June1980

T. Yasunari225

NOTES AND CORRESPONDENCE
A Quasi-Stationary
Appearance of 30 to 40 Day Period in theCloudiness Fluctuations during the Summer Monsoon
over India
By TetsuzoYasunari
TheCenterfor Southeast
AsianStudies,KyotoUniversity,
Kyoto606(Manuscript
received18 March1980,in revisedform6 May1980)
Abstract
By using the cloudiness data from 1966 to 1972, a quasi-stationary
appearance of30 to 40 day period was confirmed during the summer monsoon season over India, whichis recently found in the data for 1973 as a predominant mode of the monsoon fluctuations(Yasunari, 1979). In the same manner as the result of Yasunari (1979), the fluctuation ofthis mode shows a northward movement over India-Indian Ocean area in each year. This
periodicity seems to appear annually except for the severe drought years such as 1972.

1.

Introduction

1966 through 1972, to see the year to year
stationarity of the periodicity of around 40 days.

This is an extention of the previous study by
2. Data
Yasunari (1979), which will be hereafter be
The global daily cloudiness data (30*-30*)
referred to as Y79. The cloudiness fluctuations
associated with the Northern summer monsoon from February 1965 to July 1973 were kindly
were examined in Y79 using the data for 1973 offered by Prof. Sadler of University of Hawaii.
obtained from meteorological satellite pictures. The cloudiness values were determined at every
It was shown that a periodicity of about 40 days 2.5* latitude-longitude grid square from 0 to 9
exists predominantly and that of around 15 days levels, as defined in Sadler (1969). By use ofis also apparent as a second mode. The fluctua- these data, areas averaged cloudiness was comtion of 40-day period appeared as simultaneous posed for each square block of 5* latitudex 10*
northward movement from the southern equa- longitude to eliminate small scale cloud disturbtorial zone to the zone of about 30*,
which ances. As the data for 1965 have a larger amount
was most distinct over the India-Indian Ocean of missing data (15-30%)
compared to othersector. In contrast, that of around 15-day period years (less than 8%), those are excluded fromincluded two clockwise rotations, one over India the present analysis. The same cloudiness dataand Southeast Asia, and the other over the used in Y79 are also referred to as the data for1973.
western Pacific.
Several authors already commented on the3. Spectral analysis of monsoon cloudiness for
15 day (or quasi-biweekly) period appearing
the years from 1966 to 1973
in the Asian summer monsoon (e.g., Krishnamurti and Bhalme, 1976; Murakami, 1976 etc.)
Spectral analysis by the maximum entropy
from the data of different years. However, it method (MEM) was applied to the daily cloudihas remained unknown whether the mode of 40- ness data for the period from June 1 to September
day period happened to appear in 1973 or may 30 (122 days) of the years from 1966 to 1973.
appear stationarily in every year.
As is well known already, this method has an
In this paper, the daily cloudiness fluctuations advantage of giving high resolution, especiallyover India - Bay of Bengal region are examined for short time record (e.g., Ulrych and Bishop,
for the summer months (June to September) of 1975; Hino, 1977 etc.). In the present analysis,

226

JournalFig.

1of theSelectedareaforspectral
The area enclosedby dashedadoptedfor 1973.

MeteorologicalSocietyofJapanVol.

58, No.

3analysis.
lines isthe length of the prediction error coefficient of
25 days was adopted. As a representative areaof Indian summer monsoon, the square area overIndia through the Bay of Bengal (10*-20*,70*-100*
but 72*-96*E in the case of 1973)
was selected as shown in Fig. 1. Mean cloudiness of the square area was obtained by averaging the data of the six unit blocks as mentioned
in the previous section.
The results of power spectrum computations
for the years from 1966 to 1973 are shown inFig. 2. The area of the cloudiness measured for1973 is a little different from the other years asshown in Fig. 1, because of the difference ofsize of a unit block (refer to Y79). In all of theyears except 1972, the periodicities of 30 or 40days exist as a predominant mode. This mode Fig. 2 Piled diagrams of power spectra of cloudiness for 1966 through1973. Units are
seems to be at 30 or 40 day period with every(cloudinessvalues2.day).
The
spectral bands
passing year. In 1972, in contrast, the mostof 30 to 40 day period and 10 to 15 day
dominant mode is found at about 60 day period.
period are indicated by dotted lines.
It is noteworthy that this region (around India)
was subject to a severe drought in this year. The
drought appeared over northern and central scales.
India associated with abnormally long period of"break" from the middle of July to the beginning 4. Cross sectional
analysisof cloudinessalongof August (Kanamitsu and Krishnamurti, 1978).
Therefore, the longer periodicity of 1972 relative to other years may be attributed to the longlasting "break" of this year. Fig. 2 also shows
the yearly stationarityof the quasi-biweekly
mode over this region during the summer
monsoon season.
No other distinct modes common to most of
the years were found in the shorter period range
in Fig. 2, mainly by the filtering effect of therelatively large averaged area (10* lat. X 30*long.) for cloud disturbances of small time-space

theIndia-Indianfrom

1966

As is noticedof about

40-daymovement

overmostapparentlysector.

To

Oceantosectorfor

theyears1972

in Y79,thecloudinessfluctuation

periodshows

a markednorthward

thewholeoverexamineAsianthethismonsoon

India-Indianfeatureforeachyear,

thetime-latitudefilteredcloudinesscrosswerelongitudinal

sectorof70*-90*.

Theband-passfilters

determined

tohaveamaximum30-or40-day

frequency

wereresponseatasections
ofcomposed

area;
Ocean

band-pass
overtheperiod

June

1980

T. Yasunari

Fig.

3

Time-latitude1972. Unit

cross sectionsof contourline(or 60-day period in the case of 1972) corresponding to the frequency of maximum frequency
for the respectiveyears.

227of filteredis 0.5 and

The
are

cloudiness
fornegative values

resultsshownmovement

1966 throughare shaded .

fortheyearsin Fig.

3.

Inoffilteredthefromeveryyear

cloudiness

1966

to1972a northward
canclearly

228

Journalof

theMeteorologicalSocietyofJapanVol.

58, No.

3

be seen with the phase speed of about
1*
latitude/ day.
As mentioned
already,
the dominant periodicity
of 1972 is at about 60 days, but
a northward
movement of cloudiness can be seen
similarly
to those of the other years.
These
results are consistent with that of Y79.
5.

Conclusion

Throughdata fromandremarks

the analysis of the daily cloudiness
1966 to 1972, it is confirmed that theperiodicityof 30 to 40 days appear as a predominantmode in all the years except
1972
during the summer monsoon months over India.
As for the cases of previous years to 1966, wehappened
to find the dominancy
of the same
periodicity
in the rainfall
fluctuationsin 1962
through
1964 in the figure of Ramage (1971),
as quoted here in Fig. 4. It is noted in Fig. 4that the weekly percentage
anomaliesof rainfall
over northernand central
India(18*-27*)
fluctuate
periodicallywith roughly
a 30 to 40
day period, and that the daily pressure anomalies
at Jacobabad(as indices of the intensity of heat
low over Pakistan)
are roughly negatively
correlated to the rainfall anomalies.
Power spectral
analysis of rainfall anomalies in Fig. 4 confirmed
the dominancy
of the 30 to 40 day period asshown in Fig. 5.

Fig. 4

Daily anomalies(mb) of mean sea-levelpressure measured at Jacobabad (2801 8'N,68°28'E; full lines) and weekly percentage
anomalies of rainfall for Indian subdivisions lying roughly between 18° and 27°N
(dashed lines) for the summer of 1962,
1963, and 1964 (after Ramage, 1971).

Fig. 5

Power spectra of weekly anomalies rainfallin Fig. 4. Units are (percentage2•day). The
spectral band of 30 to 40 day period isindicated by dotted lines. Power spectra forless than 2 weeks are omitted, as theoriginal time unit of data is 1 week.

These results suggest that the periodicity of
30 to 40 days related to the summer monsoon
as described in Y79 exists quasi-stationarily as adominant mode at least for normal (or active)
monsoon years as well as the quasi-biweekly
mode. The cloudiness fluctuations in 1972 exceptionally show the mode of larger period (ofabout 60 days), which may be concerned withthe abnormally weak monsoon of that year. It
is well known that 1965 was also a severe
drought year as well as 1972, but unfortunately
we could not obtain the exact estimates of dominant modes of that year due to a lot of missing
data. Further study will be needed on theassociation between the dominant periodicities
and the monsoon activity of each year.
As commented already in Y79, the major"active" and "break" cycles of the monsoon
activity seem to be related more closely to theperiod range of 30 to 40 days than the quasibiweekly mode. From this point of view, thetime-spatial structure of the monsoon circulation
system over and around India are being investigated relevant to the fluctuation of this period
range. The results of this study will be reportedsoon.
Acknowledgements
The author wishes to express his hearty thanks
to Prof. J. C. Sadler and Prof. T. Murakami ofUniversity of Hawaii, for their kind arrange-

June

1980

T. Yasunariment and supply of the cloudiness data. Hewould like to thank Prof. C. Nakajima of Kyoto
University for his encouragements throughout
the work.

229

References

Murakami,M., 1976:
Analysis of summer
monsoonfluctuations
over India.
J. Met. Soc. Japan,
54,
15-31.
Ramage,C. S.,
1971:
Monsoonmeteorology.
AcademicPress, 36-38.
Sadler,J. C.,
1969:
Averagecloudinessinthetropics
from satellite
observations.
Int. Indian
Ocean Expedition,Meteor.
Monogr.
2.
Ulrich,T. J, and T. N. Bishop,
1975:
Maximum
entropyspectralanalysisand autoregressive
de-Hino, M., 1977: Spectral analysis (in Japanese).
Asakura Shoten, 210-226.
Kanamitsu, M. and T. N. Krishnamurti, 1978:
Northern summer tropical circulations during
drought and normal rainfall months. Mon. Wea.
Rev., 106, 331-347.
Krishnamurti, T. N. and H. N. Bhalme, 1976:
Oscillations of monsoon system. Part I, observational aspects. J. Atrnos. Sci., 33, 1937-1954.

composition.
13, 183-200.
Yasunari,T.,sociatedmonsoon.

Rev.
1979:

with theJ. Met.

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イ ン ド周辺 の 夏 季 モ ンス ー ンに お け る雲 量 変動 の30〜40日 周 期 の 準 定 常 的 出 現安成哲三

京都大学東南 アジア研究セ ンター7年
1973年

間(1966‑1972)にわ た る 雲 量 デ ー タ の 解 析 に よ り,イに つ い て の 結 果 と 同 様 に,30〜40日

ソ ド周 辺 の モ ン ス ー ン の 変 動 に は,Yasunari(1979)の周 期 が ほ ぼ 毎 年 卓 越 し て 現 わ れ,そる 様 相 で あ る こ とが 示 さ れ た 。 こ の 周 期 性 は,モの 位 相 の 動 き は ,イン ス ー ン が 非 常 に 弱 か っ た1972年

ン ド洋 上 を 北 上 すに のみ 現 わ れ て い な い。