Inter-annual variability of tides in the mesosphere and lower thermosphere

Inter-annual variability of tides in the mesosphere and lower thermosphere

G. J. FRASER*

Department of Almospheric, Oceanic and Planetary Physics, University of&ford. Oxford, U.K.

R. A. VINCENT

Physics Department. University of Adelaide. Adelaide. Australia

A. t-1. MANWN and C. E. MEEK

Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, Canada

and

R. R. CLARK

Department of Electrical and Computer Engineering. University of New Hampshire, Durham, N.H.. U.S.A.

Abstract--The inter-annual variation in diurnal and semi-diurnal atmospheric tides between 85 and 95 km has been studied for various years between 1978 and 1988. Observations comprised wind measurements from the medium frequency SA mode wind radars at Adelaide (355). Christchurch (44 ‘S) and Saskatoon (52’N) and the meteor wind radar at Durham (43’N). Although the observations include the interval between solar maximum and solar minimum, there is in general no correlation between tidai amplitudes and solar activity. fn contrast with earlier studies there does appear to be a positive correlation between solar activity and the amplitude of the senii-diuri~aI tide, but only during the southern summer and simultaneous northern winter

1. INTRODUCTION

The Atmospheric Tides in the Middle Atmo- sphere Project (ATMAP) of the Middle Atmosphere Program (MAP), with its emphasis on simultaneous global observations has led to an increased under- standing of atmospheric tides. Several of the observation programmes associated with ATMAP have been sufliciently long to show inter-annual variability in tidal parameters (FRASER. 1988; TSUDA

af &., 1988 ; VINCENT et a/., 1988). Early work on long-term variations (SPRENGRR and

SCHMINDER, 1969) showed some solar control of the semi-diurnal tide at northern mid-latitudes, the amplitude decreasing with increasing solar activity and this relationship is still considered valid (GREISIGER et al., 1987). There is also evidence, in 30 years of meteor wind results, of a 22 year periodicity in the amplitude of the semi-diurnal tide (D'YACHENKO ct ui., 1986). Here, too, the tidal amplitude is a maximum during

* Permanent address : Physics Repartinent, University of Canterbury, Christchurch, New Zealand.

years of minimum solar activity. However, FRASER

(1988) found a decrease in the amplitude of the semi- diurnal tide over a short period (198&1986) of decreasing solar activity, but these results were limited to one height (90 km) and one location (Christchurch, 44 S, 173’ E) and are discussed further below.

A recently uncovered difficulty in the study of a solar cycle component of inter-annual variability is the jnterference from the quasi-biennial oscilIation (QBO) (LABITZKI: and VAN LWN. 1988). These authors found that the correlation between solat activity. and stratospheric temperatures and gcopo- tential heights depended on the phase of the quasi- biennial oscillation.

Other periodicities have also been reported; for example PORTNYAGIN (1986) found that variations with quasi-periods of 3-.-5 years were apparent in meteor wind data from Obninsk (55”N. WE) between 1968 and 1981.

2. OBSERVATIONS

It is desirable that in an initial study small scale variations with observation height and radar location

555

556 G. J. FRASER ei 01.

such as these which might arise from interference

between tidal modes (FORBES, 1982b) should be

reduced by using observations from several locations,

averaged over a range of heights. The observations

discussed here were made by ground-based wind radars at four locations, two in the northern hemisphere and two in the southern hemisphere. These radars are the medium frequency partial reflection SA mode radars at Adelaide (35 S, 138 E), Christchurch (44s. 173 E) and Saskatoon (52 N, 107”W), and the meteor wind radar at Durham (43 N, 71 W). The data used in this study were :

(a) Adelaide: monthly mean values for January

and July, from 86, 88. 90, 92 and 94 km. (b) Christchurch: monthly mean values for Jan-

uary and July. from 85. 87.5, 90. 92.5 and 95 km.

(c) Durham : mean values for 4 to IO days in Jan- uary and July, from 85 and 95 km.

(d) Saskatoon: monthly mean values for Decem- ber and June, averaged over 87-95 km.

The observational uncertainties depend on factors

such as the number of observations in the monthly mean and the variance due to planetary and gravity waves in the month, but are typically in the range of l-4 m so ’ in amplitude and 0.2-1.0 hours in phase.

The direct approach to estimating the inter-annual variability is to study the required parameter at a given height, for a given interval, at a given location.

Using such a procedure, FRASER (1988) found that the monthly mean amplitude of the zonal component of

the semi-diurnal tide in January (summer) at Christ- church, at 90 km, decreased from approximately 20 m s- ’ in 1980-1981 to less than 5 m s- ’ in 19855 1986. The averaging interval of one month overcomes

difficulties, due to tidal variations over several days, encountered in early ATMAP campaigns (FORBES, 1986). The limitation to one observation height however may lead to undesirable fluctuations as FORBES (198213) points out that in this altitude region the semi- diurnal tide is made up of at least four modes. Model results (FORBES and GILLETTE, 1982) for 42-S in December predict a height variation in amplitude

from 12ms-‘at81 kmto56ms-‘at lOOkm.The effect of small scale variations with height have been

reduced by using averages over the range of 85 to 95 km instead of a single value at 90 km.

3. THE SEMI-DIURNAL TIDE

3.1. Southern hemisphere

Figure 1 shows the monthly mean semi-diurnal zonal amplitude at Christchurch averaged over

heights of 85.0, 87.5, 90.0, 92.5 and 95.0 km, for the

months of January (Fig. l(a)) and July (Fig. l(b)).

The height-averaged values are shown as circles and

the 90 km values (from FRASER, 1988) are shown as

pluses. The crosses are the corresponding values for Adelaide (averaged over 86, 88. 90, 92 and 94 km). In general, the height averaging has produced the expected decrease in fluctuation of the Christchurch data. The summer trend of decreasing amplitude from 1981 to 1986 remains in the height-averaged data, although of smaller slope, and is also evident in the

Adelaide data. Variations in the meridional semi-diurnal component are shown in Fig. 2. The decreasing

amplitude in summer from 1981 to 1986 closely fol- lows that of the zonal component at Christchurch and Adelaide. There is no significant trend in winter. In both summer and winter there arc irregularities on

time scales of 3-6 years, as noted by PORTNYAGIN

(1986). The phase variations corresponding to the ampli-

tude variations of Figs. I and 2 are shown in Figs. 3 and 4. There do not seem to be any significant trends.

As the phase is sensitive to vertical movement of tidal structure even if the amplitude is constant and as the amplitude of tidal forcing is the more significant parameter, further information on phase variations is omitted.

3.2. Northern hemisphere

The existence of an inter-annual variability in tidal parameters must be considered for data from comparable seasons in the northern and southern hemi-

spheres. For example, a northern winter may be com- pared with the preceding southern winter or the succeeding winter. The two comparisons will differ.

depending on the difference between the two southern winters.

The northern observations used here differ slightly from the southern observations. The Saskatoon data

are monthly means for June and December, averaged over 87795 km, while the Durham data are the means

for January and July of observations at 85 and 95 km and of 8-10 days each month, except for July 1980 in which observations were made on 4 days. Figure 5 shows the amplitude of the zonal component of the semi-diurnal tide at Saskatoon and Durham for the northern summer (a) and winter (b). In the discussion of northern hemisphere tidal components the summer and winter seasons have been specified, as in the southern hemisphere, by the months of January and July. For example, the winter points for 1983 are from the Durham data for January 1983 and the Saskatoon data for December 1982.

The summer semi-diurnal zonal (Fig. 5a) and mer-


Zonal semidiurnal amplitude, southern summer (January>

ss

5

+ Christchurch (90 km)

0 Christchurch (85-95 km)

X Adelaide (86-94 km> (a>

+ 0

+ X

X

2 + z X

0

0 0

0

+ +

0 ’ 1977

I 1902 1987

Year

Zonal semidiurnal amplitude, southern winter (July)

+ Christchurch (90 km)

0 Christchurch (85-95 km)

X Adalaida (86-94 km>

0 -t

1982 1987

Year

Fig. 1. Monthly mean semi-diurnal zonal amplitudes for the southern hemisphere radars at Adelaide (averaged over 8694 km) and Christchurch (averaged over 85-95 km) for (a) summer (January) and (b)

winter (July).

55x

flsridional samidiurnal amplitude, south-n summer (January)

o Chriotchurch (85-95 kin>

X Adelaide (86-94 km>

Year

Fig. 2. Meridional atnplitudes (details as in Fig. I).

Inter-annual variability of tides in the mesosphere and lower thermosphere 559

Zonal semidiurnal phase, southern summer (January>

0 0 0 0

0 0

*1977 1982 t987

Year

i

Zonol semidiurnal phase, southern ainter (July)

2 3 a 5

I 5 o Christchurch C85-95 km)

f

Year

Fig. 3. Zonai phases (details as in Fig. it.

G. J. FRASEK et al

Hsridionaf semidiurnal phase, southern stammer (January)

c 5 0

5

t 9

0

f

0

o Chrirtchurch (85-95 kmf

X Adelaide (X6-94 km)

Meridional semidiurnal phase, southern winter <July)

10

A

E a 5

ii

3

f

0 Christchurch (85-95 km>

X Adelaide (86-94 km)

U3>

X

0

0 X

0 0 0 0 X

0 0 5

1982 1987 Year

Fig. 4. ~~ridio~l phases (details as in Fig, 1)

Inter-annual variability of &ides in the mesosphere and lower thermosphere

Zonal ssmidiurnal amplitude, northern summer (Jun/Jul>

1. u Sdatoon (87-95 km)

A Durham (85+95 km) (1 a

‘ Oi977 19a2 1987

Ysar

Zonal semidiurnal amplitude, northern ufinter <Dee/Jan>

q Saskatoon (87-95 km>

A Durham (85+95 km)

0 0 A n

A

a

(b>

A

u a cl

IY

a

0 L 19?7

I 1982 1987

Year

Fig. 5. Monthly mean ~mi-diurnal zonal amplitudes for the northern hemisphere radars at Durham (average of X5 and 95 km observations) and Saskatoon (averaged over 86-95 km). Durham observations

for January of the year shown are paired with Saskatoon observations for the preceding December.

562 G. J. FRASER ei c/l.

idional (Fig. 6a) components do not show a systematic amplitudes in even and odd years, but as the alter-

variation with time and the variations at Durham nation amplitudes are comparable with the uncer-

and Saskatoon are similar. The winter amplitudes tainties of measurement and there may be differences

(Figs. Sb and 6b) suggest a variation with solar activity of comparable magnitude between successive months

similar to that in the southern summer. At Saskatoon in the same season it is doubtful whether the alter-

after 1982 there is an alternation of high and low nation could be attributed to a QBO contribution.

Meridional semidiurnal amplitude. northern summer (Jun/Jul)

% - o Soskotoon (87-95 km)

A Durham (85+95 km) (a> 30 - _

,-t : a- .5

Gj 20 -

3 ._I

- B I5

- 0

0

A 0

az

A i?l A 0

10 -

0 0

Cl 0

5 - 9

0 1977 1982 1981

Yeor

Meridional semidiurnol amplitude, northern winter (De&Jan>

40 I

% - q Saskotoon (87-95 km)

A Durham (85+95 km) (b> -

SO -

c\ A

f 255-

% q 20 - A 0 D A

3 0 0 0 .A

-

E” IS -

Cl

I

c 0 0

10 -

0 ,977 1922 1987

Yeor

Fig. 6. Meridional amplitudes (details as in Fig. 5).

Inter-~nnu~~l ~~r~~~ility of tides in the mesosphere and luwer tllerInos~h~re 363

4. DIURNAL TIDE relative to those at Ghristchurch in July, and the simi-

larity of Intel-a~nllai variation at &he two locations. 4. I S~~it~~~~~~ htmispllrre The ratios for the meridional components in 1984,

The inter-annual variation in the arnplit~~c of the 1985 and 1986 are, respectively, 4.1, 4. I and 4.3 : the di~r~~~ tide for ~~n~dr}r and July at Adelaide and zonal ratios are 2.1. 1.7 and 2.7. tn comparison with

Christchurch is shown in Figs. 7 and 8. Significant recent models for the diurnal tide (VIAL. 1986: FMWES

features are the large ratio of the Adelaide amplitudes and HAGAN. I%%) the results arc consistent with the

Zonol diurnal amplitude, southern summer (January>

o C~~is~c~~~ (85-95 km1

X Adelaids (86-94 km1 ( ) a

0

7 1992 Isa7

Year

Zonal diurnal amplitude, southern winter (July>

o Christchurch (85-95 km>

x Adelaide (G-94 kml (b>

X X

X

0

564 G. J. FRASER er al.

Meridional diurnal amplitude, southern summer ~January~

0 Chrinbchvrch (85-95 km)

X fidelaide (86-94 km)

0 0

0 0

X

0

0 X

0 x

8 1992 1907

Year

Meridional diurnal amplitude, southern luinter (July>

IQ

0 Christchurch C85-95 km)

1 Ql977 I

1902 1007

Year Fig. 8. Southern hemisphere meridional diurnal amplitude (other details as in Fig. 1)

rapid change with latitude of the diurnal ampIitude. The models predict a higher zonal diurnal amplitude in summer at both latitudes, but the Adelaide summer amplitudes are actually smaller than the winter amplitudes, while those at Christchurch are comparable in both seasons. VINCENT et at. (198%) showed that the zonal diurnal amplitude at 90 km at Adelaide is often a minimum of IO-15 m s-’ in December or January and in March or April rises to a maximum which may be as high as 40 m s- ’ (March, 1985). Figure 9 shows

the zonal diurnal amplitude for the summer months (December, January and February) at Adelaide and Ch~stchurch.

4.2. Northern hemisplwre

As expected from a decrease in amplitude with latitude, the diurnal amplitude at Saskatoon (WN) (Fig. IO) is less than at Adelaide (35”s) and Christchurch (44”s). VINCENT et al. (1988), in their comparison of the diurnal tide at Adelaide (35”s) and Kyoto (35”N),


o December

o Januaqj X

X Fsbrvory

X 0

Cl 0

(a> X

X 0

: a

Zonal diurnal amplitude, Adelaide, 86-94 Cm

Zonol diurnal amplitude, Christchurch, 85-95 km

1922 I927

Year

q December

0 January

X February (b>

q

X

o :: 0 X

0

1982 1927 Year

565

Fig. 9. Variation of zonal diurnal amplitude during successive summer months at (a) Adelaide and (b) Christchurch.

point out that hemispheric differences in wind and in dissipation may also contribute to the smaller amph- tudes observed in the northern hemisphere. There is no systematic variation of amplitude over the period of analysis.

DISCUSSION

Earlier studies on the inter-annual variation of tidal parameters in the middle atmosphere have shown an

increasing semi-diurnal amplitude with decreasing solaractivity (SPRBNGER and SCHMINDER, 1969; PORT- NYAGIN et al., 1977; DARTT et al., 1983; GREISIGER et

al., 1987). Most of the results presented here for part of a solar cycle, from maximum to minimum, do not show any systematic variation with solar activity. However the decreasing solar activity does appear to be accompanied by a decrease in amplitude of the semi-diurnal tide during the southern summer and

56h G. J. FRASER et al.

Zonal diurnal amplitude, northern summer (June)

CI Soskatoon (86-95 km>

(cl>

19K2 1927 Year

Zonaf diurnal amplitude, northern winter (December)

**

0 Soskatoon (86-95 km) (b>

Fig. IO. Northern hemisphere ronal diurnal amplitude (other details as in Fig. 5).

northern winter. This difference in hehaviour between variations during ATMAP campaigns (FORBES, 1986) the earlier studies and this one may indicate a hemi- this study has shown that month-to-month variations spheric difference in solar control although there are within the same season at the same location may differing opinions (MO~ANAKUMAR, 1985 ; GROVES, be comparable with the inter-annual variations for

1986) on the existence of a hemispheric asymmetry in monthly means of the same month in successive years.

solar control. The month-to-month differences in the summer diurnal

Although averages over one month were selected amplitude at Adelaide [Fig. 9(a)] may be larger than

as representing typical tidal behaviour for seasonal those elsewhere (VINCENT er al., 1988) but the sample

Inter-annual variabitity of tides in the mesosphere and lower thermosphere 567

standard deviation of the eight points for Christ- n~onths which are comparable with the inter-annuls church in Fig. 9(b) is 3.0 m s-‘, which is not sig- variations the discernment of patterns in inter-annual nificantly different from the estimated uncertainty in variations is uncertain. A study based on three*month the Fourier analysis (2.4-4.0 m s - ‘) for these points. averages may be necessary to reduce this uncertainty. In the presence of differences between successive

REFERENCFS

DAKTT D., NASTWM G. and BELMONT A. DIYACHENKO V. A., LY~ENK~ i. A. and PORTNYA~ZN

Yu. 1. FoneEs J. M. FORBES J . M. FORBES J. M. and HA~;AN M. E. i%ASEK G. J. GRWIC~ER K. M., S~HMINI~~~ R. and K&W:HNEK D. GKOVES G. V. LABITZKE K. and VAN LWN H. MANWN A. H.. MEEK C. E.. SMITH M. J. and F~AS~K

G.J. M~HANAKU~AR K. PQRTNYAGIN Yu. 1. PORTNYAC;IN Yu. I., KAWALW 0. V.. Ga~~s~~~u K. M

and SPRENGER K. SPRWGEK K. and %HMINIXR R. TSUVA T., KATZ S., i%wsn;u A. H. and MEEK C. E. VIAi. F. VIN~XNT R. A., TSWA T. and KATO S.

19x3 1986

198& 1982b 198X 1988 1987 1986 1988 1985

1969 I988 1986 1988

handbook for MAP 21, ~c~enti~c C~rnrn~l~e~ on Solar-Terrestrial Physics, Urbana, Illinois. AFCRL TR-82-O I73( l), Air Force Cambridge Research Lab, Hanscom Air Force Base, Mas- sachusetts.

Documents

Inter-annual variability of tides in the mesosphere and lower thermosphere