Journal of Atmospheric and Terrestrial Physics, Vol. 55, No. 6, pp. 843-855, 1993. MU-9169/93 $6.00+ .Ul

Printed in Great Britain. 0 1993 Pergamon Press Ltd

Dynamics of the Antarctic and Arctic mesosphere and lower thermosphere regions-II. The semidiurnal tide

Yu. I. PORTNYAGIN,* J. M. FORBES,? G. J. FRASER,~ R. A. VINCENT,~ S. K. AVERY, /( I. A. LYSENKO* and N. A. MAKAROV*

* Scientific Production Association ‘Typhoon’, Obninsk, Russia ; f Center for Space Physics and Department of Aerospace and Mechanical Engineering, Boston University, Boston, MA 02215, U.S.A.;

$ Department of Physics, University of Canterbury, Christchurch, New Zealand ; 8 Department of Physics, University of Adelaide, Adelaide, SA, Australia ; 11 Department of Electrical and Computer

Engineering, University of Colorado, Boulder, Colorado, U.S.A.

(Receivbd in final form 4 August 1992 ; accepted 4 August 1992)

Abstract-The semidiurnal tidal dynamics of the Antarctic and Arctic mesopause regions (95 f 15 km) are investigated through comparative analyses of monthly mean tidal wind fields determined from radar measurements at the Scott Base (78”S), Molodezhnaya (68”s) and Mawson (67”s) stations in the Antarctic, and the near-conjugate stations of Heiss I. (8l”N) and Poker Flat (65”) in the Arctic region. The main feature common to all stations is the fall equinoctial maximum in amplitude (10-20 m s- ‘), which is also reproduced by the most recent numerical tidal model. However, the wintertime amplitude growth with height and the shorter vertical wavelengths characterizing the model are features not reflected in the data. There is also a spring equinoctial maximum in the Antarctic data which the model does not reproduce.

Examination of interannual variability reveals characteristics similar to those noted in Part I for the mean zonal wind ; namely, some degree of year-to-year variability superimposed on apparent long-term decreases of order 0.345 m s- ’ yr ’ (depending on month) in the Southern Hemisphere semidiurnal tidal amplitudes. Numerical simulations presented herein indicate that changes of this magnitude cannot even be induced (via mode coupling) by a change in the mean zonal wind field of order 30%, and are more plausibly explained by a secular change in the tidal forcing by ozone insolation absorption. However, contrary to Part I, the annual mean tidal amplitude is not characterized by any significant secular trend, remaining within the 10.0+2.5 m SK’ range throughout the 197Gl986 period. Analyses of other data sets are required to ascertain confidence in the apparent trend reported here.

I. INTRODUCTION

The present work represents Part II of a study dealing with the dynamics of the Antarctic and Arctic meso- sphere and lower thermosphere regions. In Part I (PORTNYAGIN et al., 1993) the zonal and meridional prevailing wind systems are investigated through com- parative analyses of winds measured by radars at the Scott Base (78%) Molodezhnaya (68%) and Mawson (67”s) stations in the Antarctic, and the near- conjugate stations of Heiss I. (81”N) and Poker Flat (65”) in the Arctic region. The data are analyzed specifically to delineate hemispheric differences in mean monthly prevailing wind climatologies, and show the circulation systems in the Arctic and Ant- arctic mesosphere and lower thermospheres to exhibit significant asymmetries. Interannual variability was also examined, the main result being a near-mono- tonic decrease in the annual mean eastward wind from 2&25 to 5 m s- ’ from 1968 to 1977 over Molo- dezhnaya station. Finally, existing empirical models were evaluated against the data, and were shown to

be deficient in reproducing some salient characteristics of the high-latitude circulation systems.

In Part II, we will follow basically the same format except focusing on the semidiurnal tidal component. The same data described in table 1 of Part I are utilized here. The main properties of the semidiurnal tide needed to comprehend the results and conclusions contained herein are as follows. (a) The semidiurnal tide observed in the mesosphere/lower thermosphere region is primarily forced by insolation absorption by ozone between 40 and 60 km. (b) The mean zonal winds below 80 km modify the semidiurnal tide, pri- marily by coupling into higher-order modes which are characterized by short (- 25-50 km) vertical wave- lengths and which achieve their largest wind ampli- tudes at high latitudes (> 60”).

Both Parts I and II of our study were in fact moti- vated by the result of a numerical simulation performed by FORBES and VIAL (1989). In that work Forbes and Vial present numerical results describing semidiurnal tidal oscillations in wind and temperature in the meso- sphere and lower thermosphere for each month of

843

844 Yu. I. PORTNYAGIN et al.

the year. These calculations benefited from recently calculated heating rates due to Hz0 and O3 insolation absorption (GROVES, 1982a,b), zonally averaged wind, temperature, and pressure fields for each month of the year including hemispheric asymmetries (LABITZKE et al., 1985), and parameterizations of eddy diffusivities determined from gravity wave saturation climatologies and used by GARCIA and SOLOMON (1985) to simulate oxygen photochemistry and trans- port in the mesosphere and lower thermosphere. The LABITZKE et al. (1985) tabulations in fact form the basis below 80 km for the FLEMING et al. (1988, 1990) model which was used in Part I for comparison with the prevailing wind data.

Forbes and Vial extended the Labitzke et al. tabu- lations to 100 km by applying the geostrophic relationship to the mean zonal wind field. The seasonal progression of the zonal mean wind field at 74”N and 74”s (with a 6-month shift introduced to facilitate comparison between hemispheres) and util- ized in the Forbes and Vial tidal calculations is shown in Fig. 1. The greater intensity of mean zonal winds which prevail in the Southern Hemisphere are par- ticularly evident. These hemispheric asymmetries affect the tidal propagation by distorting the tidal structures through the excitation of antisymmetric

ZONRL MERN UINO LRT-74 N

I I

JFMRHJJASONO

components. An example of the corresponding tidal structures calculated by FORBES and VIAL (1989) at 74”N and 74”s are illustrated in Fig. 2. (For later reference, the basic features depicted in Fig. 2 are also apparent in the model results for 66” latitude.) The Forbes and Vial model indicates larger tidal ampli- tudes in the Northern Hemisphere, with a maximum in amplitude occurring above 85 km during August/ September. There is also a tendency towards a simi- lar feature during March equinox. The Southern Hemisphere results give no indication of such equi- noctial maxima. Vertical wavelengths are generally smaller during summer than winter, with a relatively smooth transition across the equinoxes. Significant differences exist between Northern and Southern Hemispheres in terms of absolute phases, but to a lesser extent in terms of phase change with height.

On the basis of these results one anticipates similar- magnitude hemispheric differences to appear in the Antarctic and Arctic data to be analyzed here. These observational data will be presented in the following Sections 2.1-2.3, and where appropriate comparisons with the FORBES and VIAL (1989) results will be made. In Section 4 a more detailed comparison of the FORBES and VIAL (1989) model with Poker Flat and Mawson observations will also be presented.

ZONRL HERN WIN0 LAT=74 S StilFTEO

JFHfltlJJRSONO MONTH.

Fig. 1. Mean zonal winds at 74”N and 74”s latitude used in FORBES and VIAL (1989) study. The 74”s contours are shifted bv 6 months to facilitate comparison with the 74”N contours.

s 95

Lfi 90 2 Z

d 85

80

Antarctic and Arctic mesosphere dynamics-11 845

ERSTHARO WIN0 LOT=74 N ERSTUAAD UINO LZ1T=74 5 SHIFTED

~~PLIT~El~/Si ~~~LITUOE~~/Sl

PHRSEIL.T.1 PHF6EfL.T. I

100 100

ii 9s 95

3 90 90 ;: r: 65 85

80 80 JFf4RflJJRSONO ~FHR~~JJRSONIJ

MONTtl MONTH

Fig. 2. Numerical simulations (FORBES and VIAL, 1989) of the semidiurnal eastward wind amplitude (top) and phase (bottom) at 74”N (left) and 74“s. Contour spacings are 5 m s- ’ and 1 .O h. The 74”s contours

are shifted by 6 months into the Northern Hemisphere to facilitate comparison with the 74”N results.

2. RESULTS

2.1. Monthly climatologies

Figures 3, 4 and 5 ihustsate, respectively, the east- ward and northward semidiurnal amplitudes and phases for Mawson (67%) Poker Flat (WN), and Scott Base (78”s). Note that the Poker Flat contours are shifted by 6 months to facilitate comparison with the Southern Hemisphere data. Before interpreting these data, a few cautionary remarks are in order. Given the relatively short vertical wavelengths char- acterizing the model results in Fig. 2, and the sen- sitivity of the tidal fields to the zonal mean wind field, the possibility must be admitted that the construction of monthly means may introduce amplitude sup pression due to destructive interference if phases change significantly over the averaging interval. On the other hand, from experience gained in the Atmo- spheric Tides Middle Atmosphere Program (see July/August, 1989, special issue of Journal of Atmo- spheric and Terrestrial Physics), it is known that averaging over 10 days or more is required in order to isolate the steady-state global-scale tidal oscillation from oscillations contributing energy at or near tidal frequencies which are associated with localized and

transient forcings (i.e gravity wave interactions). It must therefore be considered that the monthly clima- tologies in Figs 3, 4 and 5 contain only those salient features which are persistent and characteristic in the long term of that particular month.

The amplitude contours for Mawson and depicted in Fig. 3 exhibit a primary maximum in the fall equi- noctial season and a lesser one during spring equinox, similar to that appearing in the model results for the Northern Hemisphere (Fig. 2). However, the ampli- tude growth with height characteri~ng the winte~ime model results above 90 km are not apparent in the data. The phase structures for the semidiurnal tide over Mawson represent a major deviation from the numerical model, consisting of relatively little phase progression with height and no perceptible equinoctial transitions. This may be an artifact of the phase inter- ference effect noted above. The Poker Flat data (Fig. 4) only exhibit the fall equinoctial maximum (1 O-20 m s- ‘), and the phase progression with height is still weak compared to the FORBES and VIAL (1989) model. However, in the Poker Flat observations there do exist tendencies towards shorter vertical wavelengths during summer and for equin~tial phase transitions, more in line with the mode1 characteristics. Similar

Yu. I. PORTNYAGIN et al. 846

SMIOILRPRL Etwwul

mmLN wP!_ITuIE

104.0

100.0

P

5

i! c d

96.0 _

92.0 * .

Be.0

84.0

PnIoIlJlN?i NYRTtMrnO

mus.M IWUTWE

1 F I4 R I4 J J A $ 0 N 0 nmni

JFttRtlJJASONO mNln

Fig. 3. Height vs month contours of amplitude (left) and phase (right) for eastward (top) and no~hward (bottom) semidiurnal winds over Mawson. Contours are based on monthly averages of observations taken

from June 1984 to December 1986. Contour intervals are 5 m s- ’ and 2 h.

features are reflected in the Scott Base data (Fig. 5), by approximately 1000 km ; the Mawson radar uses except that the maximum occurs closer to late summer the spaced antenna drift method whereas the Molo- (February) than fall equinox (March). dezhnaya radar is of the meteor type; and results for

Molodezhnaya correspond to a nominal height of

2.2. Site comparisons 95 km, but this altitude may differ by as much as 5 km between summer and winter extremes due to

Figures 6 and 7 depict amplitudes and phases, temperature effects on the meteor ablation process respectively, for the semidiumal tidal winds near (ELFORD, 1980). Figures 6 and 7 include eastward and 95 km measured by the Mawson and ~olodezhnaya northward amplitude and phase dete~nations from radars. As noted in Part I, these stations are separated the various beam directions of the Molodezhnaya

Antarctic and Arctic mesosphere dynamics-II

sMtotlRW_ =rRsTWul SMInilRWn EFEDWn FuEsFLsT IWLITUIE rnlQR RRT PHWE

106.0

93.0

80.0 JRSONO

noWmJ FMAHJ JRSONOJFMAMJ

mNni

Fig. 4. Height vs month contours of amplitude (Ieft) and phase (right) for eastward (top) and northward (bottom) semidiurna1 winds over Poker Flat. Contours are based on monthly averages of observations taken during 1983-1984, and are shifted by 6 months for ease of comparison with the Antarctic station

data. Contour intervals are S m s ’ and 2 h.

radar, and show essentially the same results for each 96 km over Mawson occur during spring equinox, but method of determination. The Mawson data include are absent during 1986. It is also evident that the measurements from both 94 and 96 km, demon- phases during the spring of 1986 are different from strating that strong shears are not present, and those in 1984 and 1985. Both of these effects tend to furthermore that the Mawson measurements are in suppress the amplitudes of the equinoctial maxima very good overall agreement with the Molodezhnaya shown in the average contours of Fig. 3. The Molo- data. The phase data from both stations indicate that dezhnaya data tend to reflect the same maxima and quadrature between eastward (leading) northward minima, but there are occasional differences : the winds exists to a reasonable approximation, in accord- spring equinoctial maximum is missing in 1984, and ance with simple tidal theory. The data in Figs 6 and 7 while amplitudes are in excellent agreement during exhibit aspects of seasonal and interannua1 viability. summer of 1985, differences in phase of order 3 h exist During 1984 and 1985, maxima in the amplitudes at between the two stations. Such differences are larger

SEnIllImtaL rmtwAR0 Sm!IOIu?MlL NwrtMm)o

SCOTT BASE fWLIlUCflWSl SCOTT mSE PHRSE

J F bi R H J J R S 0 N 0 J F H A N J J R S 0 N 0 milti HDNTH

Fig. 5. Height vs month contours of amplitude (left) and phase (right) for eastward (top) and northward (bottom) semidiurnal winds over Scott Base. Contours are based on monthly averages of observations

taken during December t982-Nov~rn~r i 984. Contour intervals are 5 m s- ’ and 2 h.

SEMIDiURNAL TIDAL AMPLITUDES OVER MAWSON AND MOLODEZHNAYA (95 km1

‘# E

0 JJASONOJFMAMJJASONOJFMAMJJASONO

1984 1985 1986 Fig. 6. Month-to-month variability of semidiurnal tidal amplitudes from synchronous measurements made at Molodezhnaya and Mawson for 1984-1986. (a) Eastward component amplitudes: solid circle, Molodezhnaya at -95 km, sounding towards the West direction for 1984-1985, and representing an average of soundings towards the East and West for 1986; triangles, Mawson at 94 km height; open circbs, Mawson at 96 km height. (b) Northward component amplitudes: solid circles, Mofodezhnaya measurements directed towards the North direction ; triangles, Molodezhnaya measurements directed

towards the South direction ; open circles, Mawson data for 95 km.

Antarctic and Arctic mesosphere dynamics-11

SEMIDIURNAL TIDAL PHASES OVER MAWSON AND MOLODEZHNAYA (95 km)

D EASTWARD

(b)

NORTHWARD

I I I I, I III I I I I I I I I I I III I I I II III III

JJASONDJFMAMJJASONDJFMAMJJASOND

1964 1985 1966

849

Fig. 7. Month-to-month variability of semidiurnal tidal phases from synchronous measurements made at Molodezhnaya and Mawson for 19841986. (a) Eastward component phases : solid circles, Molodezhnaya at _ 95 km, sounding towards the West direction ; triangles sounding toward the East direction, and open circles, Mawson at 95 km. (b) Northward component phases: solid circles, Molodezhnaya at -95 km sounding towards North direction for 19841985 and average between North and South soundings for

1986; triangles, Mawson at 94 km height; open circles, Mawson at 96 km height.

than we would expect on strictly experimental con- siderations, leaving open the possibility that some true nonmigrating tide effects may be involved.

Figures 8 and 9 illustrate amplitudes and phases, respectively, of the zonal and meridional semidiurnal tidal winds observed over Scott Base and Heiss I. dur- ing 1983-1984. Amplitudes at Heiss I. are usually less than those at Scott Base for the same season, and in particular the summer amplitudes at Scott Base exceed those at Heiss I. by a factor of two to three. Eastward phases are generally within 2-3 h, except during the September-February 1983-1984 period where a 6-h difference exists, suggesting dominance of anti- symmetric modes over this period. Phases of the mer- idional component, which agree between the two

stations during this period and reflect significant dis- parities between stations at other times, support this conclusion.

2.3. Interannual variability

Following the format ofPart I, we will now examine the long-term variability of the semidiurnal tidal com- ponents observed over Molodezhnaya and Heiss I. Figure 10 illustrates the semidiurnal northward and eastward tidal amplitudes for 1968-1986 for the months of January, July, April and October. As for the prevailing wind component, the overall tendency in each month (except for the eastward component in January) is towards a secular decrease in amplitude from the beginning to the end of this 18-yr observing

850 YU. I. PORTNYAGIN et al.

SEMIDIURNAL TIDAL AMPLITUDES OVER HEISS ISLAND AND SCOTT BASE (95 km1

40 EASTWARD

30- ‘.

J FMAMJJA SONDJFMBMJ J A S ON D

1983 1984

Fig. 8. Amplitudes of semidiurnal tidal winds at 95 km during 1983-1984 at Heiss I. (solid line) and Scott Base (dashed line). Top : eastward component. Bottom: northward component.

SEMIDIURNAL TIDAL PHASES OVER HEISS ISLAND AND SCOTT BASE (95 km1

6 EASTWARD

3-

6-

3-

5 -0,“““““““““““”

ti

9 6 0. NORTHWARD

3-

12 -

6-

3-

011”“““““““““” 1 J FMAMJJASONDJ F M A M J JASONC

1983 1984

Fig. 9. Phases of semidiumal eastward (top) and northward (bottom) tidal winds at 95 km during 1983- 1984 at Heiss I. (solid line) and Scott Base (dashed line).

Antarctic and Arctic mesosphere dynamics-II 851

SEMIDIURNAL TIDAL AMPLITUDES OVER MOLODEZHNAYA STATION (95 km1

E 40

30

20

IO

0 70 75 80 85

(dl

OCTOBER

YEAR

Fig. 10. Year-to-year variability of the semidiurnal tidal amplitudes at Molodezhnaya and Mawson at 95 km : open circles, Molodezhnaya northward ; solid circles, Molodezhnaya eastward ; open triangles, Mawson, eastward ; closed triangles, Mawson northward. (a) January ; (b) July ; (c) April and (d) October.

period. Excluding the unusual amplitudes during 1968, 1969 and 1970, however, only a small secular trend is discernible, amounting to 0.345 m s- ’ yr- ’ for most months. Superimposed are equally sig- nificant changes over periods of l-5 yr. The cor- responding phases in Fig. 11 show relatively little change over the whole period. Annual mean ampli- tudes exhibit no perceptible trend, remaining at the 10.0+2.5 m s-’ level from 1970 through 1986.

Similar results for the semidiurnal amplitudes and phases over Heiss I. for January and July are sum- marized-in Fig. 12. Results for April and October (not shown here) share the same basic characteristics. While there is some tendency for secular decrease in the January data, a conclusion drawn as such would be dubious. Moreover, the interannual variability of the phases is quite significant over l-5 yr time scales. For January, this might be the result of year-to-year variability in stratospheric warmings, and hence in the zonal mean wind field through which the tides propagate.

Long-term changes in the tidal amplitudes at a single high-latitude site (cf. Fig. 10) may be explicable in terms of a secular decrease in the ozone forcing.

Another plausible explanation is possible in terms of a progressive distortion of the horizontal tidal structure due to model coupling effects vis-a-vis a long-term change in the zonal mean wind field. Such a change in the zonal mean wind field may itself be connected with secular changes in the ozone forcing. A decrease in ozone forcing in turn may be connected with changes in either UV solar fluxes or in the ozone concentrations. There also exists new evidence that a non-negligible in-situ heat source may exist near the mesopause due to heating by exothermic chemical reactions (MLYNCZAK and SOLOMON, 1991). However, the importance of such sources relative to the semi- diurnal oscillation at 95 km due to ozone-driven upward-propagating components is presently un- known.

Separation of the above effects and evaluation of their relative importance requires a comprehensive modeling effort, preferably one that self-consistently solves for the zonal mean and tidal fields. Such a model is not presently at our disposal. However, we have exercised the FORBES (1982a,b) model in a sensitivity study to evaluate one of the possibilities, namely the influences of a steadily decreasing zonal

852

IO

5

I-

-’ 0

6

Ku. I. PORTNYAGIN et al.

SEMIDIURNAL TIDAL PHASES OVER MOLODEZHNAYA STATION (95 km)

J N n

APRIL 1 N

o- 70 75 60 85

JULY

OCTOBER

YEAR

Fig. 11. As in Fig. 10, except for semidiurnal tidal phases.

SEMIDIURNAL TIDAL AMPLITUDES AND PHASES OVER HEISS ISLAND (95 km1

N

70 75 60 65

20 T

i IO

0

J I I I I I I I I I

55 70 75 60 65 65 70 75 60 65

(b)

:d)

YEAR

Fig. 12. Year-to-year variability of semidiurnal amplitudes and phases at 95 km over Heiss I. : solid dots, eastward component; open circles, northward component ; (a) January amplitudes ; (b) July amplitudes ;

(c) January phases and (d) July phases.

Antarctic arid Arctic mesosphere dynamics-II 853

SEMIDIURNAL EASTWARD AND NORTHWARD TIDAL AMPLITUDES

POKER FLAT

FVB9

‘\

(b)

SEMIDIURNAL EASTWARD TIDAL PHASES

2

POKER FLAT

00 MAWSON

8 ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ ’ JFMAMJJASOND

MONTH

Fig. 13. Comparison between semidiurnal tidal winds at 95 km between Poker Flat (19831984), Mawson (19841986), and the FORBES and VIAL (1989) model. (a) Amplitudes include eastward (dots, data; solid line, model) and northward (data, open circles ; dashed line, model) components. (b) Phases, for clarity of presentation, include only the eastward component phases (the northward component phases are very nearly in quadrature with eastward measurements and model). The Mawson data and model results at

66”s are shifted by 6 months to facilitate comparison with the Northern Hemisphere data.

wind field on the tidal amplitudes. Results of these numerical experiments are provided in Table 1. Here we have modified the zonal mean wind field by con- stant factors of R = 0.9, 0.8 and 0.7, and examined the eastward semidiurnal tidal amplitudes at 65”N and 65”s latitude at 93.3 and 96.6 km. Grid sizes of 5” in latitude and 0.1 scale heights in the vertical were utilized in these calculations, which correspond to December solstice conditions. We note that while the amplitude at 65”N and 93.3 km decreases by about 10% from R = 1.0 to R = 0.7, the change in ampli- tude at 96.6 km is insignificant. At 65”S, the amplitude at 96.6 km increases with decreasing R, the reverse being true at 93.3 km ; the former is accompanied by a 2 h decrease in phase while the latter undergoes a 180” phase shift as R decreases from 1 .O to 0.7. Given

Table 1. Dependence of semidiurnal amplitudes and phases at 93.3 and 96.6 km on intensity of mean zonal wind. R denotes a constant multiplicative factor by which the height- and latitude-dependent wind field is multiplied: &(z, 0) = RU,(z, 0). The model utilized is that described in

FORBES (1982a,b)

Latitude R

Height = 96.6 km Height = 93.3 km

Amplitude Phase Amplitude Phase

+65” 1.0 41.7 6.3 40.4 7.1 0.9 42.2 6.2 39.1 7.0 0.8 42.6 6.1 37.7 6.9 0.7 42.6 6.0 36.2 6.8

-65” 1.0 13.8 12.4 7.3 4.9 0.9 14.3 11.6 3.7 4.9 0.8 16.2 10.9 0.6 8.6 0.7 19.3 10.4 4.7 10.2

854 Yu. 1. PORTNYAGIN etal.

the lack of uniformity in these responses with altitude and latitude, and the relatively small amplitude change produced, it would appear that a long-term decrease in tidal forcing may be required to produce the secular trends in Fig. 10. However, definitive con- clusions should probably await more sophisticated modeling experiments.

2.4. Data model comparisons

Results from the FORBES and VIAL (1989) semi- diurnal tidal model at 74”N and S were presented in Fig. 2, and some aspects of the monthly climatological data in Figs 3-5 were compared with the model results. Amplitudes and phases from the FORBES and VIAL (1989) model at 66”N and S are now compared with Poker Flat and Mawson measurements at 95 km in Fig. 13. Note that the Mawson data and Southern Hemisphere model results have been shifted by 6 months in order to facilitate comparison with the Poker data. There are some interesting features of these data which are represented in the model results. First, at 66”N there is excellent, agreement between the model amplitudes and those measured at Poker Flat from April through October, including the maximum occurring in September and the minimum in October. Secondly, the hemispheric asymmetries in the observed phases share several salient features with the model: namely, the presence of a large winter to summer phase shift in the Northern Hemisphere, whereas in the Southern Hemisphere the phase remains more or less constant through the year. The model replicates this latter feature except that there is a constant error in the model results of order 2 h. The main discrepancies in terms of amplitudes occur during Northern Hemisphere winter, where the model indicates significantly greater amplitudes around 95 km and an increase with height.

CONCLUSIONS

The semidiurnal tidal dynamics of the Antarctic and Arctic mesopause regions (95 f 15 km) are inves-

tigated through comparative analyses of monthly mean wind fields measured by radars at the Scott Base (78”S), Molodezhnaya (68’S), and Mawson (67”s) stations in the Antarctic, and the near-conjugate stations of Heiss I. (81”N) and Poker Flat (65”) in the Arctic region. The data are analyzed specifically to delineate hemispheric asymmetries. The main feature common to all stations is the fall equinoctial maxi- mum in amplitude (l&20 m s- ‘), which is also re- produced by the most recent numerical tidal model (FORBES and VIAL, 1989). However, the wintertime amplitude growth with height and the shorter vertical wavelengths characterizing the model are features not reflected in the data. There is also a spring equinoctial maximum in the Antarctic data which the model does not reproduce. To some extent, these features tend to be suppressed by phase interference effects in the course of taking monthly means.

Examination of interannual variability reveals characteristics similar to those noted in Part I for the mean zonal wind ; namely, some degree of year-to- year variability superimposed on apparent secular decreases of order 0.3-0.5 m s- ’ yr ’ (depending on month) in the Southern Hemisphere semidiurnal tidal amplitudes. Numerical simulations presented herein using the FORBES (1982a,b) linear spectral model indi- cate that changes of this magnitude cannot even be induced (via mode coupling) by a change in the mean zonal wind field of order 30%, and are more plausibly explained by a secular change in the tidal forcing by ozone insolation absorption. This conclusion must be accepted as tentative given several remaining uncer- tainties. A more comprehensive modeling effort aimed at resolution of this problem would be best performed by self-consistently solving for the coupled zonal mean and tidal wind fields.

Acknowledgements-This work was supported in part by grant DPP-8916343 from the Divison of Polar Programs, U.S. National Science Foundation, to Boston University. This funding made possible a Distinguished Visiting Scholar appointment for Yu. I. Portnyagin at Boston University where much of this collaborative research was performed.

AVERY S. K., VINCENT R. A., PHILLIPS A., MAN~ON A. H. and FRASER G. J.

ELFORD W. G.

FLEMING E. L., CHANDRA S., SCHOEBERL M. R. and BARNETT J. J.

FLEMING E. L., CHANDRA S., SCHOEBERL M. R. and BARNETT J. J.

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