June

1979

T. Yasunari

Cloudiness

Fluctuations
Hemisphere

Associatedwith theSummerMonsoon227

Northern

By Tetsuzo Yasunari
The Center for Southeast Asian Studies, Kyoto University,Kyoto 606(Manuscriptreceived 13 September1978, in revised form 20 March 1979)
Abstract
The broad-scale fluctuations of cloudiness over the Eastern Hemisphere during thenorthern summer monsoon were investigated by using daily satellite mosaic pictures takenfrom June 1 to September 30, 1973.
Spectral analysis revealed two dominant periodicities, of around 40 days and around
15 days. Cross-spectral,time-sectional, time-lag correlation and phase-lag vector analysis
were applied to reveal the characteristics of these two modes in the time-space field.
The fluctuation of 40-day period shows marked northward
movement of cloudinessfrom the equatorial zone to the mid-latitudes (around 30 *E) over the whole Asian monsoon
area, and southward movement over Africa and the central Pacific. The northward movement is most apparent over the India-Indian Ocean sector. The fluctuation of this mode isassociated with the major "active"-"break"cycle of the monsoon over the whole Asianmonsoon area.
The fluctuation of 15-day period shows similar features to that of 40-day period, but
includes two clockwise rotations, one over India and Southeast Asia and the other over
the western Pacific. A southward movement from the equatorial zone to the Southern
Hemisphere middle latitudes is also prominent to the east and west of Australia.
The
fluctuation of this mode seems to correspond with the movements of equatorial, monsoon(or tropical), and westerly disturbances.
It is also suggested that the fluctuation of 40-day period may be closely connected withthe global-scale zonal oscillation in the equatorial zone and that of 15-day period may exist
as a result of meridional wave interactions.

1.

Introduction

In recent years, many studies on the summer
monsoon over and around India have revealed
some predominant periodicities in the day-to-day
fluctuations of rainfall, pressure and wind fields.
Recently, M. Murakami (1977) summarized theresults of these studies.
There exist at least two kinds of quasi-periodic
variation in the summer monsoonal fluctuations
over India and adjacent regions: one appears asa spectral peak with a period of around 5 days
and the other as one with a period of around 15
days. The 5-day period was revealed by spectral
analyses of wind and pressure fields (e.g.,Ananthakrishnan
andKeshavamurty,
1970:
Bhalme and Parasnis, 1975; M. Murakami, 1976
etc.). These studies show that this periodicity

is most prominent
in northern
India and is related to the frequency of monsoon lows travellingfrom the Bay of Bengal to northern India.
The periodicity
of around
15 days is foundin rainfall and lower and upper wind and pressure fields over the whole of India, including the
sub-Himalaya
region (e.g., T. Murakami,
1972;
M. Murakami,
1976; Krishnamurti
and Bhalme,
1976; Yasunari,
1976 etc.). Cloudinessover thetropical Indian Ocean also shows marked fluctuation with this period (Zangvil, 1975).
Many ofthe authors also suggest that the fluctuationofthis period range (around
15 days or quasi-biweekly) is closely related to the variation of thelarge-scale circulation
systems associated with the
SW monsoon.
However,
as M. Murakami(1977) points out,
these studies are limited to phenomena
over the228

JournaloftheMeteorologicalIndianOcean and Indian
subcontinent
region.
Therefore,
the fluctuations
over the whole SW
monsoonarea (India,
Southeast
Asia and thewestern
Pacific)
shouldbe investigatedmoreextensively
by the use of data obtained
over abroaderarea.
However,daily observations
ontemperature,moisture and wind field are still not
sufficient, especially over the tropical ocean areas.
In this sense, cloudiness data from meteorologicalsatellites can provide immediate
and invaluable
informationon the large-scale circulation systems,
since cloudinessis one of the most importantmeasuresof monsoonal
rainfallfluctuations.
Moreover,recentstudies(Krishnamurti
andBhalme,
1976; Webster
et al., 1977) have suggested that cloudiness over the monsoon area acts
as a controlling
factor of the large-scalemonsoonal fluctuations.
T. Murakami(1976) made a statistical analysisof cloudiness fluctuationin the northern summer,
by use of satellite cloudinessobservations
covering 8 years. By computationof time-lag correlations, he found that the spatial pattern of positive(negative) correlations moved northwards
from theequatorial
Indian Ocean to the Indian sub-continent at an average
speed of 1 * latitude/ day.
From the global point of view, he first revealed
the interrelatednessof large-scale cloudiness perturbations
over the tropical belt during the northern summer.
However, a more precise descriptionof monsoonal
fluctuations,
includingthe discussions of their periodicities,
has not yet beenmade.
In this paper, an attempt will be made to clarify
the time-spatial
features and dominant modes offluctuationsof the northernsummermonsoonover the Eastern
Hemisphere,by use of dailysatellite cloudiness data. We focus mainly on timescales longer than 10 days, which are thought toFig. 1Societyof JapanVol.

be associated with "break"-"active"
monsoonal fluctuations.
2.

57, No.

3phasesof

Data

Daily visible mosaic pictures (Mercator projection) of the Eastern Hemisphere (0*-180*E,
40*N-35*S), taken by the NOAA-2 satellite from
June 1 to September 30, 1973 (122 days), wereused. Monsoonal rainfall over India in this year
is considered to be nearly normal (Saha, 1978).
The brightness of each picture was measured inareas of 1 * latitude * 2 * longitude with an autodensitometer, which can measure light transmitted
through the film from 0 to 30,000 density levels.
Missing data were replaced by linear interpolationin time. Then, the number of areas where thebrightness level corresponded to cumulus or
cumulo-nimbus clouds was counted in each latitude-longitude block (5* latitude *12*
longitude). The counted values in each block were
defined as the cloudiness of each block (5* latitude
12* longitude).
*3.

Mean cloudiness and space correlations ofcloudiness during the northern summer monsoon

Before discussing temporal fluctuations, we will
describe the mean cloudiness and space correlations of cloudiness for the whole monsoon period.
Mean cloudiness of the four monsoon months(June to September) is shown in Fig. 1. Major
monsoon cloudiness stays over the area from
northern India to southeast Tibet, and overequatorial northern Africa. It is noteworthy thatthe maximum cloudiness over Asia appears overthe Yunnang region (around 20*-25*N, 100*105*E). Dense cloudiness also lies in a narrow
band over southern Japan through the western

Mean cloudiness for June through September, 1973. The
cloudiness values greater than 3 (30%) are shaded.

June

1979

T. YasunariFig. 2229

Spatial distribution of correlation
coefficients of cloudiness with a reference point over central India (17.5 * N,
78 *E). The values greater than 0.4 are shaded, and thoseless than -0.4 are dotted.

North Pacific.
This cloudinessmainly resulted
from quasi-stationary
fronts in June (Baiu Season) and September.
The two maxima of cloudiness over Asia and Africa are separated by the
Arabian desert-Arabian
Sea area.
The correlation
coefficients of cloudiness werecomputed for a reference block over central India(15*-20*N,
72*-84*E).
The result is shown in
Fig. 2. A clearly bounded band of positive correlations
extendingeastward
from central India
to Southeast
Asia and the Philippines
is noted.
From the Philippines
it bends southeastward
to
New Guinea and extends eastward far into theequatorial
Pacific. This band also extends westward to Africa.
Anotherbroad area of positive
correlations
exists from southern Australia to thesouthwestern
Pacific
and possibly
beyond.
Incontrast, on either side of the east-west oriented
marked positive band the correlationis negativewith significant values over northern India, Tibet
to the eastern North Pacific, and also over theequatorial
Indian Ocean.
It is well known thatmonsoonrainfallincreases(decreases)
overnorthern India and the southernmost
part of India
during "break" ("active")
conditions over central
India (e.g., Hamilton,
1977, etc.). Therefore,themeridional
contrast of the negative-positive-negative correlation
patternover and around India
explains precisely the typical "break" (or "active")
conditions of the Indian summer monsoon.
Thiscontrast of correlation
pattern is most remarkable
along the Tibet-Bay
of Bengal line. But it isapparent in Fig. 2 that this pattern occurs simultaneously
over East and Southeast Asia or more
widely.
Thus, it is indicated that the cloudiness fluctuations associatedwith the northern summer mon-soon are a phenomenon of the broad area fromthe Arabian Sea to the western Pacific and fromthe equator to the middle latitude zone.
4.

Zonal and meridional structures of cloudinessfluctuations

In this section, we will investigate the natureof temporal fluctuations in zonal and meridionalcross sections.
4.1

Time-longitude sections o f cloudiness

Time-longitude sections were composed foreach 5 * latitude zone. In this analysis, cloudiness data for smaller blocks (5* latitude *4*longitude) were used to examine zonal cloud
movements of smaller scale. Latitude zones from
100S to 40*N were examined, where cloudinessfluctuations associated with the northern summer
monsoon are thought to be apparent. Throughthis analysis, it is expected to find eastward or
westward moving (or quasi-stationary) cloud disturbances responsible for the monsoonal fluctuations, especially of quasi-biweekly or longer period
ranges. The characteristic features of zonallypropagating cloudiness, allow the latitude zones
to be classified into three major groups.
(a) Equatorial zone (10*S-5*N)
In this zone, eastward moving cloud disturbances are predominant throughout the monsoon
period. As a typical feature of this zone, thecross section of the 0*-5*S latitude zone is shownin Fig. 3. In this figure, almost all cloud disturbances first appear at around 60*E, develop
while moving eastward over the Indian Ocean
through Indonesia region, and finally decay ataround 150*E. Major disturbances seem toappear with a period of around 30-40 days.

230

JournalFig. 3of theMeteorologicalSocietyof JapanTime-longitude sections of cloudiness for 0*-5*S latitude zone.
Cloudiness is shown by a ten-level grey scale of half-tone display. The difference between adjacent levels corresponds tothe cloudiness value of 0.9 as shown in the scale. See text
for the symbols A and B.

Vol.

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June

1979

T. Yasunari

Minor disturbances of shorter period also appear.
These disturbances progress at a speed of roughly
4*-6* longitude/ day. The former disturbances
may be related to those noted by Madden andJulian (1972) and Zangvil (1975), since the eastward phase speed and estimated zonal wavenumber (* 2) are similar to their results. In the
0*-5*N zone, though the major eastward disturbances still can be seen, westward cloud movements of smaller scale also overlap in the time
section (not shown).

231

These features become clearer on applying a zonal
space filter to the original data. Here, the zonal
running mean of 36* longitude was applied inorder to separate the zonal scales larger andsmaller than 36* longitude width. The largerscale part thus obtained shows a quasi-stationary
fluctuation, as shown in Fig. 4(b). The smallerscale (or transient) part of cloudiness (Fig. 4(c))
shows westward movement at a phase speed of
3 *-6* longitude/ day, and with the zonal scalesof 20*-40* longitude. The features of thistransient cloudiness are similar to those of the(b) Subtropical easterly zone (5*N-25*N)
disturbances studied by M. Murakami (1976) over
The cloudiness in this latitude zone is closely the Bay of Bengal and northern India. The largerrelated to the northern summer monsoon. As a scale (quasi-stationary) part of cloudiness (Fig.
typical pattern of this zone, time-longitude section 4(b)) is located from 80*E to 100*E (over theof the 10*-15*N zone is shown in Fig. 4(a). Bay of Bengal and Indo-China), and fluctuates
Westward moving cloud systems corresponding
in its zonal width with a period of 30 to 50 days.
to the easterly wave disturbances are dominant. Fig. 4(b) indicates that cloudiness fluctuations overOverlapping these moving cloud systems, there Africa (0*-40*E), where maximum monsoon rainseems to be a broad-scale quasi-stationary cloudi- fall appears in this latitude zone, are roughly out
ness over the 60*E to 120*E longitude zone. of phase with those over Asia.

Fig. 4(a) Same as Fig. 3 except for 10*-15*N latitude zone, (b) same as Fig. 4(a) except
for zonally smoothed cloudiness, (c) same as Fig. 4(a) except for cloudiness of
smaller scale. Note only positive deviations are illustrated by the grey scale inFig. 4(b) and 4(c). The cloudiness difference between adjacent levels is 0.4 forFig. 4(b), and 4(c).

232Journalof theMeteorologicalSocietyof JapanVol.

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as shown in Fig. 6(a) and 6(c). These features
show that even in the monsoon season the area
stretching from northern India to the Tibetan
Plateau is affected mostly by the mid-latitude
westerly disturbances. In contrast, the large-scalepart of cloudiness (Fig. 6(b)) exhibits quasistationary features over the 80*-120*E zone,
with major fluctuations showing a periodicity of
about 40 days and minor fluctuations a periodicityof 10 to 15 days. It should be noted that theupper level anticyclone (Tibetan High), one of
the most dominant systems of the northern summer monsoon, lies over this area centered ataround 30*N. The relation between TibetanHigh and the westerly wave movements will be
discussed later.
4.2

Fig. 5Longitude-time sections of sea level pressure(mb) data at 20*N during July 1965 for (a)the sum of zonal wavenumbers 1 and 2, and(b) the sum of zonal wavenumbers
3-12.
(after Krishnamurti et al., 1977)

In the longitude-time sections of sea level pressure along 20*N, Krishnamurti et al. (1977) found
the quasi-stationary part (wavenumbers 1 and 2)
and the westward moving transient part (wavenumber 3 to 12) as shown in Fig. 5(a) and 5(b).
Since the year of the data and the zonal space
filter adopted are different from those used in
the present study, Fig. 4(b) and 4(c) cannot becompared directly with Fig. 5(a) and 5(b), respectively. Nevertheless, it is noted that the westward transient cloud systems in Fig. 4(c) aresimilar to the disturbances in Fig. 5(b) and thequasi-stationary part of cloudiness (Fig. 4(b)) is
located at about 20* longitude to the east of thequasi-stationarypressure trough of waves overIndia (see Fig. 5(a)).

Time-latitudesections of cloudiness

Since the broad-scale monsoon circulation has
been recognized as a local meridional (Hadleytype) circulation, it follows that the fluctuations
should also appear along the meridional plane.
To confirm this, time-latitude sections of monsoon
cloudiness were examined. To eliminate smallscale fluctuations, time-smoothing (of 5-day running mean) was applied to the original cloudinessdata and the time-mean value of each latitude wassubtracted from the smoothed data.
In the sectors from the Arabian Sea throughthe Bay of Bengal, remarkable northward movements of cloudiness are found from around 10*S
to 30*-40*N, with a period of about 40 days.
This feature is most distinct over the Indian subcontinent (72*-84*E) sector, as shown in Fig. 7.
It is noteworthy that the meridional fluctuationsof cloudiness accompanied by the major "active"and "break" phases over India appear not as
oscillations but as a repetitive northward shift ofcloudiness from the equatorial Indian Ocean to
the Himalayas. The northward phase speed isabout 1 * latitude/ day, which is almost the samevalue as that obtained by T. Murakami (1976).
The maximum (or minimum) cloudiness simultaneously stays at the equatorial region andaround the Himalayas, which is in good agreement with the well-known features of the "break"(or "active") phase of the monsoon over India.
The first phase of the northward movements may
correspond to the onset stage of the Indian summer monsoon.

(c) Mid-latitude westerly zone (25*N-40*N)
Through the whole monsoon period, eastward
In the Southeast
Asian (96*-108*E)
sector,movements of cloudiness are predominant. Only northward movements of cloudiness with a period
in the 25*-30*N zone, westward movements can of around 30 to 50 days are also recognized overbe seen in the short period from July to August, the latitude zone south of about 25*N, but over

June

1979

Fig. 6

T. Yasunari(a), (b), (c) same as Fig. 4(a), (b), (c), respectively,

233

except for 25*-30*N

latitudezone.

Fig. 7

Time4atitudesections of cloudiness for 72* -84 * E longitude zone. Time means
are subtracted from the smoothed cloudiness, and positive deviations are illustrated
by the grey scale. The cloudiness difference between adjacent levels is 0.3. See
text for the symbols A' and B'.

the higher latitude zone (than 25*N) southward
movements
occur (data not shown).
In other areas, no systematic meridional fluctua-

tions can be seen, partly because
variancesof cloudinessare veryexamine more closely the statisticalvalues andsmall.
Tofeatures of234

JournaloftheMeteorologicalthe meridional fluctuations of cloudiness, day laglatitude sections of time-space correlations weremade for all the longitudinal sectors. Time-spacecorrelation coefficients * (*) were computed following the method of T. Murakami (1976). That
is,

where

Fig. 8Society

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Vol.

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The quantity Cij(tk) denotes the smoothedcloudiness data at the reference block (i, j) on thek-th day, while Cmn(tk+ *) represents the data atother blocks (m, n) on the (k + *)-th day. *ij isthe standard deviation of Cij(k) for k=1 to T.
T represents sample size; i.e., T=122 days forthe whole monsoon period of 1973 (June 1 to
September 30). Day lag * is taken from -50 to
50. In this case, a reference block was chosenfrom each longitudinal sector.
Fig. 8(a) shows the result for the African monsoon sector. Monsoon cloudiness over Africa

Day lag-latitude sections of time-space correlation coefficients for the longitude zones of (a) 12*-24*E, (b) 72*84*E and (c) 96* -108*E.
Contour line interval is 0.4.
Areas of positive values are shaded.

June

1979

T. Yasunari

235

appears to be a standing (or slightly southward field.
moving) fluctuation with a period of 25 to 30
5.1 Dominant periodicities of cloudinessdays. A northward movement of the correlation
fluctuations over Asia and Africa
pattern is apparent to the north of 10*S over theTo
examinethe predominantperiodicities,
Yndian sector (72*-84*E), with a period of about
spectralanalysisbythemaximum
entropymethod40 days, as shown in Fig. 8(b). This feature isalso seen over the Arabian Sea (60*-72*E) and (MEM) was performed. This method has theover the Bay of Bengal (84*-96*E) sector (data advantage of giving far better resolution, especially for short-time records, than the conventional
not shown).
As mentioned, in the Southeast Asian sector, methods (Ulrych and Bishop, 1975; Hino, 1977
northward movement is limited to the south of etc.). The time-period of each set of data is 122
25*-30*N, and a southward movement is noted days (from June 1 to September 30, 1973). Powerto the north (Fig. 8(c)). In the sectors further spectra were obtained for 225 blocks over theto the east, the correlation patterns again show Eastern Hemisphere (one block covers 5 * latitude
nearly standing oscillation or slightly southward x 12* longitude).
Fig. 9 shows the result of power spectral
movement (data not shown). Throughout these
meridional day-lag cross sections from the west analysis for central India (10*-15*N, 72*-84*E),
to the east, the correlation patterns commonly which reveals strong concentrations of power, oneshow a mode of around 40 day period, except with a 40-day period and one with a period ofin the African monsoon sector. Over Africa (Fig. nearly two weeks. To examine statistically the8(a)) it seems that the major mode of oscillation spectral features of the northern summer monis a period of 25 to 30 days or, more likely, of soon area, histograms were constructed of thelargest and second largest peaks of the power
50 to 60 days.
It is noteworthy that the major northward spectrum for each block of both the Asian mon0*5-40*N)
and themovements of cloudiness from the equatorial zone soon area (60*-180*E,
in Fig. 7 are initiated by eastward moving cloud African monsoon area (0*-48*E, 10*S-20*N),
systems like those shown in Fig. 3. Namely, the as shown in Fig. 10.
Over the Asian monsoon area (Fig. 10, upper),parts of cloudiness indicated by A and B in Fig.
3 are respectively the same as those marked A' the predominant periodicities are of 40 days, 9and B' in Fig. 7. Thereafter, maximum cloudi- 15 days, and around 2-3 days. In the histogram
ness occurs in the subtropical easterly zone (5*- of the largest peaks only (denoted by shaded part25*N), as shown in see Fig. 4(b). The final stage in Fig. 10, upper), the 40-day period is preof the northward movements is attributed to the dominant, which suggests that the fluctuation withcloudiness brought by the mid-latitude westerly this period is a fundamental mode over this area.
disturbances passing over northern India and the The quasi-biweekly fluctuation is also confirmed
Tibet-Himalaya region, as referred to in Fig. 6. in the cloudiness fluctuations. The large frequency
The evidence presented here suggests the nearly
synchronized movements of the eastward moving
disturbances both in the equatorial and in the
mid-latitude zone, which may be largely responsible for the monsoonal fluctuations at least
over India and adjacent regions.
5.

Quasi-periodical variations of cloudinessduring the northern summer monsoon

In the previous section, the characteristics oftemporal fluctuations of cloudiness were discussed
in the zonal and meridional sections and thedominant periodicities of monsoonal cloudinesswere roughly estimated.
In this section, spectral and cross-spectral
analyses are applied to determine more exactly
the features of the periodicities of the fluctuations
and their phase-relationships in the time-space

Fig. 9Power
spectraofcloudinessfluctuations for a block of 10*15*N,
72*-84*E.
Units
are(cloudiness values)2 . day.

236

Journalof theMeteorologicalSocietyof JapanVol.

57, No.

3from those over Asia.
The representative periodicities of large scale
fluctuations over the Asian summer monsoon area
are thus about 40 days and around 15 days. However, the fluctuation of around 15-day (or quasibiweekly) period does not appear explicitly inthe analysis of time-space correlations (see Fig.
8). This may be due to the far larger variancesof the 40-day period, for the whole area in general, than those of the about 15-day period.
To eliminate the low frequency part of thefluctuations, a difference filter was applied to thesmoothed cloudiness data. Namely, a filtered
datum Cf(t) by this method is,Cf (tk)= C(tk) - C(tk -1)

Fig. 10where Cf(tk) is smoothed cloudiness on the k-th
day after operating 5-day running mean.
C f(tk) values show a large frequency responsewith a period range from about 8 to 15 days.
Time-space correlation coefficients were computed
again for the filtered data to verify the time-spacecorrelations of the mode of 10- to 15-day period.
The day lag-latitude section for the Indian sector
is shown in Fig. 11, revealing a remarkable northward phase shift over the Northern Hemisphere.
A northward shift was also found over the Philippine sector (data not shown). Fig. 11 indicates
Histogramsof the largest and second that the northward phase shift of this period
largest peaks of the power spectra of range is characterized by a phase speed of aboutcloudinessfor each block over Asian 2* latitude/day with a latitudinal scale of about
monsoon region (upper) and over African 20*. Thus, a mode of 10- to 15-day period is
monsoon region (lower).
Those of the also likely to exist in the cloudiness fluctuations
largest peaks are shown by shaded columns. over India and Southeast Asia.
Units are numberof peaks.

5.2of the 2-3 day period may be attributabletosmall-scaledisturbances
like those observed over
the western Pacific (M. Murakami,1971).
Over the African monsoon area (Fig, 10, lower),in contrast,
a 2-3 day period is predominant.
Orlanskiand Polinsky(1977) also foundthis

Spatial behavior of the cloudiness fluctuations with the two dominant periodicities

Here, the characteristics of the fluctuations of
the two major modes (those of about 40-day
and 15-day period) will be examined over thewhole northern summer monsoon area.
First, a cross-spectral analysis was made to get
the propagation
characteristics
of cloudinessperiod range predominant
in the spring season
movement in each period range, by the lag cor(March to May) over the African continent. They
relation method. In this computation, the total
showed that the horizontal scale of these disturbances is of the order of 2,000km.
However, the record length was 122 days, while the maximum
horizontal
scale of each unit-block(5* 12*) and lag number of 20 was used.
the time interval of the data (once per day) in
Fig. 12(a) shows the distribution of variancethe present analysis seem to be too wide to dis- for the 40-day period and its phase angle relativecuss such a period range more in detail. Though
to a reference point in central India. Highnot prominent,
fluctuationsof 60-day period oc- variance values are distributed over India, Southeast Asia, south China and the western Pacificcurred in relatively high frequency.
These results
near Japan, where the large amount of monsoon
suggestthat the characteristics
of cloudinessfluctuations
over Africa are somewhat
different
cloudiness is also found (see Fig. 1).

June

1979

T. Yasunari

Fig.

11

Day lag-latitudesections
efficientscomputedbycloudiness.

Details

appear

237

of
the

time-spacecorrelation
couse of band-pass-filteredin thetext.

Indonesia region the movement seems to be northeastward. In Fig. 12(a), only the phase angles ofcross spectra whose coherences are greater than
0.3 are illustrated. The feature of the northward
movement over the Indian Ocean again corresponds well with the result of time-space correlation analysis made by T. Murakami (1976).
Most of reports of spectral analyses so far conducted on the fluctuations of monsoon have notmentioned the 40-day period, partly because thisperiod (* 40 day) is too long in comparison withthe total length of the monsoon period (90-120
days) to be detected significantly by the conventional spectral analysis (such as lag-correlation
method). Only Dakshinamurti and Keshavamurty(1973) have reported an oscillation with a periodof around one month through the spectral analysis
of winds at 850 mb level, by use of combined
data for the four monsoon seasons. The timesectional analysis mentioned here (see Fig. 4(b),for example) reveals that the maximum (minimum) cloudiness appears at intervals of 30 to 50
days. Thus, the periodicity they noticed may berelated to that of around 40 days (or, roughly 30
to 50 days) reported here.
A phase relationship in the fluctuation of thequasi-biweekly mode was also investigated. Unfortunately, no systematic phase difference overFig. 12 Spatial distributions of variances (thin solid a broad area with respect to a single referencelines) and phase lag relations for (a) 40- block wass found, probably because the periodicity
day period, and (b) 10- to 20-day period of this mode was variable within a fairly wide
range. Reference points are plotted with
dots. Units of variances are (cloudiness period range, as expected from Fig. 10. The samefeature of this fluctuation mode is also noted byvalues)2.
M. Murakami (1976). Therefore, cross-spectral
The phaseangle
patternindicatesa very clear
estimates for a 10- to 20-day period were averaged
northward
movementof cloudiness,
especiallyto obtain more appropriate phase relationships.
over the Indian
Ocean
area,
with
a phasespeed
In this case, significant phase angle relationshipsof about
1 * latitude/
day.
Over
the Malaysiawere found around some reference blocks over

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Distributions of phase-lag vectors (see text) for (a) 40-day
period and, (b) 10- to 20-day period. Length of each
vector indicates phase-lag per one latitude-longitudedegree, and solid lines show the rough boundaries ofnorthward and southward components.

India and Southeast Asia. Fig. 12(b) shows theaveraged phase differences with the three reference blocks (over central India, Malaysia andeast Tibet). By combining these three patternsof phase difference, one may speculate that awave of this mode located over the MalaysiaIndonesia region advances northwestward throughthe Bay of Bengal to India, and on reaching the
Tibet-Himalaya region turns to the east, moving
eastward as a mid-latitude westerly wave.
As described above, the cross-spectral analysis
was partly successful in revealing the time-spatial
behaviour of the two dominant modes over India
and Southeast Asia. To examine the propagation
characteristics over the whole northern summer
monsoon area, a phase-lag vector analysis was
conducted.
Even though the spatial movementof cloudiness for each periodicity is somewhat
complicated, the cross-spectral estimates for areas
contiguous to a reference point were expected toshow a high degree of coherence. Therefore,
zonal and meridional phase-lags were calculated

for each block by averaging phase-lags for two
zonally or meridionallyneighbouring blocks .
Then, a phase-lag vector for each block was composed by using these two phase-lag values. It
follows that a wave of each periodicity willpropagate significantly where phase-lag vectors
systematically change in the space field . Composite maps of phase-lag vectors for the 40-day
period and the 10- to 20-day period are shownin Fig. 13(a) and 13(b), respectively . Averaged
coherences for each block are greater than 0.5
for most of the blocks . In these figures, thearrows indicate the direction of propagation of
the waves of each periodicity, and the length of
the arrows represents the phase-lag per unit longitude-latitude degree.
The fluctuation of 40-day period (Fig. 13(a))
tends to be propagated mainly with north-south
components over most part of the area. A broad
northward movement is prominent over the whole
Asian monsoon area, as enclosed by a solid line .
The most remarkable and systematic northward

June

1979

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239

movement is apparent over India and the Indian westward movement and the southward or southOcean area, as repeatedly noted in the previous eastward movement mentioned above appear as
sections. Interestingly, a westward component is two clockwise rotations of this period range
also marked over the Somalia-Arabian Sea region. propagating over the whole Indian monsoon area
To the east of the area of northward movement, (Bay of Bengal, India and Tibet) and over thenamely over the central Pacific, southward move- western Pacific (including South China Sea andments are predominant.
By combining these the Philippines). The similar clockwise rotation
results with those of the time-space correlation of surface pressure anomalies over India-Bay ofanalyses, the fluctuation of about 40 day period Bengal region depicted by M. Murakami (1976)
can be summarized as follows: major cloudiness may be related to the clockwise rotation of cloudiappears over the southern equatorial zone from ness over the former area. The daily mosaic
roughly 60* to 150*E in longitude (namely, over pictures show that monsoon disturbances marchmonsoon Asia in a broader sense), and gradually ing northward or northwestward from the Bay ofprogresses northward as a whole and northwest- Bengal to northern India frequently cross theward in the western marginal area (over the Himalayas and change into or merge with the
Arabian Sea). When the central part reaches mid-latitude westerly disturbances over Tibet. The
around the Himalayas (about 30*E) after about northward
movement around the Philippines
40 days, another major cloudiness appears over seems to be derived from the formation and movethe southern equatorial zone, which again moves ment of tropical depressions and typhoons overnorthward. The area where northward movement this area.
is predominant corresponds well with that of
The third difference is a systematic southward
major monsoon cloudiness (see Fig. 1). This and southeastward (or anti-clockwise) propagation
simultaneous northward (or northwestward) propa- which is apparent from the equatorial zone
gation of cloudiness with a period of around 40 around Indonesia and New Guinea. A series ofdays may be considered a quasi-periodical north- daily mosaic pictures suggests that the propagation
ward expansion of the ITCZ over the Asian mon- of cloudiness represents the movement of cloud
vortices, which progress from the equatorial zone
soon area centered at 80*-90*E.
Fig. 13(b) shows the phase-lag vectors for the mentioned above southward to the Southern
10- to 20-day period, which have similar features Hemisphere westerly zone, becoming extratopicalto those of 40-day period. However, the follow- vortices.
These results suggest that the cloudiness fluctuaing three differences can be recognized. First,
relatively large eastward or westward components tion of this period range (10 to 20 days) may be
exist over the whole area, which result from closely associated with the formation and movezonally propagating waves of this period range ment of major monsoon or tropical disturbances.
over each latitude zone. Second, the northward 6. Conclusions and remarks
movement of cloudiness over the Asian monsoon
Analysis of cloudiness fluctuations by several
area differs somewhat. In the same manner as
in Fig. 13(a), a wave progresses northward from methods confirmed the existence of two major
the southern equatorial zone over the Indian modes, with periodicities of around 40 days andOcean, but first it propagates eastward or north- around 15 days, and the behaviors of these two
eastward toward Southeast Asia, and then it turns modes in the time-space field were investigatednorthwestward toward the Bay of Bengal and over the whole northern summer monsoon area.
The fluctuation of 40-day (or, roughly 30- to
India, as was speculated from the cross-spectral
analysis (see Fig. 12(b)). By using zonal phase 50-day) period appears as a simultaneous northlags, the estimated zonal wavelength of the east- ward movement over the whole Asian monsoon
ward propagating wave over the equatorial zone area (60*-120*E) from the southern equatorial(0°-5°S) is 7,000-9,000 km (wavenumber ~4 or zone to the zone at about 30*N. The northward6). Marked northward movement also occurs over movement is most apparent over the India-Indian
and around the Philippines. In contrast, a distinc- Ocean area. The northward movement is initiatedcloud disturbances detive southward or southeastward movement occurs by eastward-propagatingover the area from south China to the central veloping over the equatorial Indian Ocean. These
Pacific, which may be attributable to the move- equatorial disturbances seem to correspond toment of mid-latitude westerly disturbances. In those suggested by Madden and Julian (1972).
their spatial distribution the northward or north- According to their study, there is a convective

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disturbance propagating eastward from the Indian period. The propagation of this wave changes
Ocean to the central Pacific, which is associated from eastward to northward over the northernwith a global-scale east-west pressure oscillation equatorial zone, then to northwestward
overof 40- to 50-day period. They also speculated Southeast Asia. Over the area from the head ofthat strong zonal circulation cells exist to the east the Bay of Bengal and south China to the western
and west of the disturbance. It follows, therefore, Pacific, a clockwise rotation is noticeable. The
that the upper level easterly and the lower level propagation of this wave is mainly related to thewesterly wind (or the surface monsoon flow in movements of major cloud disturbances. However,the Indian Ocean) may be intensified (weakened) the same disturbance does not necessarily moveafter (before) the passage of the disturbance, from the equatorial zone up to the mid-latitudes.
which is also suggested by Cadet and Olory-Togbee It is more plausible that in progressing eastward(1977). This speculation is consistent with the the equatorial wave disturbances excite the subfeatures around the India-Indian Ocean sector tropical easterly wave in some way to produce
mentioned in the present paper. That is, when monsoon disturbances over the Bay of Bengal
the disturbance approaches the Indian sector, the or tropical depressions (or typhoons) over the"break" conditions occur over India . At this western Pacific. These disturbances develop while
stage, monsoon cloudiness shifts northward to the moving northward to the Himalayas (or the
Himalayas. After passage of the disturbance to- western Pacific to the south of Japan) and thenward Southeast Asia, monsoon cloudiness over interact with the mid-latitude westerly waves to
India gradually moves northward from the south develop westerly disturbances.
In the Southernof India, and finally, in the "active" phase, almost Hemisphere, the propagation also represents thecontinuous cloudiness extends from northern India movement of some kind of tropical disturbances,southeastward to Southeast Asia and far beyond, which are generated over the equatorial area,
at which time the disturbance has reached the move southward, and finally become the midwestern Pacific region.
latitude westerly disturbances.
Very recently,
In all events, it can probably be said that the mid-latitude troughs of great amplitude staying"active" and "break" phases of the monsoon in simultaneously at around 110*E over both hemithe conventional meaning are related with thefluctuation of this period range.
As a possible cause of oscillations with thisperiod range (40 to 50 days), Madden and Julian(1972) referred to a feedback mechanism between
sea surface temperature and atmospheric circulation, which is known as atmospheric teleconnections (Bjerknes, 1969, etc.), or an evaporationprecipitation
cycle over the trade-wind zone
(Kraus, 1959). Whatever the physical process, it
is feasible that the fluctuation of this mode during
the northern summer results from a combinationof the east-west oscillation over the equatorialzone with a feedback mechanism of (sensible andlatent) heating and cooling caused by cloud coverwhich operates under conditions of meridional
land-ocean contrast, since the northward movement of cloudiness is most prominent over the
Tibet-India-Indian
Ocean Sector.
The fluctuation with roughly a 15-day (or 10to 20-day) period also appears totally as a northward movement over the Asian monsoon region.
Similar to that of 40-day period, the northward
movement seems to be initiated by the eastwardpropagating waves over the southern equatorial
Indian Ocean, whose zonal wavelengths areshorter (7,000-9,000km) than those of the 40-day

spheres were suggested to play a large role in the"break" phases of Indian summer monsoon(Ramaswamy and Pareek, 1978). This synoptic
feature may be considered to represent one phaseof the fluctuation of this mode. Therefore, thefluctuation of this period range seems to be closely
related to the mid-latitude westerly waves in bothhemispheres, as well as to the equatorial waves.
Some problems can be pointed out for future
study on the dynamical process of the meridional
interaction of waves associated with this mode.
It is plausible that monsoon (or tropical) disturbances play an important role in the developmentof the mid-latitude disturbances by injecting heatand moisture into the westerlies (Erickson andWinston, 1972), Conversely, the westerly wavesof great amplitude might trigger the formationof equatorial disturbances, which may contributeto the monsoonal fluctuation as described previously. In this case, the problem may arise of
how the westerly waves affect the equatorial flow
through the "critical latitude".* Many theoreticalstudies have been made on this problem. For
In this paper,the "criticallatitude"means
thelatitudeat whichthe phase speed of a westerly
wave is equal to the mean zonal flow.

June

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example, M. Murakami (1974) used a model including non-linear wave interaction with the zonal
flow to show that the equatorial Kelvin wave
mode can be instigated by the westerly wave.
But it remains to be proved whether the modeof the equatorial wave suggested here (around15-day period, wavenumber of 4 to 6) can be
excited. Additionally, more attention should be
paid to the fact that the Tibet-Himalayas as astrong heat-source region for the monsoon systemlies under the influence of the mid-latitude
westerlies. As is commonly supposed (Flohn,
1968; Hahn and Manabe, 1975 etc.), cumulus
convections over this region play an importantrole in the maintenance of the monsoon circulation. However, as examined in this paper, theconvective activity over this region varies depending majorly on the westerly wave movements.
In this context, the mid-latitude westerlies are
considered to be one of the main controlling
factors of the fluctuation of the monsoon system.
Here, such questions may arise as the interrelatedness of the two dominant modes. One may
suppose that the mode of the 40-day period results
simply from large amplitudes of the fundamental
mode of about 15-day period which appear every
two (or three) periods. The difference in phasespeed of northward propagation of the two modes
and in the zonal scale of the equatorial wave
related to each mode is evidence that these twomodes are independent each other, though further
discussion may be needed. In this sense, theyear-to-year features of the fluctuations of thesemodes should also be examined in connection
with the activity of monsoon for each year.
Moreover, to discuss the cloudiness fluctuations
related to the dynamics and energetics of themonsoon, a similar analysis should be made byuse of three-dimensional data on wind, temperature and moisture fields. The forthcoming
MONEX is expected to reveal more completely
the structures of the large-scale monsoonal
fluctuations treated in this paper.
Acknowledgements
The author wishes to thank the referees fortheir valuable suggestions and comments on therevision of this paper. He would also like to thankProf. C. Nakajima for his steady encouragement
and helpful advice throughout the work. Thanks
are due to Prof. I. Hirota for his critical readingof the manuscript. The author is also indebted toProf. S. Fujinami and Dr. Y. Miyake of Kyoto
University of Industrial Arts and Textile Fibers

241

for their great support
data of cloudiness.

in arranging

the digitized

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北 半 球 夏 季 モ ンス ー ン時 に お け る雲 量 変 動 の解 析
安

成

哲

三

京都大学東南アジア研究セソター
海 陸 を 含 め た 広 域 モ ソス ーン の 変 動 を解 析 す る手 段 と して,気 象 衛 星 写 真 は きわ め て 有 効 と思 わ れ る。 この 論 文 では,緯 度1*,経度2*ごとに 読 み と った 可 視 域 の 輝 度 デ ー タを も とに,北 半 球 夏 季 モ ン ス ー ン時(1973年6月‑9月)の雲 量 変 動 を東 半 球 域 に つ い て解 析 した。
ス ペ ク トル 解 析 の結 果,約40日と約15日 の 長 周 期 変 動 が 卓 越 して い る こ とが わ か っ た。

時 間 断 面 解 析,時 差 相 関 解 析,位 相 差 ベ ク トル 解 析 等 の 手 法 を 用 い た 結 果,約40日

周 期 の変 動 の位 相 は,イン ド亜

大陸 か らベ ン ガ ル湾 を 中 心 とす る ア ジ ア南 西 モ ンス ー ン域 で 顕 著 に北 上 し,ア フ リカや 中 部 太 平 洋 域 で は む し ろ,南下 す る様 相 が 明 らか とな った 。 こ の変 動 は,広 域 にわ た るモン ス ーン 活 動 域 の 変 動(又動)に 対 応 し てい る。 約15日 周 期 につ い て も,同 様 の位 相 の動 きが認 め られ るが,特け て の 地 域 で は,ベはITCZの

南 北 方 向 の変に イ ン ドか ら 中部 太 平 洋 域 にかンガ ル湾 と,フ ィ リ ピン付 近 を 中 心 とす る2つ の時 計 回 りの 回 転性 の動 きが 特徴 的 で あ る。 また,南 半 球 で は,オ ー ス トラ リア の東,西 部 の海 洋 上 に お い て,中 緯 度 へ と南 下 す る位 相 が現 わ れ て い る。 これ ら の
変 動 は,発 達 した モ ンス ー ン(又 は熱 帯)じょ う乱,中 緯度 偏 西風 じ ょ う乱 の相 互 に 関連 した動 きに対 応 して い る。

上 記 二 つ の モ ー ドの変 動 の力 学 的 説 明は 未 解 決 で あ るが,約40日
ス ト存 在 下 に お け る赤 道 域 の 大 規模 東 西振 動 の役 割 が,約15日
方 向 の相 互 作 用 が,そ れ ぞ れ重 要 な 課 題 と考 え られ る。

周 期 につ い ては,南 北 方 向 の海 陸 の熱 的 コ ン トラ

周 期 に つ い て は,中 緯 度〜 赤 道 域 に お け る波 動 の 南北