Quart. 1. R. Met. SOC. (1973), 99, pp. 73-81 551.513 : 551.557.33

Zonal wind quasi-biennial oscillations at 25-60 km altitude, 1962-69

By G. V. GROVES Department of Physics, University College, London

(Manuscript received 8 March 1972; in revised form 3 July 19iL. Communicated by Professor Sir H. S. W. Massey)

SUMMARY

Meteorological rocket observations of zonal winds at thirteen sites (or pairs of sites) have been analysed for amplitudes and phases of their quasi-biennial oscillation. The period of the oscillation was taken to be 32 months because from 1962-69 the period was relatively constant, not varying by more than &3 months. Amplitudes and phases of the oscillation over a wide range of latitude are presented and discussed. The low-latitude part of the oscillation is well-defined with maximum amplitudes at 25 to 30 km altitude, which reach about 25 ms-l at the Equator and decrease to a few ms-l at 20" latitude. At other latitudes, amplitudes are less than 5 ms-1 except above 50 km at two sites between 50' and 60" latitude, where another main part of the oscillation is indicated. More observations and sites are needed at mid and high latitudes. Observations at the highest latitude site (77') show no evidence of a diminished amplitude. Phases also depend on height and latitude and may reach gradients of 1 month km-1 with height and 2 months deg-1 with latitude respectively.

1. INTRODUCTION A quasi-biennial oscillation (QBO) in zonal winds above 30 km was first observed in

rocket data from Ascension Island ( S O S ) obtained between October 1962 and October 1964 (Reed 1965a). The oscillation was found to (i) decrease in amplitude above the 25 km level (where balloon data had shown the amplitude to be about 18 ms-l) and (ii) have a down- wards propagation of phase, as already observed at lower heights, but at a rate of 2 km monthp1, which was about twice as great as that observed below 30 km at Ascension Island.

The same two years of Ascension Island data were also analysed by Angel1 and Korshover (1965) together with zonal wind data for the period March 1969 to June 1961 from Cape Kennedy (28"N), White Sands (3Z0N), Wallops Island (38"N) and Fort Greely (64"N). At temperate latitudes it appeared that (i) although amplitudes were small (com- pared with the annual variation), they increased with height so that above 55 km they exceeded amplitudes at low latitudes at the same level, and (ii) the downward propagation of phase was faster than at low latitudes, reaching 5 or more km monthp1.

2. ANALYSIS OF DATA

Since 1964 several new rocket launch sites have been brought into operation and observations at existing sites have continued. For the analysis presented in this papzr, data up to December 1968 were available from the Meteorological Rocket Network Reports (US Dept. of Commerce 1962-70), and a few observations were available directly from experimenters and from the Exametnet Data Report Series (1968-70). Table 1 lists the data analysed. These were either in the form of individual profiles or monthly means. Data from the following pairs of stations at nearly equal latitudes were combined: Gan and Natal, Grand Turk and Barking Sands, Wallops Island and Tonopah, and Volgograd and Primrose Lake.

On analysing data at a given height (25, 30, . . . , 60 km) for a particular station (or pair of stations), the zonal wind QBO was expressed as

UQS = A COS ( 2 ~ ( M - Mo)/T) . (1) where M is time after 1 January 1966 in units of months and M o is a value of M when maximum flow from the west occurred. Values of UQB were taken equal to the difference u - U M , where u is an observed value and U M a model value for the seasonal component

73

74 G. V. GROVES

TABLE 1. ZONAL WIND DATA ANALYSED 25 TO 60 km

Number

1

2

3

4 5

6

7

8

9

10

11

12

13

Station

Gan Natal

Ascension Island

Fort Sherman Antigua Grand Turk Barking Smds

Cape Kennedy

White Sands

Point Mugu

Wallops Island

Tonopah

Volgograd

Primrose Lake Fort Churchill

Fort Greely

Thule

~

Latitude

0' 41's 50 5 5 3

70 59's

9" 20"

17' 09"

21" 26" 21" 54"

28" 27"

32" 23"

34" 07"

37' 50"

38' OO'N

48" 41"

54' 45"

58' 44"

64" 00"

76' 33"

-

Longitude Years

73" 09'E 1968-70 35" 10 'W 1966-68

14' 25'W 1962-68

1967-68

79' 59'W 1966-68

61' 47%' 1963-68

71' 09'W 1964-66 159" 35'W 1962-68

80' 32'W 1960-65 1965-68

106" 29 'W 1959-65 1965-68

119' 07'W 3959-65 1965-68

75" 29 'W 1960-68

1965-68 116' 30'W 1959-66

44' 21'E 1365-67

110" 03'W 1967-69

93' 49 'W 1959-68

145" 44 'W 1960-65 1965-68

68' 49 'W 1964-69

No. of profiles and References followed by

monthly number of launchings means

28

60

347

14.

363

159

239 443

84 7 48*

1052

48*

7110 48.

488

48* 90

67

83

200

343 48.

260

Hamilton (1970) 28 Exametnet Data Report Series (1968) 16, (1969) 16, (1970) 12; Smith et al. (1969) 12, (1970) 3; WDCA (1970) 1 WDCA (1963-69) 343; Smith et al. (1966) 4; WDCA (1967-69) 24 WDCA (1967-69) 363

WDCA (1965-67) 239

Smith (1966) 5, Smith (1969) 8

WDCA (1966-69) 48 WDCA (1962-66) 1052 WDCA (1966-69) 48

WDCA (1966-69) 48 Smith et al. (1964) 23, (1966) 16, (1969) 9, (1970) 1; WDCA (1962- 66) 439 WDCA (1966-69) 48 WDCA (1962-64) 72; Smith (1962) 16, (1969) 2 Central Aerological Observatory Bulletin (1968) 12, (1969) 55 WDCA (1968-70) 83 Stroud et al. (1960) 9; Smith et al. (1964) 5, (1966) 7, (1970) 5; WDCA (1962-66) 174 WDCA (1962-66) 343

WDCA (1964.69) 159

WDCA (1963-69) 430;

WDCA (1962-66) 847

WDCA (1962-66) 700

WDCA (1966-69) 48 WDCA (1965-70) 360

'Monthly means WDCA - World Data Center A for Meteorology.

taken from Groves (1971). A, M o were determined by the method of least squares for a chosen value of T. The analysis also provided estimates of uA, uMo, the standard deviations of A, Mo, calculated from the usual variance matrix. aA, uM, were usually within a factor of two of 1 ms-l and 1 month according to the distribution of observations and number of observations which varied greatly (from about 80 to 800) between different cases.

3. THE PERIOD OF THE QBO

In 1960 it was realized that the variations in equatorial zonal winds observed during the late 1950's were mainly cyclic with a period close to, but not generally coincident with,

ZONAL WIND QUASI-BIENNIAL OSCILLATIONS 75

2 years. From Canton Island (3"s) data, 1955-1962, it was apparent that the pzriod T changed irregularly from 21 months in early 1960 to 30 months in 1962 (Reed 1965b). The next cycle, in 1963-65, had T equal to 33 months (Ebdon 1967).

In view of these changes in T , the analysis discussed in Section 2 was carried out for T = 24, 26, . . . , 40 months at each station and height interval. For the majority of cases (66 out of 104) a value of T which minimized the sum of residuals squared could be determined. The means of these values at the different heights for each station, To, are shown in Fig. 1 (black dots) plotted against the mean date of the interval of observation. An error bar denotes the standard deviation of values from their mean To.

TIME IN YEARS

Figure 1. Variation of period of zonal wind QBO with date. Black dots denote the mean periods To deter- mined for each station in Table 1 and are plotted at the mean date of the interval of observation. Error bars denote standard deviations from To. Open circles are from 30 mb balloon data at Canton Island (3"S), May 1954-August 1967, and Gan (0" 41'S), September 1967 onwards (Ebdon 1971): values show the interval between two successive reversals in the same sense and are plotted at the date of reversal in the opposite sense.

The open circles in Fig. 1 are values for the period derived from equatorial zonal winds at the 30 mb level at Canton Island (3'9, May 1954-August 1967 and Gan (0' 41'S), September 1967 onwards (Ebdon 1971). The values show the interval between two successive reversals in the same sense and are plotted at the date of reversal in the opposite sense.

Fig. 1 shows good agreement between the rocket-sonde data which relate to a wide range of height and latitude and the balloon data which relate to one equatorial level. Plots of the rocket-sonde data against height and latitude (not presented here) show no detectable dependence of the period on these parameters. There is, however, a pronounced variation of period with date as shown by the balloon data in Fig. 1. Over the last 15 years extreme values have been about 20 and 36 months but for 1962-69 the range of variation has been much reduced to about 32 3 months. Results are therefore given below for a fixed period of 32 months, the average value for these years.

4. DETERMINATIONS OF AMPLITUDE, A Computed values of A are shown in Fig. 2 for each of the thirteen stations together

with error bars cA. A11 stations are from the Northern Hemisphere, except Ascension Is, (8's) for which the results differ very little from those at Fort Sherman (9"N). Fig. 2 shows that the level of maximum amplitude increases from below 25 km at the Equator to over 30 km at temperate latitudes - a result previously observed in the Southern Hemisphere (Tucker and Hopwood 1968). At the same time the maximum value decreases and amounts to only a few ms-l at 20" latitude. Polewards of 30" latitude maximum amplitudes are found at a higher level, i.e. 50 krn or above, and may exceed equatorial amplitudes at the same height (Angel1 and Korshover 1965).

The observed amplitudes are presented in Fig. 3 against station latitude for different heights. Continuous curves have been fitted to these points by eye which are consistent with the curves in Fig. 2. A mid-latitude maximum develops between 50" and 60" reaching values in excess of 10 ms-l. At higher latitudes, Thule (77"N) data indicate that the

76 G. V. GROVES

oscillation is still present with amplitudes of a few ms-'. A small maximum in amplitude appears to occur at most heights between 25" and 35" latitude. These features are apparent in the meridional cross-section of amplitude shown in Fig. 4.

Vrn

H:[\l LO 30 \ \ 20 L J I

t I

0 5 10 15 20 25 0 5 10 15 20 0 5 10 15 20 r n l s

Km

Go 50 c I Ht40L 20 30

0 5 10 15

rn/s

Krn

0 5 10

m I s

rn I s m I s

- 0 5

rn Is

Figure 2. Zonal wind QBO amplitudes for stations (1)-(13) in Table 1.

5 . DETERMINATIONS OF PHASE, M o Computed values of M , are shown in Fig. 5 for each of the thirteen stations together

with error bars oM,. Rapid phase changes in a narrow height interval are a feature of profiles at certain sites. Several of these changes appear to be a reversal (phase change of 16 months) as the amplitude goee through zero.

Fig. 6 shows the latitudinal variation of M o at various heights constructed from the results in Fig. 5 . A consistent variation of phase is obtained over most latitudes. The curves fitted in Figs. 5 and G are self-consistent and are also consistent with the meridional cross- section of M , shown in Fig. 7.

At low latitudes M, increases both downwards and polewards, but mainly downwards at the heights of maximum amplitudes, i.e. 25 to 30 km. At 0" latitude there is a downward

ZONAL WIND QUASI-BIENNIAL OSCILLATIONS 77

m l s

mls

f 50Km

i, 5 m /I 10 15

m Is m1s

m l s m l t

Figure 3. Zonal wind QBO amplitude at various heights. Arrows indicate station latitudes (Table I).

Figure 4. Meridional cross-section of zonal wind QBO amplitudes in ms-'. Arrows indicate station latitudes (Table I ) .

gradient of M , with height of about 1.0 month km-l. O n moving away from the Equator the downward gradient decreases above 35 km to an average of 0.5 month km-l at 10" latitude. The pattern of a downward phase gradient extends to 30" latitude above 35 km

73 G. V. GROVES

Km

LO 30 \ -\ { 1 I I I -

-20 -10 0 10 -20 -10 0 10 -30 -20 -10 0 10 -10 0 10

Mo MO M O M a

20

Km 60 * -

H15p;T-1 LO 30

-$ "'.., \.i --=-- - \ . 20 , I I I I I , , I -10 0 10 20 10 0 10 20 -10 0 10 20 30 -10 0 10 20 30

Mo Ma Ma Ma

Km 60 5yj iLj* 5- i 40

20 u - I - -10 0 10 0 K) 20 0 I0 20 30 0 10 20 30 -10 0 10

Ma Ma Ma MO Ma

H I

--* . 3o i- I101 IIII I121 ------- I131

Figure 5. Height profiles of zonal wind QBO phases, M,, the number of months after 1 January 1966 when maximum flow from the west occurs.

but at lower heights M , changes very little with height and increases .polewards. From 30" to 55" latitude the phase pattern is not everywhere well determined because either ampli- tudes are small or observing sites are few. At higher latitudes M , changes little with height and increases equatorwards. The latitudinal changes of phase are not simply described: the largest gradients are about 2 months degree latitude-'. At 50" to 60" latitude and also at low latitudes M o changes mainly with height, i.e. there is some indication that, in regions of enhanced amplitude, the phase changes mainly with height.

6. DISCUSSION By least-squares analysis W-E wind components of 32 months period have been

derived from rocket data (25-60 km altitude) for sites at various latitudes. The period of 32 months usually lies within two months of the most characteristic periodicities deter- mined for the various series of data by the least-squares .method (Fig. 1). Reversals of W-E flow in equatorial balloon data also show periodicities close to 32 months for the years appropriate to the rocket data analysed. For earlier years (1956-62) the balloon data give an average period of 26 months and in recent years (1969-70) the period has decreased to below the 26 month value again (Fig. 1). For the intermediate years the assumption of a constant periodicity (32 months) has been a better approximation than might generally have been the case.

ZONAL WIND QUASI-BIENNIAL OSCILLATIONS 79

50 Krn 55 Km 60 Km

LA1 I TUDE

Figure 6. Zonal wind QBO phases Mo against latitude. M o is the number of months after 1 January 1966 when maximum flow from the west occurs.

Km 60.

Ht

1 1 1 - 1 80° 70' 60° 50° LO0 zoo loo 0"

LATITUDE

Figure 7. Meridional cross-section of zonal wind QBO phases, Mo, the time when maximum flow from the west occurs. Numbers on curves give Mo in months after 1 January 1966. Arrows indicate station latitudes

(Table 1). Broken lines indicate that phase pattern is uncertain as amplitudes are small.

The close similarity of results for Fort Sherman (9"N) with those from Ascension Is. (8"s) is evidence of equatorial symmetry at low latitudes. Sufficient data from other Southern Hemisphere sites were not available to examine hemispherical differences at other latitudes and the results presented are therefore for the Northern Hemisphere.

A main objective of the analysis has been to examine the latitudinal dependence of the zonal wind QBO. The results presented in Figs. 3 and 6 are not without uncertainties

80 G. V. GROVES

as more sites are needed to fill in the gaps in latitude, there being only one site between 38" and 58"N; and phases become indeterminate when amplitudes are small.

The construction of Figs. 3 and6, and also of Figs. 4 and 7, is only justified if longitudinal changes are small compared with latitudinal changes, and if time variations, other than those expressed by Eq. (I), are absent. Evidence from balloon data below 25 km is that the weak QBO at higher latitudes suffers much greater variability with longitude a rd time than that at low latitudes (Dartt and Belmont 1970). The three high latitude sites (Fort Churchill, Fort Greely and Thule) cover 77" of longitude and if longitudinal effects are important the results must be taken to apply to a mean longitude (103"W). Time variability is unknown but may be less during years of nearly constant QBO period such as we have here. Evidence from balloon data (Dartt and Belmont 1970) on organized progressions of phase with latitude is that these changed markedly during 1960-61, i.e. during years of rapid change in period. Also, in the tropical stratosphere, the QBO was characterized by unusually low amplitude, short-period fluctuations during 1960-61. Hence, for the years covered by the present analysis when changes in the period were small, it is possible that time variability does not significantly affect Figs. 3, 4, 6 and 7.

ACKNOWLEDGMENTS

The preparation of data for analysis and of the results derived has been undertaken with invaluable assistance from Miss A. Harris, Miss J. Norman and Miss B. Waters. This research has been supported by a grant from the Science Research Council and Contract F 61052-68-c-0057 of the U.S. Office of Aerospace Research through the Air Force Cambridge Research Laboratories.

Angell, J. K. and Korshover, J.

Central Aerological Observatory Bulletin

Dartt, D. G. and Helmont, A. D.

Ebdon, R. A.

Exametnet Data Report Series

Groves, G. V

Hamilton, R. A. Reed, R. J.

Smith, L. B.

196.5

1968

1969

1970

1967

1971 1968

1969 1070 1071

1070 196.5a

196.5b

1962

1966 1 069

REFERENCES ' A note on the variation with height of the quasi-biennial

oscillation,' J . Geophys. Res., 70, pp. 3,851-3,856. ' Results of rocket probing of the atmosphere, Volgograd

196.5 (second half) and 1966,' Central Administration of the Hydrometeorological Szrvice, Moscow.

' Results of rocket probing of the atmosphere, Volgograd December 1966 to December 1967,' Ibid.

' A global analysis of the variability of the quasi-biennial oscillation,' Quart. J . R. Met. Soc., 96, pp. 186-191.

' Possible effects of volcanic dust on stratospheric tempera- tures and winds,' Weather, 22, pp. 24.5-249.

Private communication. Annual Report 1966. NASA SP-17.5, Washington D.C.,

Ibid., 1967. NASA SP-176, Washington D.C., 180 pp. Ibid., 1968. NASA SP-231, Washington D.C., 180 pp. ' Atmospheric structure and its variations in the region from

25 to 120 km,' Environmental Research Papers, No. 368, AFCRL-71-0410, Air Force Systems Command, USAF, 200 pp.

Private communication. ' The quasi-biennial oscillation of the atmosphere bstween

30 and 50 km over Ascension Island,' J . Atmos. Sci., 22,

' The present status of the 26-month oscillation,' Hull. Amer. Met. Sac., 46, pp. 374-387.

' Monthly wind measurements in the mesodecline over a one-year period,' /. Geophys. Res., 67, pp. 4,653-4,672.

Private communication. ' Observations of atmospheric density, temperature, and

winds over Kauai,' Sandia Laboratories Report SC-RR- 68-523. 01 pp.

185 pp.

pp. 331-333.

ZONAL WIND QUASI-BIENNIAL OSCILLATIONS 81

Smith, W., Katchen, L., Sacher, P., 1961 Swartz, P. and Theon, J.

Smith, W. S., Theon, J. S., 1970 Casey, J. F. and Horvath, J. J.

Smith, W. S., Theon, J., Katchen, L. 1966 and Swartz, P.

Smith, W. S., Theon, J. S., Swartz, P. C., Casey, J. F. and Horvath, J. J.

Stroud, W., Nordberg, W., 1960 Bandeen, W. R., Bartman, F. L. and Titus, T.

1969

Tucker, G. B. and Hopwood, J. M. 1968

World Data Center A for Meteorology 1962.1970

‘ Temperature, pressure, density and wind measurements with the rocket grenade experiment 1960-1963,’ NASA T R R-211, Washington D.C., 60 pp.

‘ Temperature, pressure, density, and wind measurements in the stratosphere and mesosphere, 1968,’ Ibid., R-340, Washington D.C., 77 pp.

Temperature, pressure, density, and wind measurements in the upper stratosphere and mesosphere, 1961,’ Ibid., R-245, Washington D.C., 84 pp.

‘ Temperature, pressure, density, and wind measurements in the stratosphere and mesosphere, 1967,’ Ibid., R-316, Washington D.C., 83 pp.

‘ Rocket-grenade measurements of temperatures and winds in the mesosphere over Churchill, Canada,’ J . Geophys. Res., 65, pp. 2,307-2,323.

‘ The 26-month zonal wind oscillation in the lower strato- sphere of the Southern Hemisphere,’ J. Atmos. Sci., 25,

‘ Meteorological Rocket Network Data Reports,’ US. pp. 293-298.

Department of Commerce, Asheville, N. C .