Oscillations in the Zonal and Meridional Winds in the Troposphere, Stratosphere and Mesosphere Over Thumba

OSCILLATIONS IN THE ZONAL AND MERIDIONAL WINDS IN THE TROPOSPHERE, STRATOSPHERE AND MESOSPHERE OVER THUMBA M. N. Sasi Space Physics Division,

Vikram Sarabhai Space Centre,

Trivandrum,

India

ABSTRACT Using M-100 rocket and meteorological balloon data collected at Thumba (8.5*N, 77°E) for a period of five years, altitude variations (of the amplitudes and phases) of the annual and semi-annual oscillations in the zonal and meridional winds are investigated. The amplitudes of the oscillations in the zonal wind vary from 2 to 50 m/s and, for the meridional wind, from 0.5 to 10 W/B. The amplitudes and phases of these oscillations over Thumba are compared with those at other tropical latitudes (10°N). INTRODUCTION Regular weekly meteorological observations from Thumba are made with M-100 rockets and meteorological balloons under a collaborative programme between ISRO, India, and the Hydro-Meteorological Services, USSR. Winds are observed from the surface to 80 km altitude. Using these observed wind profiles for a period of five years from 1971 to 1976, annual and semi-annual oscillations in the zonal and meridional winds have been studied and the results are presented in this paper. DATA AND THE METHOD OF ANALYSIS Prom the weekly values of winds for five years, monthly mean values of zonal and meridional winds were obtained. The monthly means of the wind profiles were then subjected to harmonic analysis to determine the steady, annual and semi-annual components. In this present analysis, it is assumed that the oscillations in the monthly mean winds are predominantly annual and semi-annual. The long-term variations, such as quasi-biennial oscillations, are ignored and an equation of the form s i n

^

2C0t + <

M

(1

>

X(t) . X + X sin (60t +4>l) + 2 is used, where X(t) is the value of the time-varying parameter (zonal or meridional wind) at time t, and X is the value of the steady component of the parameter. X* and X2 and c^-j and<£o ^ e the amplitudes and phases of the annual ana semi-annual oscillations respectively; x

Q

1

a

61

62

M. N. Sasi

—i—i—i—i—i—i

2

Fig.2

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6 8 AMPLITUDE (m/s)

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J U L SEP NOV JAN MAR MAY J U L P H A S E ( T I M E OF M A X )

Annual and semi-annual oscillations in meridional wind

63

Oscillations in the Zonal and Meridional Winds

A M P L I T U D E (m/s)

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P H A S E ( T I M E OF M A X )

Annual oscillation in zonal wind.

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10 20 30 NOV J A N MAR MAY J U L A M P L I T U D E (m|s) P H A S E ( T I M E OF M A X ) Fig.4 Semi-annual oscillations in zonal wind

M. N. Sasi

64

CO is the angular frequency corresponding to the annual oscillation (C0="TT/6 radians/month) and t is the time of the year (in months). Making use of a least squares technique and equation ( 1 ) , the steady components, and the amplitudes and phases of the annual and semiannual oscillations are determined. RESULTS AND DISCUSSION Figure 1 shows the altitude profiles of the amplitude and phase of the annual and semi-annual oscillations in the zonal wind, with error bars indicating the r.m.s. errors. It is seen that the annual oscillation is predominant up to the middle of the stratosphere ( ~ 37 km). In the region below, at and above the stratopause (^"48km) the domination of the semi-annual oscillation is clearly seen. In the stratosphere the phase (time of maximum) of the annual oscillations in the zonal wind occurs in the winter months. The phase of the semi-annual oscillations occurs near the equinoxes in the upper stratosphere and lower mesosphere. The altitude profile of the amplitude and phase of the oscillations in the meridional wind is shown in figure 2. It is seen that the amplitudes of the oscillations are much smaller than those of the zonal wind. Both annual and semi-annual oscillations are stronger in the mesosphere than those in the regions below. Figures 3 and 4 show comparisons of the amplitudes and phases of the oscillations in zonal winds over Thumba with those over other tropical latitudes. The monthly zonal wind profiles from 25 to 80 km for 10° latitude as given in the CIRA-1972 model [1] were analysed using the same technique as used for the wind data over Thumba, and the amplitude and phase of the oscillations were obtained. Similarly the amplitude and phase values obtained by Belmont et al. ^] for 10°N in the 20 to 65 km altitude region are also shown. It is seen that from 20 to 60 km there is a good agreement between these results and the amplitude and phase values obtained for Thumba. Above 60 km the differences are quite large, the reality of which can be ascertained only after obtaining more wind data in this region. It must be mentioned that above 60 km only one year's wind data were used in the present analysis, whereas below 60 km five years data were used. 1

ACKNOWLEDGEMENTS The author acknowledges with thanks the ready co-operation of Dr. V. Narayanan, Meteorology Section, TERLS, VSSC, India in making the M-100 rocket data available. The M-100 rockets are being launched from TERLS under a scientific collaborative programme between ISRO of India and the Hydro-Meteorological Services of the USSR. The author wishes to thank Dr. C A . Reddy, Space Physics Division, VSSC, for many helpful suggestions during the analysis of the data. REFERENCES 1. 2.

G.V. Groves, in: CIRA 1972, Akademie-Verlag, Berlin, 1972, p.155 A.D. Belmont, D.G. Dartt and G.D. Nastrom, Quarterly Journal of Royal Meteorological Society 100, 203 (1974).