Latitudinal variations of 30–70 day period waves over the tropical Indian zone

Pergamon

Journul qf Almosphrrr~ md 7-wreslr~rrl Phwc.r. Vol 56, No. Y. pp. I 135-l 145. ,994 Elsevier Science Ltd

PrInted I” Great Brltaln OO?l~Y 169:94 $7.OO+O.Oll

Latitudinal variations of 30-70 day period waves over the tropical Indian zone

K. KUMAR and A. R. JAIN*

Radio Science Division. National Physical Laboratory. New Delhi-l 10012, India

Abstract-Long series of wind data from weekly radlosonde and rocketsonde Bights have been analysed using fast Fourier transforms (FFT) to study the latitudinal variations of 30 70 day period waves over the tropical Indian zone. Zonal and meridional components of wind over three stations, namely Thumba (8.5 N, 77 E), SHAR (13.7 N. 80.2’E) and Balasore (21.5 N, 87 E) are used for the purpose. Some very interesting features of these waves are noticed at stratospheric heights which have been observed for the first time at SHAR and Balasore. The wave amplitudes at stratospheric heights are found to be much larger than at tropospheric heights at all three stations. The zonal wind amplitudes of these waves at stratospheric heights over SHAR and Balasore are found to be significantly larger than over Thumba, a near-equatorial station. The location of Balasore is unique, being in the transition zone between low and middle latitude regions. These waves do not conform to the characteristics of Kelvin waves which are observed both in the meridional wind as well as in the zonal wind. It has also been noticed that these waves are observed during the southwest monsoon as well as the northeast monsoon seasons. Wave energy density calculations for Thumba station indicate that the origin of these waves is in the troposphere, and that a part of the energy density leaks into the stratosphere.

I. INTRODUCTION

In addition to the long period oscillations (QBO, A0 and SAO) and equatorial waves (4-20 day period),

MADDEN and JULIAN (1971, 1972) identified a 40-

50 day oscillation using spectral analysis of tropical radiosonde data in the zonal wind. Since then, many efforts have been made to explore the character-

istics and origin of these oscillations (YASLJNARI,

1979; KRISHNAMURTI et ul., 1985 ; MURAKAMI and NAKAZAWA, 1985 ; LAU and CHAN, 1985. 1986, and others). Until recently these waves were observed only

at tropospheric heights. However, in the last one or two years, these waves have been found at stratospheric heights also. and even larger amplitudes of the waves have been noticed than at tropospheric heights (NAGPAL and RAGHAVA RAO, 199 1; KUMAR and JAIN.

1992). It has also been found that the phenomenon occurs

throughout the year with no systematic seasonal variation in amplitude or pcriodicity (ANDERSON pf crl..

1984). However, it was pointed out by several authors that the 30-50 day oscillation is amplified during the northern hemisphere summer over the Indian monsoon region (YASUNARI, 1981 ; KRISHNAMURTI and SUBRAHMANYAM, 1982) and over the western Pacific

(MURAKAMI, 1984 ; CHEN, 1987), and is intimately related to enhanced convection and rainfall (MADDEN,

* Present address : National MST Radar Facility, Post Box 123. Tirupati--517 502, India.

1986). HARTMANN and MICHELSEN (1989) have

reported that the precipitation exhibits a statistically significant spectral peak at about 40 days over most

of the southern half of India during summer. Madden and Julian did not find any distinct spectral

peaks in the meridional wind spectra as pronounced

as those in the zonal wind spectra. Some other obser-

vational studies yielded similar negative results in this regard (MARUYAMA, 1982). In contrast to the above- mentioned observations, there is some evidence for a role of the meridional wind in the 40-50 day oscil-

lation (YASUNARI, 1981 ; KRISHNAMURTI and SUBKAHMANYAM. I982 ; MURAKAMI et ul., 1984 ; MAD-

DEN, 1986 ; KUMAR and JAIN, 1992), indicating that it

is not solely a Kelvin wave. Therefore, a study of these waves in the zonal and meridional components of

wind is of considerable interest. In the present paper. wind data from weekly Rights

of radiosondes and rocketsondes at three stations, namely Thumba (8.5 N. 77 E), SHAR (13.7 N, 80.2 E) and Balasore (21.5 N, 87 E) are used. Rocket launchings arc conducted from Thumba with M-100 rockets. and from SHAR and Balasorc with RH-200 rockets. The location of Balasore is unique, being in the transition zone between low and middle latitude regions. In addition, it is located over the monsoon trough region which spreads over the Ganges plains of north India during the southwest monsoon season. Therefore. a study of latitudinal variations of the 30 70 day period wave in zonal and meridional wind components is carried out to examine :

1135

1136 K. KUMAK and A. R. JAIN

Table I. Wind data uxd in the unalysis

Series stamn No. I

Thumba 23 May 197% 19 September 1919

SHAR IO January 1979% 26 September 1979

Bala\ore 31 January 1979- I9 September 1979

Series No. 2

3 I December 1980 5 October 19% 22June 1983 I2 September I984

7 November 1979 2 April 1980

I4 May 1980 17 June 19X1

X February 1984 20 June 1984

7 January 19X7- 25 November 19X7

Series No. 3

SUi‘ZS No. 4

4 April l985- 20 August 19X6

Series No. 5

Common series for all stations

3 I January l979- I9 September 1979

31 January 1979-m 19 September 1979

9 December 19X7- 25 May 198X 3 I January I979 27 April 1988 I March 1989 19 September 197’)

(i) if these waves are observable at tropical lati-

tudes also, (ii) whether these waves are observed in meridional

winds also. or whether they are confined to zonal

winds only, (iii) the source mechanism of these waves through

a simultaneous study of these waves at the three

stations, and studying the wave energy density height

profile.

2. DATA AND METHOD OF ANALYSIS

Weekly flight data of winds for the three stations were scanned for reasonably long sequences with minimum gaps and containing at least one monsoon season. The series of weekly data for the three stations thus selected for further analysis are given in Table I.

It can be noticed from this table that the period 31 January-19 September 1979 is common for all three stations. The data gaps, if any, have been filled by interpolation (using the Lagrangian formula) around

the missing point using a five degree polynomial. Before taking the fast Fourier transform (FFT), the

time series of weekly wind data are reduced to zero

means to eliminate any linear trend. To lessen the effects of the discontinuities, we make the end regions (first and last 10% of the data) have smooth tran-

sitions to the mean of the measured value by multi- plying the sequence of the weekly wind data with ‘weights’ of a ‘Cosine Bell’ window. The weights are

defined by

NjX) = 1,12[l -cos (rr.X/fZ)] : 0 < .Y < f‘L

IV(X) = 1 ; ,/I. 6 .Y < L(l -,f’)

w(x) = 1/2[l -cos {rc-(L--.u)/f’L}]; I,( 1 -,f‘) < B d L

where L is the length of the data and ,f’ the desired fraction (usually 0.1).

The tapered series is extended by adding zeros towards the end to bring the number of data points

equal to 2”, where k is an integer whose magnitude depends on the length of the data series. Then the FFT is applied to obtain the spectrum of this time series. A final smooth estimate may now be found by

further frequency smoothing with a ‘Harming’ pro- cedure as follows :

C?, = 0.25G, , +0.5Gk +0.25G,+ ,.

where k = I, 2.. , (m- I), and c’, represents a smooth estimate at harmonic k to reduce the error due to truncation.

3. RESULTS

3. I. Height profile qf wace umplitude in annul wind

From the spectral analysis, it has been noticed that two dominant waves are commonly observed in the spectra at different height levels. These waves cor- respond to spectral periods of 30-50 and 50-70 days (Fig. I). The amplitude height profiles of 30-50 and

5&70 day period waves in zonal wind are shown in Fig. 2. The horizontal bars show the standard devi-

ation of wave amplitude for data sets of different time series used in the present study. It can be noticed

that these waves have significant amplitudes in the troposphere and in the stratosphere above all three stations. Two distinct peaks in amplitude have been seen in both (30-50 and 50-70 day period) waves:

one is below the tropopausc and the other near the stratopause. The wave amplitude near the stratopause height is larger than at the tropospheric heights; the amplitude of these waves at tropospheric heights is comparable at all three stations. However, the ampli-

tude of these waves at stratospheric heights is found to bc significantly larger at SHAR and Balasore than at Thumba. The maximum amplitude of these waves is noticed at Balasore at stratospheric heights in both waves. In addition, a common series of zonal wind data for all the three stations is analysed for the 3& 50 day period wave, which yields similar amplitude

height characteristics (Fig. 3).

Variations of period waves over the tropical Indian zone 1137

-16km --- 42 km --43km

6

I I

8 35 42 49 56 63 70 77

No. of days

Fig. 1. Amplitude spectra obtained after FFT of the unfil- tered time series of zonal wind data for Thumba station from 23 May 1978 to 19 September 1979. The heights shown are

16,42 and 43 km.

64

52 t

48 F

$ 36-

r’ 32- UJ - 2 28.

16 i

8

3.2. Hei,yyht prc?file of waae amplitude in meridional

,iYnds

An analysis similar to that of the zonal winds has

also been carried out for meridional winds. Height

profiles of amplitude of the 3&50 and 50-70 day period waves in meridional winds for the three stations are shown in Fig. 4. The results of common series of data are shown in Fig. 5.

Intercomparison of the zonal and meridional wind wave amplitudes show similarity in their features, with

two peaks, namely, one below the level of the tropopause and another just below the stratopause. At tropospheric heights the following features can be noticed: (i) the amplitude of the wave in the mer-

idional wind component at Thumba is smaller than at SHAR and Balasorc; (ii) at SHAR and Balasore the meridional wind wave amplitude is comparable to the zonal wind wave amplitude; (iii) at Thumba. the

meridional wind wave amplitude is smaller than the zonal wind wave amplitude; (iv) at stratospheric heights, the wave amplitude in the meridional wind is smaller than the zonal wind wave amplitude for all

the stations.

Wave period: 50 - 70 day -.- Thumba (8.5”N, 77.5”E) ---- Shar (13.7”N, 80.2”E) - Balasore (21.5”N. 87”E)

AR

Oo 2 4 6 I*lll,l,lllrll 8 10 12 14 16 18 20 111111111111.1.11111 ‘2 4 6 8 10 12 14 16 18 20 Amplitude of zonal wind (m/s) Amplitude of zonal wind (m/s)

Fig. 2. Amplitude -height profile from FFT for 30-50 and 50 -70 day period waves in zonal winds over Thumba. SHAR and Balasore.


60- 3.3. Hci,qht pmfile of’~mve ener,q_v densit?

56-

52-

To understand the origin and propagation of these

waves, the energy density of waves (30-50 and 5C-70

day periods) are calculated using the average density

from the atmospheric model for Thumba (SASI and SENGUPTA. 1979). and the wave amplitude is taken from the spectra of both the wind components for Thumba station. This can be written as

40-

2 36- - 3 32- a,

= 28-

and

Wave period: 30 - 50 day

24- ,J._

16T

12

where /-’ is the average density (kg m ‘) ; Vz and Vm

are the mean values of the zonal and meridional wind

amplitudes (m s- ‘) of the 30-50 and 50-70 day period waves taken from the spectra of Thumba station, respectively. The wave energy density calculation is

restricted to Thumba station because the largest series of data is available for this station. Figure 6 shows that the energy density of the 30-50 and 50-70 day period waves at tropospheric heights is about 15 dB higher than at stratospheric heights. It indicates that, probably, part of the wave energy density from the troposphere is leaking through the tropopause into

the stratosphere. This figure also gives an indication

I I I

Oo 2 4 I I I I 1 I

6 8 IO 12 14 16 Amplitude of zonal wind (m/s)

Fig. 3. Same as Fig. 2, but for the common series for all three stations in the zonal wind.

Wave period: 30 - 50 day

----Thumba (8.5”N, 77.5”E) --- Shar (13.7”N, 80.2”E) - Balasore (21.5”N. 87”E)

O’,“““““‘.““” 0 2 4 6 8 10 12 14 16 18

Amplitude of meridional wind (m/s)

E = I ,lZp I”

v? = If-_‘+ vrn2,

Wave period: 50 - 70 day -.--Thumba (8.5”N, 77.5”E) ---Shar (13.7”N, 80.2”E) -6alasore (21.5”N, 67”E)

I I, 5 I, I I I I I

2 4 6 8 IO 12 14 16 18 Amplitude of meridional wind (m/s)

Fig. 4. Same as Fig. 2, but for the meridional wind


60 1

56-

40-

$ 36-

E 32- .P

I” 28 1 24

7

0 ’ .), , , , , , , , 0 2 4 6 8 10 12 14 16 18

Amplitude of meridional wind (m/s)

Fig. 5. Same as Fig. 2, but for the common series for all three stations in the meridional wind.

as to whether the wave is growing or dissipating. In

the stratosphere. between 22 and 50 km, the wave energy density is almost constant and above 50 km,

which is close to stratopause, the wave energy density dissipates rapidly.

- 30 - 50 day period wave --- 50 - 70 day period wave

12-

4-

-02.0 I I I

-1.0 0.0 1.0 2.0 Log of wave energy density (Joules rnm3)

Fig. 6. Wave energy density height profile for Thumba station for both the period waves, i.e. 3&50 and 50-70 day.

3.4. Plots of’ the time height cfwind components

The time series of weekly data for both the wind

components is passed through band-pass filters with

a limited purpose to see (i) the phase variation with

height. and (ii) the consistency of the waves between different heights. Two independent filters are used to study the two waves. In the first case the waves of periodicity lower than 30 days and greater than 50

days are removed using the filter. Similarly, in the second case the waves of periodicity lower than 50 and greater than 70 days are removed. Because of this

filter WC are losing a total of 14 points (7 at each end)

for the 30-50 day period wave and a total of 34 points (I 7 at each end) for the 50--70 day period wave. The responses of the band-pass filters are shown in Fig. 7. The length of weekly wind data sets at SHAR and Balasorc is not enough to make time-height plots of

the 50-70 day period waves, and hence such plots arc made only for Thumba.

Figures 8, 9 and IO arc the plots of the filtered time

series and show the strong activity of 3&50 day period waves in the zonal and meridional wind components at Thumba, SHAR and Balasore, both at tropospheric and stratospheric heights, rcspectivcly. The hatched rectangle on the x axis scale indicates the southwest monsoon season. From these figures, it can

be noticed that (i) 30-50 day period waves are seen

prominently at all three stations, and (ii) these waves arc not confined to the southwest monsoon season. but are equally strong in the northeast monsoon

season. The phase of these waves is advancing down- ward at all three stations as shown by the dashed lines. These waves are not only evident in the zonal wind

but in the meridional wind also. Figure I I shows the

plot of 5&70 day period waves at Thumba ; characteristics similar to those of 30-50 day period waves have been noticed for these waves also.

The cross-correlation technique has been applied to the filtered time series common to all the stations of

zonal winds for 30-50 day period waves at I5 and

50 km heights to study the propagation of the waves. Figure 12 shows the cross-correlation at I5 and

50 km heights, respectively, in the zonal winds between Thumba and Balasore and between SHAR and Bala- sore. The delay indicates that the phase is advanced at Balasore compared to Thumba and SHAR. Although

the length of the filtered data is not large it does suggest that the wave is propagating from Balasore to Thumba (i.e. from a higher latitude to a lower latitude).

4. DISCtiSSION AND CONCI,USIONS

Some interesting features of 30-70 day period waves in the zonal and meridional wind components at three

I140

l.O-

0.8-

iz 6 p 0.6- B a

0.4-

I I \

I I \ II I I \ I I

70 46 35 28 23 No. of days

Fig. 7. Response of the band-pass filters; solid line shows 30-50 day period, and dotted line shows 5@70 day period.

Thumba station

30 - 60 day period wave

Meridional wind

I Zonal wind

58

56

54

52

50

48 II,’ * - y Y- “’ D

40

38

36

34

32

30

16

14

12

10

k107 +I 8

0 E ;1O -1

0

No. of weeks

Fig. 8. Time -height section for 30-50 day period wave for zonal and meridional wind components over Thumba Station. On the x axis, 0 stands for 18 February 1981. The wind scale for the first height is given in the middle of two graphs, in m s ‘. The wind values for the subsequent heights are displaced vertically

by adding 10 m s- ’ at each height.

SHAR station

30 - 50 day period wwe

Zonrsf wind

No. of weeks

Meridiorkal wind

Fig. 9. Same as Fig. 8, but for MAR station. On the .X axis, 0 startds for 28 February 1979.

if42

Bafasore station

30 - 50 day period wind

No. of weeks Fig. IO. Same as Fig. 8, hut for Baktsore station. Ch the\- x&., 0 n&w& for 25 Fehru~try 1987.

Zonal wind

No. of we#?ks


-l.OL 1 I I I ,

0 2 4 6 6 10 No. of lags in weeks

Fig. 12. A cross-correlation in the filtered time series of zonal winds. Thumba and Balasore (-). SHAR and Balasore (-- -) at 15 km. At 50 km, Thumba and Balasore (--O-O-),

SHAR and Balasore ( x ~ x -).

stations in the Indian zone are as follows :

(i) The height profiles of the wave amplitude at the

three stations have shown that these waves are not confined to tropospheric heights only, as had been reported earlier. In fact, the wave amplitude in the zonal wind at stratospheric heights is about 223 times

larger than at tropospheric heights. To understand whether the source of these waves is in the troposphere

or in the stratosphere, the wave energy density has been computed for Thumba. The results indicate that the source of these waves is in the troposphere and that a part of the energy is leaking into the stratosphere through the tropopause.

(ii) Simultaneous studies of these waves in zonal

and meridional wind components would help in identi- fying the mode of waves. Since the oscillations are observed both in the meridional wind as well as in the zonal wind, these waves do not seem to conform to

the characteristics of Kelvin waves. Further studies are needed to understand the nature of these waves.

ANDERSON J. R., STEVENS D. E. and JULIAN P. R. CHEN T. C. HAKTMANN D. L. and MICHELSEN M. L. KRISHNAMURTI T. N., JAVA KUMAR P. K..

SHENG J., SURGI N. and KUMAR A. KRISHNAMURTI T. N. and SUBRAHMANYAM D. KUMAR K. and JAIN A. R. LAU K. and CHAN P. H. LAU K. and CHAN P. H. MADDEN R. A.

(iii) These waves are not confined to an equatorial station (Thumba) but are seen to be equally promi-

nent at the tropical stations (SHAR and Balasore). In

fact, the wave amplitude in the zonal wind at strato-

spheric heights is larger at Balasore, a station in the region of transition from the tropics to mid-latitudes.

(iv) These waves have been reported in the litera- ture during the southwest monsoon season (YAW-

NARI, 198 I : KRISHNAMLJRTI and SUBRAHMANYAM,

I982 ; MURAKAMI and NAKAZAWA. 1985 ; HARTMANN

and MICHELSEP~, 1989). However, some earlier studies (MADDEN, 1986; KUMAR and JAIN. 1992) and the

present study confirm that these waves arc not con-

fined to the southwest monsoon season. but are also observed in the northeast monsoon season.

(v) To understand the propagation of these waves, cross-correlations have been computed between fil-

tered time series at Thumba, SHAR and Balasore. The results show that the waves are propagating from Balasore to Thumba. In a recent paper. NAGPAL and

RAGHAVA RAO (1991) have suggested that the long period waves may be Rossby modes originating some- where in the extratropical region and penetrating into

the tropics. To confirm this, observations arc required over a wider latitude zone.

The present results show that these waves are not confined to the equatorial region; they are also seen

in tropical latitudes, and are propagating from

Balasore to Thumba. The origin of these waves is

in the troposphere, with some energy density of the waves leaking through the tropopause into the stratosphere.

Ac,knon,/~,c!yPm~nf.s~The authors express their sincere thanks to Dr V. Narayanan, TERLS. VSSC. Trivandrum, for providing some of the latest data used in the present analysis. Thanks are also due to the IMAP data centre, Bangalore, for providing most of the earlier data used in creating the database used in the present study. The authors are also grateful to Dr B. M. Reddy, National Physical Laboratory, New Delhi. for his valuable suggestions. They would like to thank the referees for their constructive suggestions.

REFERENCES

19x4 Mon. Wealh. Rev. 112, 2431. 1987 Mon. Wrath. Rev. 115, 1589. 1989 J. u(mos. Sci. 46, 2838. 1985 J. (~fmos. Sci. 42, 864.

1982 J. utmos. Sci. 39, 2088. 1992 Indian J. Radio Space Phy.7. 21, 42. 1985 Mon. Weath. Rec. 113, 1889. 1986 Mon. Weath. Rea. 114, 1354. 1986 J. NIIIIOS. Sci. 43, 3 138.


MADDEN R. and JULIAN P. R. MADDEN R. and JULIAN P. R. MURAKAMI M. MAKUYAMA T. MURAKAMI T. and NAKAZAWA T. MURAKAM~ T., NAKAZAWA T and HE J. NAGPAL 0. P. and RAGHAVA RAO R. SASI M. N. and SENGUPTA K.

1971 1972 1984 1982 1985 1984 1991 I919

J. utmos. Sci. 28, 702. J. atmos. SC. 29, 1109. J. met. Sot. Japan 62, 88. J. met. Sot. Japan 60, 172. J. atmos. Sci. 42, 1107. J. met. Ser. Jupatz 63, 250. J. atmos. tcrr. Phys. 53, 1181. Scientific Report ISRO/VSSC/SR/19/79, Indian Space

Research Organisation, Bangalore. India. YASU~SARI T. 1979 J. met. Sot. Jupurl 57, 227. YASLJNAKI T. 1981 J. met. Srr. Japan 59, 336.

Documents

Latitudinal variations of 30–70 day period waves over the tropical Indian zone