336

Journal

StructureoftheMeteorologicalof an IndianaroundSummer40-Dayby Tetsuzo

Societyof JapanMonsoon
PeriodSystemVol. 59,

No. 3

with

Yasunari

The Center for Southeast Asian Studies, Kyoto University, Kyoto 606, Japan(Manuscript received 9 September 1980, in revised form 1 February 1981)

Abstract
Through the analysis of the geopotential and wind fluctuation during the summer
monsoon season over and around India, the predominant periodicity of about 40 days was
confirmed especially in the lower and upper troposphere as a major active-break cycle of themonsoon activity. As well as the cloudiness fluctuation of the same period (Yasunari, 1979;
Yasunari, 1980), these elements except the geopotential height in the upper level showed amarked northward propagation from the equator toward the Himalayan region. The geopotential height at 200 mb showed, in contrast, a standing oscillation over India. Cross-spectral
analyses revealed that the vertical structure of the atmosphere is gradually modified during
the northward phase shift of the anomaly cloudiness with the 40-day period. This modification is most apparent in the temperature field. The modification of the vertical structurefrom the south to the north over India seems to be attributed to the gradual enhancement of
cumulus convection from the south to the north. It is also suggested that this cycle is causedby the periodical cold air outbreak over the middle and upper troposphere of the equatorialzone which may be associated with the index cycle of the westerly wave movements in theSouthern Hemisphere.
Finally, a multicell structure of the anomaly Hadley circulation regime
over India through the Indian Ocean was proposed, which gives reasonable interpretations on
various aspects of the active-break cycle of monsoon activity with this mode.

1.

Introduction

Through the analyses of cloudiness fluctuations
by the present author (Yasunari, 1979; Yasunari,1980), a predominant periodicity of 30 to 40 days
was confirmed as a quasi-stationary mode over
and around India during the summer monsoon
period. The fluctuation of this periodicity showsa marked northward phase propagation from theequatorial zone to the mid latitudes (around 30°E)
over Asian monsoon area, most apparently overIndia-Indian Ocean sector.* The fluctuation ofthis mode seems to be correspondent with the
major active-break cycle of monsoon activity. It
also seems that the cloudiness fluctuation of thisperiod appears as a periodical northward shiftof ITCZ cloud band at the longitudinal sector
over India through Southeast Asia.
* Recently , Sikka and Gadgil (1979) found very
similar features by using the satellite cloudinessdata from 1973 to 1977.

It was also pointed out in the former paper(Yasunari, 1979) that the fluctuation in the meridional plane is closely related to the eastward
moving equatorial disturbance studied by Madden
and Julian (1971, 1972). Recently, Chang andWang (1976) found that the surface pressure fieldover ITCZ of the western Pacific also shows thedominant oscillation of the same period range(5 to 7 weeks).
The synoptic feature of each stage of the major
active-break cycle was formerly suggested byRahmatullarh (1952), though he did not commenton the time scale of the cycle. However, thehorizontal and vertical structure of the monsoon
circulation regime associated with this mode has
not yet been clarified.
Many studies have already described the difference between the "active and the "break" phasein the atmospheric field over India (e.g., Alexander et al., 1978; Hamilton, 1977, etc.). However, as pointed out already (e.g., Krishnamurti

June

1981

T. Yasunari

and Bhalme,
1976; M. Murakami,1976), there
exists another dominantperiodicity
of about 15
days as an active-breakcycle, and the spatial
structure of this period range shows a somewhat
different feature from the 30-40 day period mode
(Yasunari, 1979). As most of the previous studies
did not separate these two modes each other, thetime scales and related space structure of "break"(or "active")phases were sometimes
confusing.
As for the quasi-biweekly
mode,, a few studies(M. Murakami,
1976; Yasunari,1979) revealed
the spatial structureof the lower level over andaround India, though the complete three-dimensional structure
has not yet been analyzed.
In this paper, we focus to clarify the horizontal
and vertical structure of the fluctuation of about
40-day period,
especiallyin the latitude-heightsection over India.
Some considerations
on theequatorial
and the Southern
Hemisphericcirculations will also be made, which are supposed tobe the most important exciting force of this modeover India.
Finally, we will present one modelof the meridional(or local Hadley) circulations
along the line from India to the Southern Hemisphere associatedwith the active-breakcycle ofthis period range.
A brief description
will bemade on the structure of the quasi-biweekly
mode,
only for a comparison
with the mode of 30 to
40-day period.
2.

Data

337

the surface synoptic conditions. To cover thearea over Indian Ocean and Southeast Asia, theupper air data over Thailand, Vietnam, Malaysia,
Singapore and at Gan Island were additionallyused. The Southern Hemisphere 500mb charts(published by the Australian Bureau of Meteorology) were used for the analysis of the Southern
Hemisphere. The period for the analysis is chosenfrom June to September 1969, when the meanmonsoon activity is considered to be nearly normal. The stations over and around India usedin this study are shown in Fig. 1.
3.

Fluctuation of the 40-day period in the geopotential fielda) Spectral analysis of the geopatential field
The fluctuations in the geopotential (or pressure) field may be important indices of monsoon
activity, since it is well known that the monsoon
trough in the lower troposphere and the TibetanHigh in the upper troposphere change their intensity and position associated with the activebreak cycle.
To examine the dominant periodicities, powerr
spectral analysis of MUM method (e.g., Ulrich
ind Bishop, 1975, etc.) is applied to the timeseries of geopotential field during the monsoon
period (122 days from June 1 to September 30).
The advantage of this method was commented
already in the previous paper (Yasunari; 1979).
Maximum lag number of 25 days is adopted,

Daily upper air data over India (from "Aerological Data of India", India Meteorological
Department)
is a main data source for the present
analysis.
The surface weather
maps (from "Indian daily weather report") were used to examine

Fig.

1

Observation

stations

over

and

aroundIndia used in this study.
Cross section for
Fig. 5, 7, 9 and 14 is indicatedwith the linefrom Gauhati
and Gan.

Fig.

2

Vertical

distributionof the power

spectraof the geopotential
height at
Nagpur.
Units are m2• day and thevaluesmore
than
100 are shaded.

338

JournaloftheMeteorologicalwhile the total record length is 122 days. The
results of spectral analysis on the geopotential
field at Nagpur are shown in Fig. 2. A remarkable spectral peak of around 40-day period exists
at the lower level (700 to 850 mb), which seems
to be closely related to the oscillation of monsoon trough over northern India. The spectral
peak of around 15 days is also prominent throughout the troposphere.
b) Horizontal structure of the geopotential heightfluctuation
As already noted in the previous papers (Yasunari, 1979; Yasunari, 1980), cloudiness fluctuation of 30 to 40-day period shows a northward
phase shift most apparently over the longitudinal
sector of India-Indian Ocean area. Fig. 3 shows
the latitude time section of the filtered cloudinessfor 1969 along the longitudinal sector of 70*E9.0*E (refer to Yasunari, 1980). Precisely speaking, the phase gradually tends to become parallelto the direction of the mean monsoon trough or
the great Himalayan range over northern India,
as shown in Fig. 12(a) of Yasunari (1979). Basedon this result, we examined the cross sections offiltered pressure field along the line nearly normal
to the monsoon trough as shown in Fig . 1. To
separate the fluctuations of about 40-day period
and the quasi-biweekly period, two time filters
are adopted. The frequency responses are indicated in Fig. 4. The time cross-sections of thefiltered geopotential height at 850 mb are shownin Fig. 5(b) and 5(c). Five-day moving smoothedheight is also illustrated in Fig. 5(a). Fig . 5(a)shows the quasi-stationary existence of monsoon
trough over northern India, fluctuating quasiperiodically with a 10 to 20-day period but witha large "break" phase at some time . The intensities of its maximum (or minimum) phases arealso quite different from one to the other . It is

Fig.

3

Time

latitudesectionSocietyof JapanVol. 59,

No. 3

noteworthy that the mixed feature with a quasiperiodical and a random fluctuation in Fig. 5(a)can clearly be explained as the composite fluctuation of a northward movement of 40-day period(Fig. 5(b)) and a quasi-standing oscillation of
about 15-day period (Fig. 5(c) ). The oscillationof about 15-day period in Fig. 5(c) appears actually as a northwestward propagation of wave
from the Bay of Bengal toward northwestern
India nearly normal to the section line in Fig. 1.
The difference of propagation between the two
periodicities mentioned above was confirmed in
the spatial distribution of phase lag obtained by
the cross-spectral analysis (Fig. 6). In Fig. 6, thereference point is chosen at Nagpur in central
India.
Krishnamurti and Ardanuy (1980) noted thatthe surface pressure fluctuation of the 30 to
40-day period is correspondent with the eastward
propagation of zonal wave number 1 and 2 at
the monsoon trough latitudes. The phase-lag
distribution of the 40-day period in Fig. 6 may
be compatible with their observation.

Fig.

of filtered4
Frequency
responseof the two
The peak of filter 1 exists at 40-day
and that of filter 2 is at aboutcloudinessperiod along the longitudinal
sector of 70*-90*E.
line is 1.0 and negative values are shaded.

far

the

40-dayUnit of contour

filters.
period14-day period.

June

1981

T. Yasunari339

Fig. 5 Cross sections of the geopotential height at 850 mb along the line shown in Fig. 1,
by using (a) 5-day moving averaged data (b) data by filter 1, and (c) data by filter 2.
Contour intervals are (a) 20 m, (b), (c) 5 m. The areas of negative values are shadedin (b) and (c).

It was remarkably noticed that all the monsoon depressions* of this year (as indicated with
"D" in Fig . 5(b) and 5(c) appeared uniquely inthe phases of negative pressure anomaly of the
40-day period as well as the biweekly period.
Additionally, the two intense depressions, whosepressures around the center area indicate morethan 8 mb below the normal, occurred just at theminimum phases (e.g., July 30-31 and September
7) of the 40-day period.
In contrast with the feature of 850 mb level,
the filtered geopotential height of 200 mb level
along the Gauhati-Ganline (refer to Fig. 1)changes its sign nearly-simultaneously from 30*N
to the equatorial zone, as shown in Fig. 7. PreFig. 6 Phase lag relations of the geopotential
cisely speaking, the center area of the maximum
height at 850mb for the 40-day periodvariance exists at 15*-20*N zone and the vari(solid lines) and the 13.3-day period (dashedance is relatively small to the north and southlines). The reference point is located atof it or it has the opposite sign to the center area.
Nagpur.
That is, the geopotential height in the upper levelshows a standing oscillation with the maximum
variance over central India. If we compare Fig. 7 pled with the trough (or ridge) at 850 mb. However, when we made the anomaly data from thewith Fig. 5(b), it is evident that the maximum
daily mean height along the section line (refer(or minimum) height over central India is cou- to Fig. 1) and then composed time section with
Following the definition by Rao (1976), the low the same manner as Fig. 7, we would noticepressure system is referred to as "monsoon depres- though weakly the tendency of the northwardsion" when surface winds exceed 17 kt over the phase shift of the anomaly height (not shown).
sea.
This implies that during the course of the north-

340

JournalFig.

7
Cross section of theFig. 1. Contour
intervalof theMeteorologicalfilteredgeopotential
is 20m.
Negativeward shift of the transientanomaly
the arealmean field changes
with far larger
amplitude
than the transient anomaly.
In other words, there
may exist the interaction
between the transientfield and the (areal) mean field with the maximum rate over central
India.
4.

Fluctuations
fieldof the 40~day period

in the winda) Spectral analysis of the wind field
The mean monsoon circulation is characterizedprincipally as a meridional (or Hadley type) circulation containing the southwest monsoon flow
and the easterly jet in the lower and upper troposphere respectively. Consequently, the wind fieldshould be an important indicator of the fluctuation of the monsoon circulation.
The MEM spectral analysis was applied to thewind field at some stations, to check the dominantperiodicities in the wind fluctuation. We selected
the data of T'rivandrum, Nagpur and Calcutta,
as a representative point of southern India, central India and the monsoon trough region ofnorthern India, respectively. The results of thezonal and meridional wind component at thethree stations are shown in Fig. 8 in terms of thevertical distribution of power spectral densities.
At Trivandrum the strong concentration of power
at 40-day period is prominent at around 200 mb
and 700 mb levels in the zonal wind component.
The secondary maximum is seen at the wide
period range from 10 to 20 days both in theupper and lower troposphere. The similar featureon the 40-day period exists in the meridional
wind component. In the mean state of the summer monsoon period, Trivandrum is located nearly under the center core of the easterly jet in
the upper troposphere.
Henceforth, the total
spectral feature of wind field over this stationshows that both southwest monsoon flow in thelower troposphere and the easterly jet in theupper troposphere fluctuate predominantly withthe 40-day period. At Nagpur the total feature
is similar to that of Trivandrum, though the powerSocietyofJapanheight at 200 mb
areas are shaded.

spectraldensities

along

are

Vol. 59, No. 3

the

line

shown

in

distributed

with

a

wide

period range (longer than 40 days) in the low
frequency band. This may be due to a relatively
large variance of the seasonal march component
over this area. At Calcutta the concentrationofpower is seen only in the lower troposphere(700850 mb level), which may be attributed
to therelatively small variance of the upper wind overthere.
The spectral
peak in the quasi-biweekly
period is also prominent
mainly in the lower halfof the troposphere.
At Nagpur
and Calcutta,
the power densitiesof the quasi-biweekly
mode are relatively large
in the zonal component,
which may suggest that
this mode is related to the frequency
of someweather phenomena
dominatingover and aroundthe monsoon trough region such as the monsoon
depressions.
b) Horizontal

structureof the wind fluctuation

Next, we examined the horizontal
structure of
the 40-day period of the zonal wind component
along the same cross-section
as Fig. 5, at thelower troposphere(850 mb) and upper troposphere (200 mb). The result of 850 mb is shownin Fig. 9(a). Very similarly to Fig. 5 of the geopotential field, the phase of the fluctuation of thezonal wind shows a distinct northward
movement.
It is apparent
in Fig. 5(b) and Fig. 9(a) that the
maximum westerly (or easterly) and the minimum(or maximum)
geopotential
height are roughly inphase over the 10*-20*N latitude zone.
The cross-sectionof the filtered zonal wind at
200 mb is also shown in Fig. 9(b). Very similar
to the feature of the lower level, a northward
movementof the filtered wind could be depicted
most distinctly to the south of about 15*N. The
disappearanceof the northward
propagationtothe north of it may be closely connectedwiththe dominancy
of the standing oscillationof thegeopotential
in Fig. 7.

heightover centralIndia

as shown

It should be noted that the phase relationshipbetween the lower and the upper levels changes

June

1981

T. YasunariFig. 8 Vertical distributions of the power spectra of the zonal (left) and the meridional (right)
wind components for (a) Trivandrum (b_)Nagpur and (c) Calcutta. Units are (m/sec)2•day
and contour intervals are 200 (solid line) and 100 (dashed line).

341

342

JournaloftheMeteorologicalSocietyof JapanVol. 59, No. 3

Fig. 9 Cross sections of the filtered wind component along the line shown in Fig. 1 at(a) 850 mb (b) 200 mb. Contour interval is 2m/sec
and negative areas are shaded.

easterly) flow occurs in the lower level the maximum easterly (or westerly) counterflow appears
about 10 days later in the upper level. It is wellknown that the "active" (or "break") phase of
monsoon circulation regime over central or northern India (around 20*N) is associated with thewesterly maximum (or minimum) of the lower
level monsoon flow. At the same stage, the upperpropagation (or preferably to show standing oscil- level easterly wind over south India is also maxilation), while the lower level anomaly still propa- mum (or minimum). The distribution of the zonal
gates northward. This may suggest that the ver- wind in Fig. 10 (upper) is consistent with thetical structure of the wind field gradually changes typical "active" and "break" phases mentioned
during the northward shift of the anomaly circu- above. It also seems that the altitude level ofthe maximum zonal wind in the lower-half tropolation regime.
sphere changes in the course of the seasonalc) Vertical structure of the wind fluctuation over march from June to October. The time crossTrivandrurn
section of the meridional wind (Fig. 10, lower)
As mentioned already, the wind field over Tri- clearly shows that the wind regime of thisvandrum is considered to represent the fluctuaperiod range in the upper and lower half tropotions of both the southwest monsoon flow in the sphere is different each other. The upper windlower troposphere and the easterly jet in the upper regime shows a feature of the prominent upward
troposphere (refer to Fig. 8(a) ). In this subphase tilt while the lower one shows the downsection, the vertical structure of the wind fluctu- ward phase tilt.
ation will be examined on the periodicity of 40
In the lower level the variance of the anomaly
days. The vertical time cross-sections of the fil- wind depend mostly on the zonal wind compotered zonal and meridional wind component are nent. In the upper level, in contrast, the meridioshown in Fig. 10. A quite different feature of nal wind component is relatively large comparedfluctuation is found in the zonal and meridional to the lower level, which is also seen in the power
wind component. In the fluctuation of the zonal density as shown in Fig. 8(a). It is of interest
wind component, a downward phase movement that in the upper level (especially at 200 mb)
of anomaly wind is apparent with the maxima the maximum northerly (or southerly) wind ap(or minima) in both 200 mb (or higher) and pears about 90* out of phase with the westerly
around 700 mb levels. It should be noted, how- (or easterly) maxima. That is to say, the anomaly
ever, that the actual mean zonal wind is westerly wind vector related to the 40-day period showsin the lower half and easterly in the upper half a clockwise rotation at the easterly jet core level.
troposphere. That is, the wind regime is different At around 300 to 400 mb level, on the contrary,
between the lower and upper half troposphere. the maxima of westerly (or easterly) and the
Additionally, if we take account of the Hadley maxima of northerly (or southerly) are roughly
type circulation over India., it may be preferable
correspondent in their phases each other, which
to express that after the maximum westerly (or means the anomaly wind vector alternates its
from the south to the north. In the equatorialzone the maximum lower westerly (or easterly)
and the maximum upper westerly (or easterly)
are well in phase each other. During the northward propagation of the anomaly system somephase lag seems to appear between the two levels.
To the north of central India (about 15*N) the
upper level anomaly seems to show no systematic

June

1981

T. Yasunari343

Fig. 10 Vertical cross sections of the filtered zonal (upper) and meridional (lower) windcomponent over Trivandrum.
Contour interval is 1m/sec, and negative areas are shaded .

direction from NW (SE) to SE(NW) abruptly.
A clockwise rotation of the anomaly wind vector may remind us of the effect of the BruntDouglas' issalobaric wind tending to adjust thestanding oscillation of the geopotential field between the areas over Indian subcontinent and theIndian Ocean, as referred to Fig. 7. However,a rough estimation by using the geopotential dataof 200 mb level at Vishakhapatnum, Madras andTrivandrum suggested that the magnitude of theissalobaric wind is too small to explain the total
variance of the actual meridional wind component. It is more probable, therefore, that theclockwise rotation of the wind vector is resulted
from the northward movement of the anomaly
circulation regime. Additional discussions will
be made later.

mb or 700mb level. Then, the relative phase
positions of the maxima (or minima) of zonal
wind, geopotential, temperature and humidity are
rearranged with reference to the maximum cloudiness, to exhibit distinctly the difference of thetime-spatial structure related to the "active" or"break" phase of eachstation. The result foreach station is shown in Fig. ll.
As mentioned previously, the phases of thegeopotential and zonal wind fluctuations of thisperiod range at the lower troposphere show distinct northward movements (refer to Fig. 5 andFig. 9). Henceforth, the difference of the verticaltime structures of several elements at the threestations from south to north can be identified as
the transformation process of the space structureaccompanied by the northward phase movementof the maximum (or minimum) cloudiness. At
5. Composite time-space structure of the 40-day
Trivandrum, the phases of the temperature field
period over India
at each level changes from the lower to the upper
In this section, we will discuss the composite level. At the maximum (or minimum) cloudinesstime-space structure of the fluctuation of the the warm (or cold) core exists at the lower level40-day period, especially along the meridional(more than 850 mb), but the middle tropospherevertical plane over India. The inter-level cross- (700 to 300 mb) is just at the boundary fromspectral analysis was applied to verify the rela- cold (or warm) to warm (or cold) air , and thetive phase relationships among the geopotential, cold (or warm) core is apparent at 200 mb level.
temperature, wind and moisture fields of different The vertical trough (or ridge) line is located near
altitude levels, and cloudiness. The data at Tri- the cold (or warm) maximum (or minimum) ofvandrum, Calcutta and Nagpur are again chosen the middle troposphere. In contrast, at Nagpur
respectively as representative points of southern, the maximum cloudiness is accompanied by thecentral India and northern India (or monsoon cold core in the lowermost troposphere (850 mb
trough area). The relative phase relations of the level) and the relatively warm air above it . The
40-day period among the maxima (or minima) maximum westerly and trough line in the lowerof different elements were calculated at these troposphere are also followed almost in phase
three stations with reference to the wind at 850 with the maximum cloudiness. Unlike the case344

Journalof theMeteorologicalSocietyof JapanVol. 59,

No. 3

at Trivandrum,
the maximum
cloudinessexistsunder the minimum
tonalwind (the strongest
easterly wind) in the upper troposphere.
At Calcutta the featureof the temperatureand g,eopotential field is similar to Nagpur, but the penetration of the cold (or warm) air under the warm(or cold) air becomes more distinct at the maximum (or minimum)
cloudinessstage.
In addition, the troughin the lower level is clearly
coupled with the ridge in the upper level at the
maximum cloudinessstage. That is, the air column is warmest at this stage. The zonal windat the maximum (or minimum) cloudiness is intermittent or weak easterly in the lower level, probably because the westerly flow is forced to change
to the easterly around here by the effect of theorographyof the Arakan and the Himalayas.
Ateach station the phase of the maximum (or minimum) moisture at the lower troposphereis roughly in phase with the maximum(or minimum)
cloudiness.
As commented already, the periodicity of about
two weeks dominantly
appears over the monsoon
trough resion. Here, we will compare the structure of the 40-day period with that of the about
two weeks (13.3 days). Fig. 12 shows the composite structureof the 13.3-day period over Calcutta. A salient feature is the vertically-standing
trough (or ridge) line, while that of the 40-day
periodshows the (southward)
tilting (refer toFig. 11(c) ). It is noteworthy
that the composite
structureof the 13.3-day period is comparable
totally to the synoptic structureof the monsoon
depression analyzed by Krishnamurti
et al. (1975).
For example, the cold core is apparent
below
500 mb and there is a reversal of thermal struc-Fig. 11 Vertical structures of the geopotential height(thick solid lines), zonal wind component (thinsolid lines), temperature (dashed lines) and moisture (dotted lines) for the 40-day period in reference to the maximum cloudiness over (a) Trivandrum (b) Nagpur and (c) Calcutta,

Fig.

12
period

Same
over

as

Fig. 11 but

Calcutta.

for

the

13.3-day

June

1981

T. Yasunariture above that level. They pointed out that the
influence of the storm in the vertical structureof the zonal wind is noted up to 400 mb. Fig. 12
suggests the similar result to their description.
Since the easterly wind is dominant in the mean
field above 300 mb, the westerly anomaly windof these levels over the trough may represent therelatively weak easterly wind over there. Besides
most of the minimum phases of 850 mb heightanomaly for the biweekly mode (see Fig. 5(c) )embed respectively one monsoon depression. Howver, the phase lag relation of the same heightanomaly (refer to Fig. 6) suggests that the horizontal scale of the biweekly mode (N 10,000 km)
is far larger than that of one monsoon depression(. 2,000 km). Some other studies (Krishnamurti
et al., 1977; Krishnamurti and Ardanuy, 1980)
presented the evidences that this mode is attributed to the westward propagating waves with thescale of 810,000 km. Murakami (1976) also
pointed out that this mode is of large space scale
covering the whole area over the Bay of Bengal,
Indian subcontinent, Indo-China and Tibetan Plateau. It is sufficient, therefore, to understand thatthe composite structure in Fig. 12 may represent
the structure of some kind of wave or oscillationof large scale oriented along the line from Indochina via the Bay of Bengal toward the monsoon
trough area embedding one monsoon low or depression in each cycle. As commented already,however, whether each storm may develop to themonsoon "depression" or may remain at themonsoon "low" stage seems to depend principallyon the 40-day period oscillation.
We can notice some apparent change of thevertical structures, especially of the geopotential
and temperature field from the south to the north.
Most remarkable change is a reversal of the temperature anomaly at the lower and upper troposphere from southern India (Trivandrum) to central and northern India (Nagpur and Calcutta).
The trough-ridge
line throughout the tropospherealso shows gradual southward tilting to the higher
level, and finally over the monsoon trough region
the lower trough is coupled with the upper ridge.
This stage is identified as the most active phaseof monsoon over India with the well-establishedTibetan High and the intense monsoon trough.
That is, at the maximum cloudiness stage theupper level cold trough over southern Indiachanges to the warm ridge over central andnorthern India, while the lower level warm troughchanges to the cold trough. Since the upper level
warm ridge over central and northern India may

345

be formed by the upward transport of latent heatby cumulus convection,
it is suggested that the
cumulus convection
is gradually enhanced
fromthe south to the north with the northward
shiftof the maximum
cloudinessas shown in Fig. 3.
Here, the divergence
field was estimatedforeach altitude level over the square region by using
the zonal and the meridional
wind data of thefour stations at central India (Nagpur, Bombay,
Bangalore and Vishakhapatnum).
The areal mean
divergence
was computed
following the Gauss'theorem:

where S is the area of the square, Yn the windvector component
normal to a side of the square,
Vni the mean of Vn at two stations for ith side,and [ ] denotes the areal mean. To reduce theobservational
error, the smoothed wind data of
5-day runningaverage were adopted.
The cross
spectrabetweenthe divergencefor each level
and the areal mean geopotential height and cloudiness were, then, computed.
Fig. 13 indicates thephase diagram in reference
to the trough at 850
mb obtained
for the 40-day period.
The trough(or ridge) of 850 mb corresponds
well with the
maximum
convergence(or divergence)in thelower-halftroposphere
and with the maximum
divergence(or convergence)in the upper half
troposphere.
This implies
that the maximum
cloudiness over central India represents the maximum upward
motion in the whole troposphere

Fig. 13 Vertical structure of the divergence fieldover central India (the area encircled by
Nagpur, Bombay, Bangalore and Vishakhapatnam) for the 40-day period. The position of the 850 mb trough (and ridge) at
nearly the center of the area is also indicated.

346

Journal

Fig.

of

the

Meteorological

14 Cross section of the filtered
the line shown in Fig. 1. Contour
negative

areas

Societyof Japanzonal wind shear
intervals
is 5m/km

Vol. 59, No. 3

along
and

are shaded.

relevant to this periodicity. These features suggest that the conversion from available potential
energy to kinetic energy (*T'<0)
in the middle
and upper troposphere gradually increase in thecourse of northward shift of the maximum cloudiness and may reach its maximum over central
India through the monsoon trough region. The
appearance of the low level cold core at the maximum cloudiness, may also be an evidence for thedominancy of cooling effect of raindrop evaporation resulted from the enhanced cumulus convection.
As mentioned above, the composite verticalstructure of temperature, geopotential and windfield for the 40-day period is modified drasticallyfrom the south to the north over India by deepening and enhancement of cumulus convection. It
may be speculated that the northward shift ismainly attributed to the northward shift of the
maximum monsoon flaw in the lower level. It issure that the maximum cloudiness is well inphase with the maximum westerly wind in thelower level (i,e., monsoon flow) at central India(Nagpur). The maximum cloudiness at southern
India (Trivandrum) is, however, about 90* outof phase with the maximum westerly (namely at
the transitional stage from the easterly to thewesterly). This may suggest that not only thestrength of convection but also the physical process which induces the cloudiness (or rainfall) isdifferent each other over southern India and overcentral or northern India. A brief consideration
will be made here on the actual process related
to the 40-day period. It is well known that during
the monsoon season over India especially overcentral and northern India the atmosphere of thelower level is conditionally unstable in a meanstate (e.g., Krishnamurti and Bhalme, 1976, etc.).
A penetration of relatively moist air into the lower
troposphere by the southwest monsoon flow overthe Indian subcontinent will cause the intensification of moist static instability. The shallow
convection by daily heating near the ground could

be a trigger to release the increased moist static
instability to develop deeper convections.
However, if we review the satellite cloud pictures forindividual days, it seems that the cloudiness fluctuation of the time scale treated here appears as
the slow northward
shift of the main route ofsuccessively passing cloud clusters with the horizontal scale of about 500 to 1,000km
or more.
The category of cloud clusters here includes monsoon lows or depressions
passing from the Bayof Bengal through northern India as well as eastward travelingcloud disturbancescrossing overthe equatorial
Indian Ocean and southern India..
Consequently,
as for the alternation of monsoon
activity of around 40-day period, we should nottake accountof a simple processof cumulus
convection by releasing conditional
instability but
the process of large scale upward
motion suchas the C'ISK mechanism.
It would be worthwhile
to state that the anomaly
of zonal wind shear
at 850 mb shows a northward movement as shownin Fig. 14.
The low-level convergencenecessary for initiating the CISK mechanism
may be provided
atthe stage of maximum
cyclonic shear. Actually,
the anomaly cyclonic shear is in good agreementwith the anomaly
positive cloudiness
in Fig. 3.
It is of interest that the maximum stage of anomaly cyclonic shear tends to occur at about 15 °N
and secondly at 0*-5*N region. The former region may correspondwith the cyclogenesis
areaof monsoon lows and depressions
over the Bayof Bengal, and the latter region is associated withthe main passage of the eastward
moving disturbances over the equatorial zone. It was alreadypointed out (Yasunari,ment of the equatorial

1979) that
disturbances

the develophas an im-

portant role to initiate the northward
shift of thecloudinessof the 40-daymode.
The verticalstructureat the maximum
cloudinessover Trivandrum, therefore,
may be affected by the equatorial disturbancespassing over there or to thesouth of it.

June

6.

1981

T. Yasunari

347

Some considerations on the equatorial disturbance and the Southern Hemisphere circulation

It is of interest to note that in Fig. 3 themaximum (or minimum) cloudiness over theequatorial zone occurs well in phase with the
maximum (or minimum)
cloudiness in theSouthern
Hemisphere
where
nearly-standing
oscillation is dominant over the area from theequator to 30*S. This may suggest that theformation or development of the major equatorial
cloud disturbances over the Indian Ocean isclosely linked with the mid-latitude westerly
disturbances and/or fronts in the Southern Hemisphere which affect even the equatorial zone.
To verify the influence of the Southern Hemisphere circulation on the equatorial disturbances,
the time series of some elements at Gan Island(0*41'S, 73*09'E) over the equator and Tananarive (18*54'S, 47*32'E) at Madagascar Island
were examined. Fig. 15 shows that the maximum
cloudiness (associated with the eastward moving
disturbances) and the maximum zonal windcyclonic shear over the equator occur at theminimum stage of the geopotential height andtemperature at 500 mb. The geopotential height
showed nearly the same tendencies throughout
the troposphere, which is also the case with
Trivandrum. In addition it should be noted thatthe tendency at 500 mb over Gan is highly correlated with that of the temperature
overFig. 16 Power spectra of the temperatureat
500 mb over Tananarive
by using MEM
method.
Units are (*)2• day.

Tananarive, which has the predominant periodicity of the 40 days as shown in Fig. 16. Thatis, the formation or development of the major
equatorial disturbances related to the 40-day
period seems to be triggered by the cold air
penetration in the middle (and maybe the upper)
troposphere from the Southern Hemisphere subtropics.
Here, in order to examine the long-term
fluctuation (>10 days period) at the Indian
Ocean sector, the cross section of smoothed
500 mb height for the mid-latitudes along 50°E
is composed as shown in Fig. 17. The perturbation shows a distinct time scale of 30 to 40
days especially at 20-30*S zone and the maxima
of cold stage at Tananarive (shown with thewhite arrows in the figure) from May to October
are well correspondent with the trough of around
20*S. It also seems that the regional index (assupposed in the intervals of the contours between
30*S and 50*S) has thee same periodicity andthat the cold stage coincides with the low index
phase.
Thus, we may speculate that the fluctuationof monsoon cloudiness with the 40-day period
is closely linked with the cold air outbreak associated with the large scale fluctuation of thewesterly wave movements in the Southern Hemisphere. Unfortunately, only a few studies havebeen made on the large-scale perturbations ofFig. 15 Time series of filtered cloudiness (thick solid the Southern Hemisphere mid-latitude circulaline) for the 40-day period, zonal wind shear tion including the consideration of time scales,(dotted line) over the equatorial zone (0°-5°N), mostly because of the insufficiency of the data.
temperature (solid line) and geopotential height(1965) analized the long waves of(dashed line) at 500 mb over Gan Island, and Anderssen
temperature
at 500 mb over Tananarive(solid wave number 1 to 6 by using the 500-mb geoline). Note that all the data except for the potential data. He commented that the waves ofcloudiness are smoothed with 7-day running wave number 1 is quasi-stationary in a roughmean and are plotted upside down.
sense but its most frequent phase velocity at348

JournalFig.

17 Crossof 50*E.

of theMeteorologicalsection of the geopotential
The data are smoothed

Societyof JapanNo. 3height at 500 mb along the linewith 9-day running
mean.
The

phases of minimum
temperature
at 500 mb in the 40-day
Tananarive
are also shownwith the white arrows.

50*S is nearly 10* longitude per day. Thisimplies that the transient part of wave number
1 has the inherent periodicity of 30 to 40 days.
He also noticed that there is some relationshipbetween the amplitude variation of the wave
number 1 and the development of cut-off lowsover south Africa.
Kao et al. (1970) madeanalysis of the kinetic energy of large-scale
motion in wave number-frequency
space.
Though they did not stress the dominantperiodicities of each wave mode, their results
show that the zonal and meridional kinetic
energies of the longest waves (wave number 1to 3) at the higher latitude (60S) seems to concentrate at the spectral band of 30 to 50 day
period especially in winter season. By utilizing
the EOLE satellite data set. Webster and Keller(1974, 1975) found the dominant periodicity of
18-23 days in zonal indices, momentum flux andmean and perturbation kinetic energies at 200 mb
level. However, their results on the wave number-frequency
domain also suggest the existence of longer period (>30 days) especially for
ultra long waves (wave number 1 and 2). At all
events, conclusive results have not yet been obtained on the time-space characteristics of the
mid-latitude circulation in the Southern Hemisphere.
The difference of the synoptic situation of theSouthern Indian Ocean between the maximum
cloudiness (i.e., formation of cloud disturbances)and the minimum cloudiness (i.e., no formationof cloud disturbances) over the equatorial zone
can be seen contrastively in the satellite pictures
as shown in Fig. 18. The successive 4 days
during the maximum cloudiness stage (Aug. 1619) and the minimum stage (July 21-24) wereselected by referring to Fig. 15. In Fig. 18(a)cloud clusters (with the scale of 500-1,000 km
or more) are continuously formed and developed
over the equatorial zone (0*-10*S), while in Fig.
18(b) cluster formation is rarely seen. It shouldVol. 59,

periodover

be noted that cold fronts (or frontal zones)
are elongated toward the equator in Fig. 18(a),while they are relatively short and are limited in
the mid-latitudes in Fig. 18(b). The occurrenceof cellular convective clouds is noticeable overa broad area at the back side of the fronts in
Fig. 18(a), but it is not seen or is limited in asmall area in Fig. 18(b). These features suggestthat at the maximum cloudiness stage (Fig. 18(a))
the cold air outbreak from the polar region toward the tropics is remarkably associated withthe extended trough of the westerly waves, whileat the minimum stage (Fig. 18(b)) the cold air
mass is not strong and the westerly waves are
confined within the mid-latitudes. In other words,
the maximum cloudiness stage may correspondwith the low index phase, and the minimum
cloudiness stage the high index phase of thewesterly waves, respectively.
The increase of convective instability by thepenetration of cold air in the middle (and upper)
troposphere over relatively warm and moist air
may facilitate the development of disturbances.
In addition, it is strongly suggested from Fig.
18(a) (especially from the case for August 16
and 18) that some dynamical interaction between
the extended troughs (or fronts) and the equatorial waves may be important for the formationand development of disturbances.
The equatorial disturbances once developed may trigger
the northward shift of monsoon cloudiness during
the eastward propagation
along the equator.
More discussion on this process will be madelater.
Although the analysis made in this section isstill preliminary, it is tempting to say that the
major cold air outbreak toward the lower latitudes associated with the index cycle (or vacillations) of the mid-latitudes in the Southern
Hemisphere should be a pace-maker of the 40day period in the monsoon activity.

June

1981

T. YasunariFig. 18 Quadrant mosaic pictures (ESSA 9, visible) of the southern hemisphere over the
Africa-Indian
Ocean sector' for the period of (a) the maximum, cloudiness (Aug. 16-19)
and (b) the minimum cloudiness (July 21-24) over the equator.

349

350

7.

JournaloftheMeteorologicalSocietyof JapanVol. 59, No. 3

Discussion

The mean monsoon circulation over Indiai
Ocean through the Asian continent is considerec
to be a northward-extended local Hadley cell o
the Southern Hemisphere. The branch of thedownward motion may be located at the sub
tropical high zone over the southern Indiar
Ocean, and that of the upward motion may bE
at the major summer monsoon area centerec
over northern
India through the Himalayas
These two branches are connected each otherby the cross-equatorial monsoon flow in the lowe]
troposphere and the easterly jet in the upper
troposphere.
However, it was revealed that the major
active-break cycle of monsoon activity occur
with the 40-day period associated with the northward movement of the maximum (or minimum;
cloudiness from the equatorial zone, and thatthe atmospheric structure over India shows aprominent change especially in the geopotential
and temperature field in the course of the northFig. 19 Schematic model of the 40-day period oscilward movement of the cloudiness. This oscillation of the anomaly Hadley circulation. The
lation was also supposed to be triggered throughmaximum stage of the anomaly wind directions
the formation of the equatorial disturbances by
are indicated in the circles. Relatively warm
the outbreak of the cold air mass followed byareas are shaded.
the major trough of the mid-latitude westerly
waves in the Southern Hemisphere.
with that described by Madden and Julian (1971,
Taking account of the total aspect mentioned 1972), which has a strong zonal circulation cells
above, one schematic model of the anomaly to the east and west of it. It is plausible, thereHadley regime is proposed
as a working fore, that as well as the intensified southern
hypothesis associated with the active-break cycle Indian Ocean high the development of theof about 40-day period. The model shows a disturbance over the eastern half of the equatorial
multicell structure along the meridional section Indian Ocean may cause the strengthening offrom the Himalayas to the sub-tropical high area the southwest monsoon flow over the Arabianover the Indian Ocean. The four stages of the Sea side.
oscillation are illustrated in Fig. 19, in the order
The cumulus convection may be intensified
as A-B-~C---D
and again from D to A cyclicly. during the northward shift of the upward portion
When the cold air outbreak occurs over the and may reach its maximum at around 20*N
Southern Hemisphere (refer to stage A), the (near the monsoon trough region) as shown atequatorial disturbance is formed near the head stage C. During the stage from B to C the southof the cold air mass. At this stage, the whole west monsoon flow may reach its maximum andof India except the foot of the Himalayas is bring about the maximum rainfall over the west
dominated by the "break" condition with the coast of India. The cold core will graduallymaximum downward motion over central India. appear into the lower troposphere by the coolingAt stage B the "break" condition over central of rainfall, while the warm air will gradually
India still continues while a part of upward spread over it by latent heating. Thus, atmotion of the Southern Hemispherecell is stage C the upper tropospheric warm anticyclone
approaching to southern India. The equatorial over central (or northern) India may become
disturbance is, on the other hand, moving east- most intensified. At nearly the same time (stage
ward during the stage from A to B. As pointed C) the less-disturbed high-index type circulationout previously (Yasunari, 1979), this eastward with a relatively warm air may be dominatedmoving disturbance may be closely connected over the Southern Hemisphere, as noticed in

June

1981

T. YasunariFig. 3 and Fig. 17. This implies that at themature stage of the active monsoon the southwest monsoon flow in the lower level has alreadygone down in its source region. At stage D thearea of the maximum convection shift to the
Himalayas, while the "break" condition again
spreads over the whole of India and the equatorial zone, but this stage will move to stage A
when the next cold air outbreak occurs in theSouthern Hemisphere.
It may be reasonable to say that the clockwise
rotation of the anomaly upper wind vector over
Trivandrum (or the easterly jet core region) as
shown in Fig. 10 is attributed to the northward
shift of the cells. That is, when the upward
portion (maximum cloudiness) of the anomaly
cell is located near the equatorial zone such asstage B and D in Fig. 19, the effect of the coriolis
force is relatively small. But when the cell islocated over higher latitude such as stage A andC, the effect is relatively large. As a result, themeridional circulation regime may have lesszonal component of wind over the equatorialzone, and reversely more zonal component overthe higher latitudes.
It is well known that at the "break" period
over central India the monsoon rainfall increases
over the equatorward region and the foot areaof the Himalayas. This situation is well correspondent with stage A. Additionally, it is suggested that the intensities of the convection (andrainfall) of these two areas may be different eachother at the individual stage from.D to B. Namely, when the equator side is more active in rainfall, the Himalayan side will be less active, andvice verse. This phenomenon has been recently
noticed observationally (Riehl, 1979).
Thus, the model of multicell structure presented
here could explain many aspects of the activebreak cycle of the monsoon activity with about
40-day period over and around India, thoughsome problems may still be reserved. Interestingly, a similar multicell structure was presented
in the numerical simulation of the mean monsoon
circulation and its temporal variation (viz., activebreak cycle), only when the effect of orographyof the Himalayas is included in the model (Hahn
and Manabe, 1975). This may imply, in otherwords, that the structure of the meridional circulation as shown in Fig. 19 should be most apparent along the longitudes of the HimalayaTibet region (about 70-100*E). In fact, the timelatitude sectional analysis of cloudiness along
the different longitudinal sectors (Yasunari, 1979)351

offered an evidence for this idea.
The active-break cycle with around the 40-day
period should be compared in the process andspatial structure to that of the winter monsoon
with the 20-30 day period over east and southeast Asia studied by Murakami (1979). He commented that there exists a marked periodicity of
20-30 days in the cold air bursting out of theSiberian high and associated temperature oscillation over the east China sea is nearly in phase(vertically) within the atmospheric layer fromthe surface to 500 mb (or even higher) but theoscillation over the south China sea exhibits alarge vertical phase difference between the lower(below 850 mb) and upper (above 500 mb) troposphere. The structure change from the north tothe south mentioned above seems to resemble
quite well with that of the 40-day period fromthe south to the north, but a distinctly different
situation of the summer monsoon oscillation fromthat of the winter monsoon may be an existenceof the equatorial disturbances as a medium of theinteraction between the two hemispheres.
The maximum cloudiness (viz., the upward
portion of the anomaly cells) seems to correspondwith the maximum cyclonic shear zone in thelower troposphere. In this respect, a northward
shift of the cloudiness may be considered as anorthward shift of the convergence zone (or
ITCZ). In connection with the mechanism of
the "northward shift", Webster and Chou (1980)
suggested in the numerical model the possibilityof the feedback process between the hydrologic
cycle and the differential heating between theocean and the continent although their simulation was focused to the biweekly mode oscillation. However, it is noteworthy to state thatthe type of disturbances seems to be different
each other over the equatorial zone (10*S-10*N)
and over central and northern India (10*-30*N)
although the cloudiness (or rainfall) shifts northward continuously from the equator to the
Himalayan side. As mentioned already, the condition for the development of equatorial disturbances may be provided by the cold air penetration from the Southern Hemisphere associatedwith the increase of the cyclonic shear flow.
Some recent preliminary surveys of the summer
MONEX 1979 have revealed that the cyclonicvortex (or the synoptic-scale disturbance) overthe south Arabian Sea embedded in the strong
cyclonic shear zone has an important role on
the onset of the monsoon over India. Furthermore, it has been strongly suggested that this352

JournaloftheMeteorological"onset vortex" may be formed through the baro-

SocietyofJapan

Vol. 59, No. 3

point may hold in the study of interanual fluctuatropic instability process (e.g., Krishnamurti et tion of monsoon activity over and around India.
al., 1980; Kuettener and Unninayar, 1980). This Hahn and Shukla (1976) stressed the negative
process may be feasible even for the formation correlation between the snow covered area overof the equatorial disturbance related to the active- Eurasia in spring and the monsoon rainfall overbreak cycle of the monsoon, though the convec- India. However, Chang and Yasunari (1981)
tively unstable condition by the cold air penetra- has recently noticed some relationships between
tion to the middle (and upper) troposphere may the surface pressure patterns in the Southernalso be necessary. Over central and northern Hemisphere and the monsoon rainfall over India.
India, on the contrary, the cloudiness may be For example, in 1972, the severe drought year
majorly due to the monsoon depressions as de- over India, a mean position of the southern
scribed in section 3. Recent dynamical studies Indian Ocean high (and may be related upper
have revealed that the combined barotropic-barotroposphere pattern) shift far eastward towardclinic instability (Krishnamurti et al., 1976) and/ Australia with weaker intensity compared toor the CISK-barotropic-baroclinic
instability normal years, while the spring snow covered area
(Shukla, 1978) are important for the formation over Eurasia was also largely extended (Hahn
and growth of the monsoon depressions. Their and Shukla, 1976). Consequently, an abnormallystudies also suggest that these instabilities are severe drought (or flood) year of monsoon should
inherent over and around the monsoon trough be considered as a result of coupling with bothregion. That is, the type of dynamic instability the anomalous circulations over the Northern
for the development of disturbances seems to be and the Southern Hemisphere.
replaced from southern India, to central and8. Concluding remarks
northern India, though a necessary condition for
the full-development such as a sufficient moistureThrough the analysis of the geopotential andsupply through the southwest monsoon flow may wind fluctuation during the summer monsoon
be preserved from the south up to the north. season over and around India, the predominant
On the other hand, some former and recent periodicity of about 40 days was confirmed
studies noted the influence of the mid-latitude especially in the lower and upper troposphere
westerly waves of the Northern Hemisphere to as a major active-break cycle of the monsoon
the monsoon activity over India. Raman and activity. As well as the cloudiness fluctuation
Rao (1980), for example, pointed out that the of the same period (Yasunari, 1979; Yasunari,"break" condition over central and northern 1980), these elements except the geopotential
India is closely linked with the circulation pattern height in the upper level showed a marked northof blocking ridge over east Asia coupled with ward propagation from the equator toward thethe cold trough to the north of India. This con- Himalayan region. The geopotential height atdition may provide more convective precipitation 200 mb showed, in contrast, a standing oscillation(and cloudiness) over the Himalaya-Tibet region. over India. Cross-spectral analyses revealed that
Therefore, we are tempted to say that the active- the vertical structure of the atmosphereisbreak cycle of the 40-day period associated with gradually modified during the northward phase
the northward transition of maximum cloudiness shift of the anomaly cloudiness with the 40-day
zone is resulted from a chain-interacting process period. This modification is most apparent in theof laterally coupled waves initially driven by the temperature field. The modification of the verticalSouthern Hemisphere westerly wave movements. structure from the south to the north over India
At all events, further theoretical and observa- seems to be attributed to the different degree oftional studies will be required on the problem cumulus convection by different types of disturbwhether the active-break cycle of the monsoon ances. It is also suggested that this cycle isdiscussed here is characterized as a free oscil- caused by the periodical cold air outbreak overlation with some feedback system or as a forced the middle and upper troposphere of the equaoscillation by the extratropical influences.
torial zone which may be associated with the
From the point of view of long-range fore- index cycle of the westerly wave movements in
Finally, a multicell
casting of the monsoon activity, the tendency of he Southern Hemisphere.
temperature field over the Southern Hemisphere;tructure of the anomaly Hadley circulation
associated with the large scale fluctuation of regime was proposed, which gives reasonable
westerly waves should be noted more. This view- nterpretations on various aspects of the fluctua-

June

1981

T. Yasunari353

active monsoon over southern Asia during sumtion of this mode.
mer. Tellus, 29, 335-344.
As a next step, a more detailed observational
and theoretical studies should be made on the Kao, S. K., R. L. Jenne and J. F. Sagendorf, 1970:
The kinetic energy of large-scale atmosphere moprocess of interactions (including the energetics)
tion in wavenumber-frequency
space: II. Midamong the Southern Hemisphere circulations, thetroposphere of the Southern Hemisphere. J. Atmos.
equatorial disturbances and the monsoon activity.
Sci., 27, 1008-1020.
FGGE data and MONEX data now being pro- Krishnamurti, T. N., M. Kanamitsu, R. Godbole, C.
cessed are desirable for these studies. The reB. Chang, F. Carr and J. H. Chow, 1975: Studylationship between the year to year fluctuationof a monsoon depression (I) Synoptic structure.
of the monsoon activity over Asia and that ofJ. Met. Soc. Japan, 53, 227-240.
the mid- and high-latitude circulation regime inand, 1976a: (II) Dynamic structure. J.
the Southern Hemisphere will also be an interestMet. Soc. Japan, 54, 20'8-225.
ing problem for future.

Acknowledgements
The author wishes to thank Prof. C. Nakajima
for his constant encouragement
and helpfuladvices throughout this work. He would also
like to express his sincere appreciation to Prof.
I. Hirota for many valuable comments during thecourse of completing this work and for his carefulreading of the manuscript. He is greatly indebted
to Dr. Ts. Nitta, University of Tokyo, and theother refrees for their constructive suggestions
and advices on the revision of this paper. Thanks
are extended to Prof. T. Watabe and Prof. Y.
Takaya for their encouragements throughout thework. He is also grateful to Miss Y. Ishii andMrs. T. Baba for typing the manuscript.
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theSouthern
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モ ン ス ー ン に み ら れ る 約40日

安

Japan

哲

周 期 の 変 動 の 構 造

三

京 都 大 学 東 南 ア ジ ア研 究 セ ン タ ー
イ ソ ド周 辺 の夏 季 モ ン ス ー ソに は,約40日
る こ とは,気周 期 の 大 きなactive‑breakの

サ イ クル が ほ ぼ毎 年 の よ うに 見 られ

象 衛 星 か らの 雲 量 の解 析 で す で に 明 らか に した(Yasunari,1979;1980)。

今 回 は,主と して高 層

資 料 を用 い て,こ の 周 期 性 に と もな う大 気 構 造 の 時 間 的空 間的 変 動 の 様 相 も調 べ た。
ス ペ ク トル解 析 の 結 果,対 流 圏 上 層 と下 層 を 中 心 に,高 度場 と風 の場 に は約40日 周 期 が 卓 越 して い る こ とが わ かっ た。 そ して,こ れ らの 変 動 は,対 流 圏 上 層 の高 度場 を の ぞ き,雲 量 の変 動 に 見 ら れた よ うに,赤 道 付 近 か ら ヒ
マ ラ ヤへ と北 上 す る位 相 を示 して い る。 対 流 圏上 層 の高 度 場 のみ は,イン ド中部 に最 大 振 幅 域 を も つstanding

oscillationと し て現 わ れ て い る。
ク ロス ス ペ ク トル の 手法 を用 い て,温 度,湿 度,高 度,風 の 場 に お け る,こ の 周 期 帯 の 鉛 直 構 造 を 調 べ た結 果,と くに 温 度 場 と高 度場 の構 造 が,雲 量 の極 大(又は極 小)の 位 相 の 北 上 に と もな っ て,南 イ ン ドか ら北 イ ン ドへと,大 き く変 質 さ れ て い く こ とが 明 らか に な った 。 た とえ ば,雲 量 の 極 大 域 に と もな って,赤 道 イ ソ ド洋上 か ら
南 イ ン ドに か け て は,対 流 圏 上 層 に cold core,最 下 層 に warmcoreが存 在 し てい た の が,モ

域 の中 ・北 部 イ ン ドへ と北 上 す るに つれ,対 流 圏 の中 ・上 層 に Warmcore

ソス ー ソ トラ フが拡 が り,下 層 に coldcoreの存

在 す る構 造 へ と変 質 して い く。 高 度 場 に おい て も,対 流 圏 を 通 して,ほ ぼ 鉛 直 に 立 って い た トラ フが,北 上 に とも な っ て,次 第 に南 へ tilt して い き,北 イ ン ドで は,下 層 の トラ フ と上 層 の リ ッジが coupling

した 構 造 へ と変質 して い く。 こ の雲 域 北 上 に と も な う鉛 直 構 造 の 変 化 に は,北 へ い くに した が って 対 流 活 動 が よ り活 発 に な る こと,お よび 雲 量(降 水 量)をもた らす じ ょ う乱 が,南 イ ン ドと中 ・北 部 イ ン ドで 異 な った 構 造 で あ る こ と,な どが 深 く関 連 して い る と推 測 され る。
この 周 期 の active‑break

サ イ クルを 支 配 す る機 構 に は,赤道 方 面 へ の 大 規模 な寒 気 の 吹 き出 しを とも な う,

南 半 球 中 ・高 緯 度 の偏 西 風 波 動 が 重 要 な 役 割 を 果 して い る こ と も,:気 象 衛 星 資料,高層 天 気 図 の解 析 か ら示 唆 され た。
以 上 の 解 析 結 果 を も とに,約40日で,複 数 の anomaly

周 期 の モ ンス ー ソの変 動 は,南

半 球 中 緯度 か ら ヒマ ラヤに いた る子 午 面 上

cell 構 造 を もつ ハ ドレー循 環 系 の南 北 方 向 の変 位,とし て把 握 で き る こ とを 提示 した。