower
OLR values found in the northern
low latitudes,
except those over southern
China to the peninsulaof
Indochina.
An area of relatively
large standardde-where SD(30-60) is the standard deviation of thetime series of the 30-60 day period component obtained through use of the band-pass filter mentioned
above and SD is the standard deviation of the original time series.
In the low latitudes high ratios are distributed
along 10*N, especially over the Bay of Bengal andthe South China Sea and also in the area centeredat 90*E and 5*S.
In the middle latitudes another area of high ratios is noted along 20-30*N over the western Pacific(140-160*E). This area agrees with the northern region of the area of high OLR values (Fig. 2a), i.e.
the zone of the subtropical high. The fluctuation on
this time scale may be, therefore, associated withthe strengthening and weakening of the subtropicalhigh. The existence of a 30-60 day oscillation overthis area can also be confirmed in the power spectraof OLR by Nakazawa (1986). Takeda and Ikeyama(1985) and Ikeyama and Takeda (1988) found thata period of about 30 days is dominant in the cloudiness field over this area (around 30*N and 150*E)
during the summers of 1978 through 1984. Relatively high ratios (>30 %) are also found over thearea between 140*Eand 150*E, along 40*N and in
the area north of the Tibetan Plateau (50-60*Nand70-80*E). Although the 30-60 day oscillation in the
OLR field is generally distinct in the low latitudes,
it is very important that this variation is also seenin the middle and high latitudes.
3.2 LaggedCorrelation Pattern
In order to investigate the variability of the atmospheric circulation in the low and middle latitudes
associated with intraseasonal oscillationof the summer Indian monsoon, the reference point at 15*N
and 87.5*E (in the Bay of Bengal) was chosen. The
filtered time series of OLR at 15*Nand 87.5*E (solidline) obtained by an 11-day moving averageis shownin Fig. 4. The lagged correlations for the OLR field
were computed from lag=-21 to +21 days for every
three days (not shown here). In this case the bandpass filtered time series, obtained by subtracting the
61-day moving average from the 11-day moving average, were used in order to remove shorter periodvariations as well as the seasonal trend from eachgrid point value.
Over the area from India to the Bay of Bengal,
the high correlation pattern propagates northward

132

Journalof the MeteorologicalFig. 3. Spatial
distributionof the ratio of
the 30-60 day period variance of OLR
against the standard deviation for May 1
to September 30, 1979. Shaded areas are
greater
10%.

than

40 %. The contour

interval

isFig. 4. Time series of the 11-day moving average OLR at 15*N and 87.5*E (reference
point, solid line), at 25*N and 147.5*E(broken line) and at 55*N and 75*E (dotted line) for May 1 to September 30, 1979.
The unit is W/m2.

with about a 40-day period. In the middle latitudes
the areas of large variation of the correlation coefficient are apparent to the south of Japan (130-160*E
along 30*N) and to the north of the Tibetan Plateau(in the vicinity of 55*N and 75*E). The distribution
pattern for lag=+21(-21) day is opposite to thatfor lag=0. These facts indicate that an oscillation
with about a 40-day period occurs in phase withthat of the Indian monsoon over the entire domain.
Large negative values are found to the south ofJapan and to the north of the Tibetan Plateau for
lag=-6 days, whereas large positive values appear
in these regions for lag=+15 days. The spatial distribution of the correlation coefficient for lag=-6
days is shown in Fig. 5. That is, when the cloudSocietyof JapanVol. 68, No. 2band which propagates northward arrives around
10*N in the region over India to the Bay of Bengal, the cloudiness is at a minimum over above twoareas. This feature is clearly seen in the time series (11-day moving average) of OLR at 25*N and147.5*N (broken line) in Fig. 4, which is negatively
correlated to that of the reference point with several days lag. In this region, the maximum values
denote the development or northward shift of thesubtropical high, while the minimum values denotea dissipation or southward shift.
The areas of large positive values mainly expandin the low latitudes of the northern hemisphere,
which suggeststhat the variation of convectiveactivity along the ITCZ over the northwestern Pacific is
positively correlated to that of the Indian monsoon
with several days lag. Another positive area is apparent along 35-55*Neast of 120*E,which may suggest that the activity of the extratropical cyclones
over this area is also associated with the oscillationof the Indian monsoon. These coherent fluctuationsof OLR around Japan may be associated with thevariation in the Baiu front (i.e. the eastern part of
the frontal zone). These facts will be further investigated in Section 4.3 through the use of synoptic
fields.
3.3 Time sequenceof the OLR anomalies
In the above subsection it was suggested that acyclic variation in the atmospheric circulation associated with the Indian monsoon occurs around
Japan and also to the north of the Tibetan Plateau.
In this subsection an investigation is made of thevariation in the spatial pattern of the 30-60 day
component, with special note of the evolution of thispattern in the seasonal cycle.
In order to examine the meridional variations,latitude-time sections of the longitudinal mean 3060 day component of OLR are composed for 8590*E and 145-150*E, as shown in Fig. 6a and 6b,
respectively. Figure 6a shows the section of the Indian monsoon region. The onset of the monsoon
in central India during 1979 occurred on June 19(Sikka and Grossman, 1980). In mid June the active convective areas, propagating northward, arrive
between 10*N and 20*N. Although the active convection with a 40-day period occurs four times inthe equatorial zone, the northward propagation occurs three times, i.e. in June, late July to Augustand in late September. After the second northward
propagation, in late August, the 40-day cyclebreaks
down. This is known as the monsoon withdrawal(Ding et al., 1982). It is also interesting to notethat the convection zone does not propagate northward during early May, in spite of the considerable
intensity of the convective activity. This feature of
the northward propagation agrees well with thoseof the filtered wind fields at 850 mb examined by

April 1990

N. Hirasawa

Fig. 5. Spatial distribution of correlation coefficientsof OLR for lag=6 days. The reference point is found at 15*N and 87.5°E(shown by •). Shaded areas are negativevalues. The contour interval is 0.2.

Fig. 6. Latitude-time sections of the standardized anomalies of the 30-60 day periodof OLR for longitudinal means (a) over 8590*E and (b) over 145-150*E. Shaded areas are negative anomalies. The contour
interval is 10 W/m2.

Krishnamurti and Subrahmanyam (1982).
Figure 6b shows the section of the subtropicalhigh region around Japan. The variations, with a
40-day period, of the areas along 40*N and along
25*N are out of phase. That is, the frontal activity
around Japan in the Baiu season is closely associated with the 40-day oscillation in the tropics. The
anomaly pattern seems to represent the southward
propagation from 40*N to the equatorial zone, especially south of 20*N. In order to confirm this southward propagation of the actual convection field, thelatitude-time section of low-pass filtered OLR along
the longitudes of 145-150*E is shown in Fig. 7. Thisand T. Yasunari

133

Fig. 7. Latitude-time section of the 11-day
moving average OLR for the longitudinalmean over 145-150*E for May to November, 1979. Shaded areas are less than 230
W/m2. The contour interval is 20 W/m2.

propagation is very distinct during August and late
September to October while indistinct in June. The
southward propagation was also obtained for the 30day period of cloud amount during 1980 by Takeda
and Ikeyama (1985). It was also shown by M. Murakami (1984) that a northward propagation of theactive convective area between 120*E and 130*E,
was concurrent with that over India to the Bay of
Bengal. It is very interesting to investigate the difference of the dynamical processes between the regions west and east of approximately 130*E.
Figure 8 shows the series of spatial distributionsof the normalized OLR anomalies of the 30-60 day
component for June 7 through August 16, for every
10 days. The negative anomaly areas are shaded,
which generally agree with the large cloudiness areas. To the south of Japan, the negative anomalies (July 7-17 and August 16-21) appear when thepositive anomaly band is present along 10*N overthe India to the Bay of Bengal, i. e. slightly before
the break phase of the Indian monsoon. Conversely,
the positive anomalies (June 12-22 and July 27August 1) appear when the negative anomaly bandis present along 10°N over that region, i.e. slightly
before the active phase. The fact that the amplitudes of the variation to the south of Japan and in
the area along 40*N are large to the east of 130*E
indicates that the cyclic variation with a 40-day period is present in the activity of the subtropical highand the Baiu front but not in the Mei-yu front. Thechange of the horizontal structure of the subtropicalhigh during the middle of June pointed out by Kato
(1989) and Kato and Kurihara (1989) seems to be
associated with the change of the anomalies, fromnegative to positive, to the south of Japan. The
positive anomalies between July 27 and August 1
seems to be associated with the end of the Baiu.
To the north of the Tibetan Plateau, the positive
and negative anomalies appear alternately with a
40-day period. The phase of the variation over this

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Vol. 68, No. 2

Fig. 8. Spatial distributions of the normalized anomalies of the 30-60 day period of OLR (through use of
the band pass filter) for June 7 to August 16, for every 10 days. Shaded areas are negative anomalies.
The contour interval is 0.2. A hundred times value of the contour is shown in figure.

area (dotted line in Fig. 4) appears to slightly precede the phase to the south of Japan (broken line in
Fig. 4).
Over the southern region of the Tibetan Plateau
the 30-60 day period is also dominant. When active

convection
appears along 10*N over India to the Bayof Bengal, the convection
over the southeastern
partof the Tibetan
Plateaualso becomes
active.
However, the relationshipbetweenthe convective
activity over this region and that associatedwith theApril 1990

N. Hirasawa

and T. Yasunari

135

Fig. 9. Spatial distributionof the ratio of the
30-60 day period variance of the 700 mb
geopotential
heights against the standard
deviation for May 1 to September 30, 1979.
Shaded areas are greater than 40 %. The
contour interval is 10 %.

northward propagations is not clear.
It was shown in this section that cloudiness is
a minimum to the south of Japan (the subtropicalhigh region) and to the north of the Tibetan Plateau(centered at 55*Nand 75*E)when active convection
exists along 10*N over the Indian monsoon region.
Conversely,there is maximum cloudiness over theseareas when minimum cloudiness exists along the
10*Nregion. Since the amplitude of the variation of
the OLR field around Japan is large to the east of
130°E, this variation may be closelyassociated with
the intraseasonal variation of the Baiu front and thesubtropical high. On the other hand, the Mei-yu
front does not show an intraseasonal variation ofthis time scale. Moreover, it should be noted that
this 40-day oscillation becomesindistinct simultaneously over these three areas after late August (notshown). These facts strongly suggest the link of theintraseasonal fluctuations among these regions andalso the strong modulation of this fluctuation in theseasonal cycle.
4. Fluctuations
heightin the 700 mb geopotential

It was indicated in the above section that the 3060 day oscillation of the OLR, which was well correlated with the variation of the Indian monsoon,
existed over a broad area. In this section it will beshown that the variation of the 700mb geopotential height is one of the characteristics of the atmospheric circulation which exhibits such an oscillation.
Figure 9 shows the spatial distribution of the ratio of the standard deviation of the 30-60 day component of 700 mb geopotential height to the total
standard deviation. This ratio was calculated in thesame manner as that of the OLR (Section 3.1). The
areas of greater than 40 % are shaded and the con-Fig. 10. Latitude-time sections of anomaliesof the 30-60 day period variation of the
700 mb geopotential height for longitudinal means (a) over 78.75-90E and (b)146.25-157.5E. Shaded areas are negative
anomalies. The contour interval is 10m.

tour interval is 10%.
Large values of the ratio are found in the low latitudes, especially over the equatorial zone, where relatively low ratios are apparent in the OLR field (Fig.
3a). In the extratropical regions large values of theratio are apparent to the southeast of Japan, which
is located to the north of the high ratios in the OLR
field. High values also found in the vicinity of 60*N,
180*E. The high ratios over the area to the southeast of Japan are probably due to the variation inboth the Baiu front and the subtropical high pressure area.
Figure lOa and lOb show latitude-time sections ofanomalous 700 mb heights having a 30-60 day periods for longitudinal means over 78.75-90*Eand over146.25-157.5*E,respectively. The negative anomalous areas are shaded. In Fig. 10a the anomalies
appear to propagate northward from the equatorial zone to 55*N. Although the alternate northward propagation of anomalous ridge and troughpatterns from the equatorial zone to the foothillsof Himalayas correspond well to those of the OLR
data, the northward propagation from 30*Nto 55*N
is not confirmed in the OLR data.
In Fig. 10b the anomalies propagating southwardfrom 60*N to 30*N appear from June to early August. In low latitudes, the anomalies of the OLR
show a southward propagation but those of the 700
mb geopotential heights exhibit a standing-type oscillation with a node at about 25*N rather than ameridional propagation.
Figure 11 exhibits the spatial pattern of anomalies

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Journalof the MeteorologicalSocietyof JapanVol. 68, No. 2

Fig. 11. Spatial distributions of anomalies of the 30-60 day period variation of the 700 mb geopotential
height (through use of the band-pass filter) for June 7 to August 16, for every 10 days. Shaded areas
are negative anomalies. The contour interval is 10 m.

for the 30-60 day period for June 7 to August 16
for every 10 days. The positive (negative) heightanomalies correspond well to the positive (negative)
OLR anomalies (Fig. 8), particularly over the regionof the Indian monsoon, to the north of the Tibetan

Plateau and to the southeast of Japan.
The positive (negative) extremes of anomalies inthe extratropics, which are centered to the north of
the Tibetan Plateau and to the south of Japan, areevident from mid June to mid August. It should

April1990

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137

Fig. 12. Composite maps of the fronts and the centers of both tropical (•) and extratropical (*) cyclones
for a pentad, with an interval of a pentad for June 10 to August 3, 1979. The thick line is 3175 m
contour of 700 mb geopotential height, averaged for a pentad, and black triangle indicates the point at
25*N and 147.5*E.

be noted that extratropical variations of this typeare distinct only during the period when the convective area repeats the northward propagation overthe India-Bay of Bengal region, i. e. the season of
the Indian monsoon. As seen in Fig. 11, the anomalies to the north of the Tibetan Plateau slightly precede those near and to the south of Japan. Thisphenomenon may be at least partly related to theeastward propagation of the 30-60 day mode along
the equator as shown in studies by Lorenc (1984),
Chan (1987) and the others.
The southward propagation, seen in Fig. lOb, occurs east of 130*E from 60*N to 30*N, while theanomalies propagate northward from the equatorialzone to 55*N west of 130*E. The propagation pattern of the anomalies exhibits a clockwise rotation.
In this section it is confirmed that a 40-day oscil-lation is present in the atmospheric circulation fieldin the same manner as in the OLR field. A largeamplitude area to the southeast of Japan indicatesthe evident 40-day variation of the subtropical highduring the Baiu season. It was also shown by Nakanishi (1972) that the 30-60 day component was one ofperiodicities of the variation of the subtropical highduring the Baiu season.
5. Frequencyof extratropical

cyclones

In this section the investigation of the cyclic variation in the anomalies of the OLR and 700 mb geopotential heights around Japan is accomplished by useof synoptic weather charts, in order to examine therelationship between the variation of the subtropicalhigh and that of the real weather regimes around
Japan. Figure 12 exhibits the composite maps of

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Journalof theMeteorologicalfronts and the centers of both tropical and extratropical cyclones for a pentad, with an interval of apentad, based on JMA's synoptic charts. The thickline is a contour of 3175m of 700 mb geopotential
height, averaged for everyypentad, and the black triangle is the point of 25*N and 147.5*E.
In mid June (upper left panel in Fig. 12), the Baiu
front is established off the southern coast of Japan,
which first develops in late May and shifts graduallynorthward (not shown). This gradual shift is associated with the slow developmentof the subtropicalhigh (see Fig. 4). When the OLR indicates maximum values around June 20, the frontal zone shifts
further north (middle left panel). In the OLR field(see Fig. 8) the positive anomalies appear south ofJapan while the negative anomalies simultaneously
appear along 40*N. At this time the area enclosedby the 3175m contour line is spread. It continued tobe hot and sunny in the middle of Japan until June
27, when the frontal zone again shifts to the south
coast of Japan. Here, this phase is defined as thebreak phase of Baiu over the southern part of theJapan Islands. From early July to mid July the values of the OLR are small south of Japan, while thefrontal zone shifts southward and stagnates along
the southern coast of Japan (lower left and upper
right panels). The negative anomalies to the southof Japan and the positive anomalies along 40*N are
seen during July 7 in Fig. 8. Severe local rainstorms
are often observed in the western region of Japan
during this period. This Baiu event finished withthe second development of the subtropical high inlate July (middle and lower right panels), i. e. thearea enclosed by 3175m contour shifts northward.
These results indicate that the 40-day variation of
OLR at 25*N and 147.5*Eis induced by the meridional shift of the frontal zone and the subtropicalhigh. Chen and M. Murakami (1988) also pointed
out the north-south movement of deep convective
clouds over the western Pacific during this period.
From the results in this section, it is suggested
that the meridional position of the Baiu front varies
in association with the 40-day oscillationof the subtropical high. This variation of the meridional position is closely associated with the change of weather
around Japan during the Baiu season.
6. Summary and remarks
This papaer investigated the intraseasonal variation (30-60 day) in the OLR and 700mb geopotential height anomalies over the Asian continent
through the western Pacific associated with the Indian monsoon activity. Large anomalies are foundin some phases of the monsoon cycleover the area tothe north of the Tibetan Plateau and around Japan.
The features of this variation over these areas are
summarized as follows:
The strengthening and weakening of the subtrop-

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2ical high pressure area (20-30*N and 130-160*E)
was associated with the intraseasonal variation of
the Indian monsoon. It was also shown that themeridional shift of the Baiu front was associatedwith the strengthening and weakening of the subtropical high. Namely, when the subtropical highshifts northward slightly prior to the active phase of
the Indian monsoon, cloud bands do not exist overthe southern part of Japan, and vice versa. Lau
and Chan (1986) have shown similar results for the
relationship between the subtropical high and Indian monsoon for 1975 to 1982. In contrast to theirresults, the position of the subtropical high in thepresent study shifts further northward. Since thepresent analysis is made for the single year of 1979,
this difference may be at least partly due to the interannual variation. The subtropical high in this region during summer seems to be modulated by thelarge scale convection in the tropical western Pacificby means of the Rossby wave response, rather thana northern branch of the Hadley cell, as suggested
by Kurihara and Tsuyuki (1987) and Nitta (1986,
1988). The position and strength of the subtropicalhigh is, therefore, very sensitive to the fluctuationof ITCZ to the south. This sensitive difference inthe meridional position may be very important forweather around Japan during the Baiu season. In
practice, Lau and Chan pointed out that it was during the active phase of the Mei-yu trough when thesubtropical high shifted northward, contrary to thepresent results.
In addition, the difference of the meridional variation of Baiu (east of 130*E) and Mei-yu (west of
130*E) fronts is found in this study. The differenceof the characteristics of the circulation fields of these
two front was also pointed out (Kunashima and Hiranuma, 1971; Akiyama, 1973).
It has also been confirmed clearly that the phaseof the 40-day oscillation over the area centered at
55*N and 75*E precedes that over the subtropicalhigh region by several days.
Figure 13 schematically shows the results mentioned above. Figure 13a represents a snapshot justprior to the active phase of Indian monsoon. The
contours are the 700 mb geopotential heights of an
11 day average centered on June 17. The shaded
area represents values less than 3100 m, with a contour interval of 25 m. The major convective zones
are located along 10*N over the Bay of Bengal andalong 15*N over the western Pacific. Over the area
to the north of the Tibetan Plateau the ridge isstrengthened. The subtropical high develops andshifts northward while the frontal zone also shifts
northward around Japan. This phase corresponds tothe break and the ending phase of the Baiu aroundthe southern part of the Japan Islands. Figure 13b
shows a schematic snapshot just prior to the breakphase of the Indian monsoon (July 7). The ma-April1990

N. Hirasawa

and T. Yasunari139

Fig. 13. Schematic pattern of circulation and cloudiness for the 40-day oscillation. (a) pattern slightly
prior to the active phase of monsoon over central India (June 17). (b) pattern slightly before the breakphase of monsoon over central India (July 7). The contour represent the 700 mb geopotential heights
of an 11 day average centered on June 17 and July 7, respectively. The contour interval is 25 m.

jor areas of convection are located over the equatorial Indian and the western Pacific Ocean, andthe anomalous ridge remains over the Indian Ocean
along 10*N.The trough deepens to the north of the
Tibetan Plateau. The subtropical high weakensand
shifts southward. This phase corresponds to the active phase of the Baiu around the southern part ofJapan Islands.
Here, the association of the regional scale featurein the present results to the evolution of the global
aspect of the 40-day oscillation is briefly examinedby using the time series of the 700mb geopotential height during the northern summer. The global
composite maps for category 1, 3, 5 and 7 are shownin Fig. 14. Here, categories 1 and 5 correspond to
the active phase (June 27 and August 6 in Fig. 11)
and the break phase (July 17 in Fig. 11) of monsoon

over central India, respectively. The map on June 17
and July 27 in Fig. 11 (the break phase of the Baiu)
correspond to category 7 and that on July 7 (theactive phase of the Baiu) correspond to category 3.
The wavenumber-1structure is dominant over theequatorial zone and near both poles, especially near
the north pole. It should be noted that the areaof large anomalies to the north of 70*N appears to
propagate westward.
Over the northern middle latitudes, a wavenumber-3 structure is dominant during the active andbreak phase of the monsoon (category 1, 5) whilea wavenumber-1structure is dominant between theactive and break phase (category 3, 7). The large
amplitude anomalies over the area to the north of
the Tibetan Plateau and in the subtropical' highregion emerge at the latter phases (category 3,7).

140

Journalof theMeteorologicalSocietyof JapanVol.

68, No.

Fig. 14. Category 1, 3, 5 and 7 of global composite maps of the 30-50 day mode in the 700mb geopotential
heights (One cycle of the Indian monsoon is divided into 8 categories). Shaded areas are negative
anomalies. Category 1 and 5 correspond to the active and break phase of the monsoon, respectively.

2

April1990

N. Hirasawa

andTherefore, it is confirmed that the variations overthese two areas are part of the local characteristicsof the global variation (wavenumber-1), associatedwith the variation.in the Indian monsoon.
The intraseasonal variation of the trough-ridge
system over the two area (i. e. to the north of the Tibet and to the south of Japan) is distinct during theperiod from the onset to the withdrawal of the Indian monsoon. Although Krishnamurti and Gadgil(1985) showed that the 30-50 day mode was alsopresent at higher latitudes throughout the FGGE
year in the wind and temperature fields, the results here indicate that the feature of the circulation pattern in the Indian monsoon season seems
to be largely different from that in any other seasons. That is, in the Indian monsoon season the heatsource moves northward cyclically, while in otherseasons it does not show a meridional fluctuation.
Thus the present results fundamentally seem to haveconfirmed the hypothesis by Yasunari (1986) whichsuggests that the intraseasonal circulation changes
in the mid latitudes are induced by the different
types of responses to a slowly northward-moving
monsoon heat source, though some questions still
remain (e.g. Though the feature in Fig. 13b looks
like the response in the "advective limit", the heatsource seems to shift most northward in the active
phase over northern India, i. e. before the phase of
Fig. 13b.).
Further studies of interannual variability areneeded with respect in the relationships among
the variation of the Indian monsoon, the troughridge system over the Eurasian continent throughthe western Pacific including the Mei-yu and Baiu
fronts, and the ITCZ over the western Pacific.
Acknowledgments
The authors wish to express their thanks to Prof.
T. Kawamura and Dr. Kobayashi for valuable suggestions. The authors would like to thank Dr. T.
Nakazawafor providing OLR data. Thanks are extended to Dr. M. Murakami and Dr. K. Kato forvaluable discussions. This study was partly sponsored by the Grant-in-Aid for Scientific Research,
the Ministry of Education, Science and Culture
No.01460055.
References
Akiyama,T., 1973:The largescaleaspectsof the characteristicfeaturesof the Baiu front. Pap. Met. Geophy.,24, 157-188.
Chang, C.C., 1981: A contrastingstudy of the rainfall
anomaliesbetween central Tibet and central India
during the summer monsoonseasonof 1979. Bull.
Amer.Meteor.Soc.,62, 20-22.
Chen, T.C., 1987: 30-50 day oscillationof 200-mbtemperature and 850-mbheight duringthe 1979northernsummer. Mon. Wea.Re*.,115, 1509-1605.

T. Yasunari

141

Chen, T.C. and M. Murakami, 1988: The 30-50 day variation of convective activity over the Western Pacific
Ocean. Mon. Wea. Rev., 116, 892-906.
Chen, T.C., M.C. Yen and M. Murakami 1988: The water
vapor transport associated with the 30-50 day oscillation over the Asian monsoon regions during 1979
summer. Mon. Wea. Rev., 116, 1983-2002.
Ding, Y.H., T. Iwashima and T. Murakami, 1983: Temperature changes over Eurasia during the late summerof 1979. Scientia Atmospherica Sinica, 7, 1-12 .
Hoskins, B.J. and D.J. Karoly, 1981: The steady linear
response of a spherical atmosphere to thermal andorographic forcing. J. Atmos. Sci., 38, 1179-1196.
Ikeyama, M. and T. Takeda, 1988: Seasonal and interannual changes in cloud amount over the western Pacific.
-changes in 30-day period variation . J. Meteor. Soc.
Japan, 66, 291-307.
Kato, K., 1989: Seasonal transition of the low-level Circulation systems around the Baiu front in China in 1979
and its relation to the northern summer monsoon. J.
Meteor. Soc. Japan, 67, 249-265.
Kato, K. and T. Kurihara, 1989: Case study on intraseasonal variations of the subtropical high as a moisturetransport system and the Baiu cloud distributions(comparison between the two periods around the middle of June 1979). Tenki, 36, 221-232 (in Japanese).
Knutson, T.R., KM. Weickmann and J.E. Kutzbach,1986: Global- scale intraseasonal oscillations of outgoing longwave radiation and 250 mb zonal wind during
northern hemisphere summer. Mon. Wea. Re*., 114,
605-623.
Krishnamurti, T.N. and D. Subrahmanyam,1982:
The 30to 50 day mode at 850 mb during MONEX. J. Atmos.
Sci., 39, 2088-2095.
Krishnamurti, T.N. and S. Gadgil,1985: On the structureof the 30 to 50 day mode over the during FGGE.
Tellus, 37, 336-360.
Krishnamurti, T.N., P.K. Jayakumar, J. Sheng, N. Surgi,
and A. Kumer, 1985: Divergent circulation on the
30-50 day time scale. J. Atmos. Sci., 42, 364-375.
Kurashima, A. and Y. Hiranuma, 1971: Synoptic and climatological study on the upper moist tongue extending from Southeast Asia to East Asia. Water Balanceof Monsoon Asia, Univ. Tokyo Press, 308pp.
Kurihara, K. and M. Kawahara, 1986: Extremes of East
Asian weather during the post ENSO years of 1983/84-Severe cold winter and hot dry summer - . J. Meteor. Soc. Japan, 64, 493-503.
Kurihara, k. and T. Tsuyuki, 1987: Development of thebarotropic high around Japan and its association with
Rossby wave-like propagations over the north Pacific142

Journalof theMeteorological

Madden, R.A. and P.R. Julian, 1972: Description of
global scale circulation cells in the tropics with a 4050 day period. J. Atmos. sci., 29, 1109-1123.
Murakami, M., 1979: Large-scale aspects of deep convective activity over the GATE area. Mon. Wea. Re*.,
107, 994-1013.
Murakami, M., 1984: Analysis of the deep convective activity over the western Pacific and Southwest Asia.
Part 2: Seasonal and intraseasonal variations during
northern summer. J. Meteor. Soc. Japan., 62, 88108.
Murakami, M., 1985: Atmospheric circulations and 3040 day oscillation in 1979 summer. Gross Wetter, 23,
19-38 (in Japanese).
Nakanishi, 1972: Some aspects of structure of subtropical high and its relation to the weather in summer
in Japan.
Manual of seasonal weather forecastingin Japan., 2, Japan meteorological agency, Technical note for long-range forecast, No. 13, 245-279 (inJapanese).
Nakazawa, T., 1986: Intraseasonal variation of OLR in
the tropics during the FGGE year. J. Meteor. Soc.
Japan, 64, 17-34.
Nitta, T., 1986: Long-term variations of cloud amount in
the western Pacific region. J. Meteor. Soc. Japan, 64,
373-390.
Nitta, T.,1988: Convective activities in the tropical western Pacific and their impact on the northern hemisphere summer circulation. J. Meteor. Soc. Japan,
65, 373-390.
Raman, C.R.V., Y.P. Rao and S.M.A. Alvi, 1980: The
role of interaction with middle latitude circulation in
the behavior of 1972 and 1979. Curr. Sci., 49, 123129.
Raman, C.R.V. and Y.P. Rao, 1981: Blocking highs over
Asia and monsoon droughts over India. Nature, 289,
271-273.
Sikka, D.R. and S. Gadgil, 1980: On the maximum cloudzone and the ITCZ over Indian longitudes during
southwest monsoon. Mon. Wea. Rev., 108, 1840-

Societyof Japan

Vol . 68, No.

4853.
Sikka, DR. and R. Grossman, 1980: Summer MONEX
chronological weather summary. International
MONEX management center, New dehli, India.
Suda, K. and T. Asakura, 1955: A study on the unusual"Baiu" season in 1954 by Means of northern hemisphere upper air mean charts. J. Meteor. Soc. Japan,
33, 233-244.
Takeda, T. and M. Ikeyama, 1985: Time variation ofcloud amount with about 30-day period in the western
Pacific region. J. Meteor. Soc. Japan, 63, 997-1012.
Tanaka, M., 1983: Interaction between the active-breakcycle of the summer monsoon and the circulationin Eurasia and the western pacific. J. Meteor. Soc.
Japan, 61, 455-463.
Tao, S.Y. and Y.H. Ding, 1981: Observational evidence of
the influence of the Qinghai-Xizang (Tibet) Plateau
on the occurrence of heavy rain and severe convective
storms in China. Bull. Amer. Meteor. Soc., 62, 2330.
Yasunari, T., 1979: Cloudiness fluctuations associatedwith the Northern Hemisphere summer monsoon. J.
Meteor. Soc. Japan, 57, 227-242.
Yasunari, T., 1980: A quasi-stationary appearance of 30
to 40 day period in the cloudiness fluctuations during
summer monsoon over India. J. Meteor. Soc. Japan,
58, 225-229.
Yasunari, T., 1981: Structure of Indian summer monsoon
system with around 40-day period. J. Meteor. Soc.
Japan, 59, 336-354.
Yasunari, T., 1986: Low-frequency interactions between
the summer monsoon and the northern hemisphere
westerlies. J. Meteor. Soc. Japan, 64, 693-708.
Webster, P.J., 1981: Mechanisms determining the atmospheric response to sea surface temperature anomalies. J. Atmos. Sci., 38, 554-571.
Webster, P.J., 1982: Seasonality in the local and remote atmospheric response to sea surface temperature
anomalies. J. Atmos. Sci., 39, 41-52.

イ ン ドモ ン ス ー ンの30‑60日

振 動 に 関・連 した

ア ジ ア 〜西 太 平 洋 に お け る 大 気 循 環 の 変 動
平 沢 尚 彦 ・安 成 哲 三(名古屋 大 学水 圏科 学 研究 所 ・筑 波大 学地 球 科学 系)

1979年の0:LRと700mb高

度 場 の 資 料 を用 い て 、 イ ン ドモ ン ス ー ン の30‑60日

周 期 変 動 と関 連 した ア

ジ ア か ら 太 平 洋 に お け る大 気 循 環 場 の 変 動 に つ い て 調 べ た 。
OLRの30‑60日

周 期 成 分 の解 析 か ら以 下 の こ とが わ か っ た 。 西 太 平 洋 上 の 北 緯15度

中 央 部 が モ ン ス ー ン の 活 発 期(不西 太 平 洋 上(東緯140度

活 発 期)に〜150度)のは い る直 前 に 、 対 流 活 動 が 活 発 期(不雲 量 極 大 域(極小 域)は40日に伝 播 して い くモ ー ドを持 っ て い る。 チ ベ ッ ト高 原 北 方(北(日 本 の 南 東)の期)に

度 場 の30‑60日、 東 緯75度

活 発 期)に付 近で は、 イ ン ド

活 発 期)との 周 期 で 北 緯15度

緯55度

雲 量 は 、 イ ン ド中 央 部 が モ ン ス ー ン の 活 発 期(不な る。700mb高2

な る。 こ の付 近 か ら 赤 道 へ と南付 近)と

亜熱 帯 高気 圧域

は い る直 前 に 、 極 小 期(極大周 期 成 分 の 解 析 か ら以 下 の こ とが わ か っ た。 チ ベ ッ ト高 原 北 方 でApril1990

N. Hirasawa

and T. Yasunari

143

は、 雲 量 極 大 期 と トラ フ の 停 滞 、 雲 量 極 小 期 と リ ッ ヂ の 強 化 が 対 応 し て い る。 亜 熱 帯 高 気 圧 の位 置 は 、 イ
ン ド中 央 部 が モ ン ス ー ン の 活 発 期 に は い る直 前 に 最 も北 偏 し(北最 も南 偏 す る(北 緯10‑20度)。

緯25‑30度)、

ま た 、 チ ベ ッ ト高 原 北 方 の30‑60日

不活 発 期 には い る直 前 に

振 動 の位 相 は亜熱 帯 高 気圧 域 の位 相に比 べ 数 日先 行 す る。 これ ら の 結 果 は 、 イ ン ドモ ン ス ー ン 、 西 太 平 洋 上 の 対 流 活 動 、 ア ジ ア 大 陸 上 の 偏 西風 波 動 、 亜 熱 帯 高 気 圧 が 密 接 な 関 係 を持 ち な が ら 季 節 内 変 動 を して い る こ と を 示 唆 して い る。
これ ら大 規 模 場 の30‑60日
え て い る。 梅 雨 前 線(東振 動 に関 連 し た相 互 作 用 は梅 雨 期 に お い て 日本 付 近 の 天 候 に 大 き く影 響 を与経130度

以 東 の 部 分)は

亜 熱 帯 高 気 圧 が 南 に偏 っ た時 に北 緯30度分)は

亜 熱 帯 高 気 圧 が 北 に偏 っ た 時 に北 緯40度

付 近 に と南 北 変 動 を す る。 メ イ ・ユ 前 線(東梅 雨 前 線 の よ うな 周 期 的 な 南 北 変 動 は 示 さ な い 。

付 近 に、 また、

経130度

以 西 の部