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Variability of mesospheric dynamics observed at Yambol (42.5°N, 26.6°E) by meteor radar
Pergamon
Variability of mesospheric dynamics observed at Yambol (42.5’N, 26.6’E) by meteor radar D.
PANCHEVA
and PL.
M~KHTAR~V
Geophysical Institute, Bulgarian Academy of Science, “ Acad. G. Bonchev” 3. Sofia I1 13, Bulgaria (Received injkd
form I5 June 1993 ; accepted
I July 1993)
Abstract-On the basis of a best fit model, containing all subharmonics of a solar day, with variable amplitudes and generalized phases, the characteristics of the prevailing wind, tides and the quasi-2-day variations over Bulgaria for the time interval January 1991-June 1992 are obtained. An attempt is made to study the influence of the winter stratospheric warming on the tidal variability. The result obtained suggests an interaction between tides and a planetary wave.
1. INTRODUCTION
A meteor radar has operated regularly in Bulgaria (Yambol) since the end of 1989 and the measurements have been processed and analysed in the Geophysical Institute, Sofia since the end of 1990. The investigation of dynamical regime in the upper mesosphere and lower thermosphere over southeast Europe was set up in the frame of a bilateral co-operation between the Geophysical Institute, Sofia (Bulgaria) and the Scientific Production Association “Typhoon”, Obninsk (Russia). The meteor radar has been described by LYSENKO et al. (1988). The Yambol meteor radar does not possess an altitude meter; that is why the wind data obtained are related to the mean height of the meteor zone, about 95 f 2 km. The continuously operating equipment produces time series of mean hourly values for zonal and meridional wind components. The most outstanding feature of the motions at meteor altitude is the prevalence of solar tidal oscillations. In the lower the~osphere, on average, more wind energy resides in tidal motions than in the prevailing wind. Our basic aim is to study the regional peculiarities of the prevailing zonal and meridional wind, as well as the variability of the main tidal parameters over southeast Europe. Of particular interest is the problem connected with the investigation of the effects of the winter polar stratospheric warmings on the circulation at midlatitude lower thermospheric levels and the tidal variability induced by interaction with planetary waves at stratospheric levels.
2. METHOD
OF ANALYSIS
AND PREVAILING
WIND
In the beginning of our work the most important problem connected with data analysis was: how to
extract the tides from hourly time series, exhibiting sometimes various types of gaps. Analysing MEM or amplitude spectra, obtained from time series of hourly zonal and meridional wind measurements with different length (e.g. lo- or 30-day intervals) it can be seen that: (a) the well-known semidiurnal and diurnal tides do not always have exactly 12- and 24-h periods (especially diurnal tides) and (b) very frequently, fluctuations with periods of about 6,8,16 and 48 h appear also. Figure 1(a) presents the amplitude spectra of the zonal and meridional wind, obtained by the method of correloperiodogram analysis (KOPECKY and KUKLIN, 1971) for April 1991. This method allows the assessment of the amplitude, phase and probability of presence simultaneously (i.e. the significance of the fluctuation) for every harmonic component. The horizontal solid lines on the figure mark the 99% probability of presence and the dashed lines the 75% probability of presence. The best expressed Auctuations for the zonal wind are : semidiu~al tide with a mean period for the investigated month of 11.6 h ; diurnal tide with a mean period of 24.7 h, as well as 9- and 19.8-h fluctuations. For the meridional wind, the semidiurnal tide has a mean period of 12.5 h, but the diurnal tides are not well defined (there are several peaks between 20 and 30 h and the most prominent are those around 21.6 h). Sometimes, usually during the winter months, the 8-h fluctuations are better expressed than the semidiurnal or diurnal tides [Fig. 1(b)]. One of the first, clearly expressed feature of the atmospheric tides over Bulgaria, is their variabiiity on a day-to-day basis. However, as pointed out by VIAL et al. (1991), ‘day-to-day variability of the observed tidal component is another way of saying that the period of the oscillation, if it is indeed associated with a tide, is not exactly determined.’ This was the reason the best fit method was used, containing
1271
D. PANCHEVAand PL. MUKHTAROV
1272
Period
(hours)
(4
Fig. I (a). Amplitude spectra of hourly values of zonal and meridional wind, measured in Yambol (Bulgaria) for April 1991. The horizontal solid lines mark 99% probability of presence and the dashed lines 75%. (b)
The same as (a), but for January 1992. all subharmonics
of a solar day (MUKHTAROV and
PANCHEVA, 19921,
which is a development of the above-mentioned correloperiodogram analysis. This continuous spectral method allows us to define the period of the semidiurnal and diurnal tides. The dayto-day variability of tides is investigated through spectra later. Figure 2 presents the mean zonal and meridional wind, observed at Yambol during the interval 1 January 1991-16 June 1992. While the prevailing meridional wind does not show a clear seasonal course
-,d,,,,,..,,,,.,,l,,,,,,,~,,,,,,,~,,,,,,,~,,,, 0
2
Mo)nthsa(Jart
‘9;“-J::e
16
ia
‘;2)
Fig. 2. Seasonal course of prevailing zonal (solid) and meridional (dashed) wind over Bulgaria for the time interval January 199LJune 1992.
(except for a slight during winter), the during the summer, lOm/s, and weakly noctial months, with
increase of the northward wind zonal one is definitely eastward with a mean velocity of about westward wind during the equia speed of not more than - 2 m/s.
3. CLJMATOMGIES OF DIURNAL, SEMJDJURNAL, TERDJURNA~ AND QUAR~RDJURNAL TJD~ Figure 3 presents the seasonal courses of the amphtudes and phases of the investigated tidal components. The results obtained show the following special features. From Fig. 3(a) : (1) the amplitudes of the meridional tidal components are usually higher than the zonal ones, except during the summer months, when they are almost equal. (2) The meridional amplitudes of the diurnal, terdiurnal and quarterdiurnal tides manifest a well expressed seasonal course : an amplification during winter. when the mean amplitudes of diurnal and terdiurnal tides are almost equal; the zonal amplitudes of the above-mentioned tides do not display a seasonal course, except for a slight growth during summer. This result demonstrates that the terdiurnal tide is a permanent feature of the mesospheric dynamics over Bulgaria. (3) The zonal, as well as the
Variability
of mesospheric
dynamics
1273
F
c!
a
;a”
-&mddqd6mdtii --f4hlmd~~ileu
-1
(4
Fig. 3(a). Monthly mean amplitudes of zonal and meridional diurnal, semidiurnal, terdiurnal and quarterdiurnal tides. (b) Monthly mean phases of zonal and meridional diurnal, semidiurnal, terdiurnal and quarterdiurnal tides.
1214
D.
PANCHEVA
and
meridional amplitudes of the semidiurnal tides have the same seasonal course: amplification during summer. From Fig. 3(b) : (4) The phases of the investigated tides present the following characteristics : (i) the meridional component of the semidiurnal tides always leads the zonal one (except for November, but because of several data gaps, the results are uncertain). The mean phase difference is about 2 h. The phases of diurnal tides show a clear seasonal course: the meridional component overtakes the zonal one during winter months and lags behind during summer. (ii) The phases of the terdiurnal and quarterdiurnal tides do not display definite features. The zonal and meridional components of the diurnal and semidiurnal tides create the rotating vectors of the wind velocity with rotating periods of 24 and
25
%F\
:
-
*-I
Vmox d wmidiwnol t&s Vmin of nanidbnd tii
(mean Vmax=t3.46)
PL. MUKHTAROV
12 h, respectively, and with elliptical polarization. The maximum speed of the Yambol mean semidiurnal tide [Fig. 4(a)] is about 13.5m/s, which occurs around 06LT at a southwest-northeast direction (the mean angle is 60’ from the west-east direction). The prevailing direction of rotation is clockwise. The maximum wind speed of the diurnal tide is about 12 m/s [Fig. 4(b)], which occurs around 11 LT at the mean angle of 47’. The direction of rotation depends on the season: during the winter and equinoctial months, the prevailing direction is clockwise and during the summer counter-clockwise. 4. QUASI-2-DAY
The monthly amplitude spectra obtained for the hourly values of zonal or meridional wind very fre-
_____ ____ ____ __ --_ _----I-
*- -
\
L
,gl5{
x .ti
.ZlO.
p
s-
0
10
0” us >
1”“1”“6”“8”“““““““““”
MONTHS (Jm&J:‘,e.‘~)
a.
Vmax d diuml tidea (meanVmor=12.14) Vmio al diurnal tides
07I.5
:
:
VARIATIONS
-
rims(Ll)
d s&diunwl (me.313 time
Vmm appe.arwe
6.03LT)
A
/I
, /’ L’
I’
I ’ : IC L-4
,’
I
’ ’ \ I
h “ I 1 I \,’
l6
-
Time (LT) d diurnal Vms oppwrancs
(man time 11.2 Ll)
Fig. 4(a). Monthly mean characteristics (I’,,,,. and I’,,,. time of appearance and mean direction cast) for semidiurnal tidal vector. (b) The same as (a), but for the diurnal tidal vector.
north of
Variability
of mesospheric
quently show visible fluctuations with mean periods of 4654 h. The basic parameters of these quasi-2day variations are obtained on the basis of IO-day averaged time intervals by using the best tit 4component model. It contains mean, terdiurnat, semidiurnal, diurnai and quasi-Zday components. The prcvaihng periods of the 2-day fluctuations change predominantly between 46 and 54 h (a result from amplitude or MEM spectrum analyses), and in our model we do not previously fix them. The results for 199 1 are presented on Fig. 5. Figure 5(a) demonstrates
dynamics
an example of amplitude spectrum for August 199 I, when the quasi-2-day fluctuations in the meridional wind, together with the semidiurnal tides are the only fluctuations. Figure S(b) presents the time course of these ~~ctuatio~s and Fig. S(c) the amplitudes of the investigated quasi-Zday waves in the zonal and meridional wind components for 1991. The strongest increase of these waves is observed during midsummer (July-August) and the secondary one during winter. An interesting result is that the amplification of these waves in the meridional component leads that of the zonal one by about JOdays.
5. PLANETARY
fc) Fig. 5(a). Amplitude spectrum of meridional wind (‘w’ambol) for August 1991 (solid line marks 99% probabitity of presence and dashed one 75%). (b) The time course of quasi-2day variations in the meridional wind during the interval 15 July-15 August 1991 (c) Seasonal course of the amplitudes of quasi-2-day variations in the zonai and meridional wind during 199 1~
I275
WAVE
VARIABILIITY OF TIDES
BERNARD (198 1) showed that variations with periods of a few days are often observed in the tidal parameters. He suggested that long-period variations are related to variations of the tidal excitation source, or of propagating conditions in the middle atmosphere. That is why the next problem of interest in the present paper is the study of the planetary wave (PW) changes of the above-mentioned tides. We obtained first the time series of daily tidal amplitudes and phases and then calculated the spectra. Figure 6 presents the amplitude spectra obtained far the whole investigated time interval {I January 1991-.I6 June 1992) for the different tidal amplitudes. These results show that all amplitudes of the zonal or meridional tides possess almost the same PW fluctuations. However, the only peaks that are considered consistently significant (close to 99”/,) are 9 days (with weaker peaks of 2X-30 days) for the zontal tidal amplitudes ; and 40 days (with weaker peaks around 8 days) for the meridional amplitudes. Other clusters of peaks (often similar for the different tides and components) are of lower significance and are not consistently seen for al1 tidal periods. The most pronounced peaks in the amplitude spectra of the tidal phases (not shown in a figure) are l&20,50 (zonal) and 38.-4Odays (meridional). An interesting result is that the strongest variations in the zonal tidal amplitudes and phases do not coincide. The time series of the meridional tidal amplitudes and phases have well-marked fluctuations around 40 days. The planetary wave var~abi~jty of the tidal amplitudes and phases is stronger during winter and equino& months. Figure 7 describes an example of PW activity, observed in the amplitudes of zonal diurnal and semidiurnal tides in the winter of 1991/1992 (1 October 1991-31 March 1992). The amplitude spectra of Fig. 7(a) show that the strongest waves have mean periods of about 9.5-l 0 days, followed by the 24-30-
D.
1276
PANCHEVA
and
PL.
MUKHTAROV
a0
a0
m 0
.
’
loo
Pe%d (da;)
COB :
.p 3
________________I
____
s 0.0
Period
(days)
Fig. 6. Amplitude spectra, obtained for the whole time interval (January 1991LJune 1992) for the different tidal amplitudes in zonal and meridional wind (solid lines mark 99% probability of presence and dashed ones 75%).
1277
Variabiiity of mesospheric dynamics
the possible mechanisms of this strato-mesospheric interaction could be related to the fact that the interaction between the variations of the mean zonal circulation and the quasi-stationary planetary waves (Hi or HZ) leads to the generation of travelling planetary ~u~tuations (HIK~TA. 1971) manifesting themselves in the modulation of tidal amplitudes with planetary wave periods. TEITELBAUMand VIAL (1991) experimentally and theoretically explained this phenomenon as a result of non-linear interaction between tides and planetary waves.
PERIOD (hours)
(a) 6.
CONCLUSIONS
On the basis of a best fit model, containing all subharmonics of a solar day, with variable amplitudes and phases (MUKHTAROVand PANCHEVA,1992), the characteristics of the prevailing wind and tides over Bulgaria, for the time interval January 1991-June 1992 are obtained. DAYS (1.Oct.‘9l-31 .Mar.‘92)
(b)
-
HI
30hF3
6ON
(4 Fig. 7(a). Amplitude spectra of zonal sem~diurna~ (solid) and diurnal (dashed) amplitudes for the time interval I October 1991-31 March 1992. (b) The instantaneousamplitudesof9IO-day fluctuations in zonal semidiurnal (solid) and diurnal (dashed)
amplitudes. (c) The instantaneous amplitudes the height wave H 1 at 30 hPa pressure level.
of
day fluctuations. Figure 7(b) shows the instantaneous amplitudes of 9%IO-day waves, obtained by the complex demodulation method (BLOOMFIELD, 1976). Substantial amplification of the investigated waves is observed during the middle of January and in the first half of February, when the height-wave Hl in the stratosphere shows intensification also [Fig. 7(c), NAUJOKATet al., 19921. This result suggests an interaction between tides and planetery waves. One of
l While the prevailing meridional wind does not show a clear seasonal course, the zonal one is definitely eastward during the summer, with a mean velocity of about lOm/s and weakly westward during equinoctial months, with the speed not more than - 2 m/s. l The Yambol mean semidiurnal tide possesses a speed maximum of about 13.5m/s, which occurs around 06 LT in the direction southwest-northeast (60’ from the cast-west direction), with clockwise direction of rotation. The mean speed maximum of the diurnal tide is about 12 m/s, at the mean angle of 47 The direction of rotation depends on the season : clockwise during the winter and equinoctal months and counter-clockwise during the summer. l On the basis of IO-day averaged time segments, by using the best fit 4-component mode!, the main characteristics of the quasi-2-day variations are found. The strongest increase of these w-aves is observed during mid-summer and the secondary one during winter. The amplification of the quasi-2-day waves in the meridional component leads that in the zonal one by about 10 days. l The PW changes of the tidal amplitudes and phases for the whole investigated time interval (January 199 I-June 1992) are studied. The most pronounced ~uctuations observed in the tidal amplitudes are 8-10,28-30 and around 40 days, while in the tidal phases they are 18-20 and 4C50 days. The 30 day fluctuations of the tidal characteristics could probably be connected with the variations of solar flux with a period of about 27 days. l An attempt is made to study the influence of the winter stratospheric warming on the tidal variability.
1278
D.
PANCHEVA and PL. MUKHTAROV
Substantial amplification of the 9-l 0 day variations in the zonal diurnal and semidiurnal tides during the winter of 1991/1992 is observed when the height-wave
H 1 at the 30 hPa pressure level shows intensification also. This result suggests an interaction between tides and planetary waves.
REFERENCES BERNARD R. BL~~MFIELD P.
1981 1976
J. atmos. terr. Phys. 43, 663. Fourier Anulysis of Time Series:
an Introduction.
J.
Wiley, New York. HIROTA 1. KOPE~KY M. and KUKLIN G LYSENKO Y., MIHAJLIK P., PETROV B., S~ASOV H. and MANEV P. MUKHTAROV P. and PANCHEVA D. NAUJOKAT B., LABITZKE K., LENSCHOW R., PETZOLDT K. and WOHLFART R.-C. TE~TELBAUMH. and VIAL F. VIAL F., FORBESJ. and MIYAHARA S.
1971
J. met. Sot. Japun 49,439.
1971 1988
Issled. geomagn. aeronom. Russian). Bul9. Ge0ph.w. J. 14, 34.
1992 1992
C. r. Acad. bu!q. Sci. 45, N 11. Beilage zur Berliner Wetterkarte,
1991 1991
J. geophys. Rex 96, 14,169 J. geophys. Res. 96, 1215.
Fiz.
Solntsa
SO 15/92
2,
167 (in
Variability of mesospheric dynamics observed at Yambol (42.5’N, 26.6’E) by meteor radar D.
PANCHEVA
and PL.
M~KHTAR~V
Geophysical Institute, Bulgarian Academy of Science, “ Acad. G. Bonchev” 3. Sofia I1 13, Bulgaria (Received injkd
form I5 June 1993 ; accepted
I July 1993)
Abstract-On the basis of a best fit model, containing all subharmonics of a solar day, with variable amplitudes and generalized phases, the characteristics of the prevailing wind, tides and the quasi-2-day variations over Bulgaria for the time interval January 1991-June 1992 are obtained. An attempt is made to study the influence of the winter stratospheric warming on the tidal variability. The result obtained suggests an interaction between tides and a planetary wave.
1. INTRODUCTION
A meteor radar has operated regularly in Bulgaria (Yambol) since the end of 1989 and the measurements have been processed and analysed in the Geophysical Institute, Sofia since the end of 1990. The investigation of dynamical regime in the upper mesosphere and lower thermosphere over southeast Europe was set up in the frame of a bilateral co-operation between the Geophysical Institute, Sofia (Bulgaria) and the Scientific Production Association “Typhoon”, Obninsk (Russia). The meteor radar has been described by LYSENKO et al. (1988). The Yambol meteor radar does not possess an altitude meter; that is why the wind data obtained are related to the mean height of the meteor zone, about 95 f 2 km. The continuously operating equipment produces time series of mean hourly values for zonal and meridional wind components. The most outstanding feature of the motions at meteor altitude is the prevalence of solar tidal oscillations. In the lower the~osphere, on average, more wind energy resides in tidal motions than in the prevailing wind. Our basic aim is to study the regional peculiarities of the prevailing zonal and meridional wind, as well as the variability of the main tidal parameters over southeast Europe. Of particular interest is the problem connected with the investigation of the effects of the winter polar stratospheric warmings on the circulation at midlatitude lower thermospheric levels and the tidal variability induced by interaction with planetary waves at stratospheric levels.
2. METHOD
OF ANALYSIS
AND PREVAILING
WIND
In the beginning of our work the most important problem connected with data analysis was: how to
extract the tides from hourly time series, exhibiting sometimes various types of gaps. Analysing MEM or amplitude spectra, obtained from time series of hourly zonal and meridional wind measurements with different length (e.g. lo- or 30-day intervals) it can be seen that: (a) the well-known semidiurnal and diurnal tides do not always have exactly 12- and 24-h periods (especially diurnal tides) and (b) very frequently, fluctuations with periods of about 6,8,16 and 48 h appear also. Figure 1(a) presents the amplitude spectra of the zonal and meridional wind, obtained by the method of correloperiodogram analysis (KOPECKY and KUKLIN, 1971) for April 1991. This method allows the assessment of the amplitude, phase and probability of presence simultaneously (i.e. the significance of the fluctuation) for every harmonic component. The horizontal solid lines on the figure mark the 99% probability of presence and the dashed lines the 75% probability of presence. The best expressed Auctuations for the zonal wind are : semidiu~al tide with a mean period for the investigated month of 11.6 h ; diurnal tide with a mean period of 24.7 h, as well as 9- and 19.8-h fluctuations. For the meridional wind, the semidiurnal tide has a mean period of 12.5 h, but the diurnal tides are not well defined (there are several peaks between 20 and 30 h and the most prominent are those around 21.6 h). Sometimes, usually during the winter months, the 8-h fluctuations are better expressed than the semidiurnal or diurnal tides [Fig. 1(b)]. One of the first, clearly expressed feature of the atmospheric tides over Bulgaria, is their variabiiity on a day-to-day basis. However, as pointed out by VIAL et al. (1991), ‘day-to-day variability of the observed tidal component is another way of saying that the period of the oscillation, if it is indeed associated with a tide, is not exactly determined.’ This was the reason the best fit method was used, containing
1271
D. PANCHEVAand PL. MUKHTAROV
1272
Period
(hours)
(4
Fig. I (a). Amplitude spectra of hourly values of zonal and meridional wind, measured in Yambol (Bulgaria) for April 1991. The horizontal solid lines mark 99% probability of presence and the dashed lines 75%. (b)
The same as (a), but for January 1992. all subharmonics
of a solar day (MUKHTAROV and
PANCHEVA, 19921,
which is a development of the above-mentioned correloperiodogram analysis. This continuous spectral method allows us to define the period of the semidiurnal and diurnal tides. The dayto-day variability of tides is investigated through spectra later. Figure 2 presents the mean zonal and meridional wind, observed at Yambol during the interval 1 January 1991-16 June 1992. While the prevailing meridional wind does not show a clear seasonal course
-,d,,,,,..,,,,.,,l,,,,,,,~,,,,,,,~,,,,,,,~,,,, 0
2
Mo)nthsa(Jart
‘9;“-J::e
16
ia
‘;2)
Fig. 2. Seasonal course of prevailing zonal (solid) and meridional (dashed) wind over Bulgaria for the time interval January 199LJune 1992.
(except for a slight during winter), the during the summer, lOm/s, and weakly noctial months, with
increase of the northward wind zonal one is definitely eastward with a mean velocity of about westward wind during the equia speed of not more than - 2 m/s.
3. CLJMATOMGIES OF DIURNAL, SEMJDJURNAL, TERDJURNA~ AND QUAR~RDJURNAL TJD~ Figure 3 presents the seasonal courses of the amphtudes and phases of the investigated tidal components. The results obtained show the following special features. From Fig. 3(a) : (1) the amplitudes of the meridional tidal components are usually higher than the zonal ones, except during the summer months, when they are almost equal. (2) The meridional amplitudes of the diurnal, terdiurnal and quarterdiurnal tides manifest a well expressed seasonal course : an amplification during winter. when the mean amplitudes of diurnal and terdiurnal tides are almost equal; the zonal amplitudes of the above-mentioned tides do not display a seasonal course, except for a slight growth during summer. This result demonstrates that the terdiurnal tide is a permanent feature of the mesospheric dynamics over Bulgaria. (3) The zonal, as well as the
Variability
of mesospheric
dynamics
1273
F
c!
a
;a”
-&mddqd6mdtii --f4hlmd~~ileu
-1
(4
Fig. 3(a). Monthly mean amplitudes of zonal and meridional diurnal, semidiurnal, terdiurnal and quarterdiurnal tides. (b) Monthly mean phases of zonal and meridional diurnal, semidiurnal, terdiurnal and quarterdiurnal tides.
1214
D.
PANCHEVA
and
meridional amplitudes of the semidiurnal tides have the same seasonal course: amplification during summer. From Fig. 3(b) : (4) The phases of the investigated tides present the following characteristics : (i) the meridional component of the semidiurnal tides always leads the zonal one (except for November, but because of several data gaps, the results are uncertain). The mean phase difference is about 2 h. The phases of diurnal tides show a clear seasonal course: the meridional component overtakes the zonal one during winter months and lags behind during summer. (ii) The phases of the terdiurnal and quarterdiurnal tides do not display definite features. The zonal and meridional components of the diurnal and semidiurnal tides create the rotating vectors of the wind velocity with rotating periods of 24 and
25
%F\
:
-
*-I
Vmox d wmidiwnol t&s Vmin of nanidbnd tii
(mean Vmax=t3.46)
PL. MUKHTAROV
12 h, respectively, and with elliptical polarization. The maximum speed of the Yambol mean semidiurnal tide [Fig. 4(a)] is about 13.5m/s, which occurs around 06LT at a southwest-northeast direction (the mean angle is 60’ from the west-east direction). The prevailing direction of rotation is clockwise. The maximum wind speed of the diurnal tide is about 12 m/s [Fig. 4(b)], which occurs around 11 LT at the mean angle of 47’. The direction of rotation depends on the season: during the winter and equinoctial months, the prevailing direction is clockwise and during the summer counter-clockwise. 4. QUASI-2-DAY
The monthly amplitude spectra obtained for the hourly values of zonal or meridional wind very fre-
_____ ____ ____ __ --_ _----I-
*- -
\
L
,gl5{
x .ti
.ZlO.
p
s-
0
10
0” us >
1”“1”“6”“8”“““““““““”
MONTHS (Jm&J:‘,e.‘~)
a.
Vmax d diuml tidea (meanVmor=12.14) Vmio al diurnal tides
07I.5
:
:
VARIATIONS
-
rims(Ll)
d s&diunwl (me.313 time
Vmm appe.arwe
6.03LT)
A
/I
, /’ L’
I’
I ’ : IC L-4
,’
I
’ ’ \ I
h “ I 1 I \,’
l6
-
Time (LT) d diurnal Vms oppwrancs
(man time 11.2 Ll)
Fig. 4(a). Monthly mean characteristics (I’,,,,. and I’,,,. time of appearance and mean direction cast) for semidiurnal tidal vector. (b) The same as (a), but for the diurnal tidal vector.
north of
Variability
of mesospheric
quently show visible fluctuations with mean periods of 4654 h. The basic parameters of these quasi-2day variations are obtained on the basis of IO-day averaged time intervals by using the best tit 4component model. It contains mean, terdiurnat, semidiurnal, diurnai and quasi-Zday components. The prcvaihng periods of the 2-day fluctuations change predominantly between 46 and 54 h (a result from amplitude or MEM spectrum analyses), and in our model we do not previously fix them. The results for 199 1 are presented on Fig. 5. Figure 5(a) demonstrates
dynamics
an example of amplitude spectrum for August 199 I, when the quasi-2-day fluctuations in the meridional wind, together with the semidiurnal tides are the only fluctuations. Figure S(b) presents the time course of these ~~ctuatio~s and Fig. S(c) the amplitudes of the investigated quasi-Zday waves in the zonal and meridional wind components for 1991. The strongest increase of these waves is observed during midsummer (July-August) and the secondary one during winter. An interesting result is that the amplification of these waves in the meridional component leads that of the zonal one by about JOdays.
5. PLANETARY
fc) Fig. 5(a). Amplitude spectrum of meridional wind (‘w’ambol) for August 1991 (solid line marks 99% probabitity of presence and dashed one 75%). (b) The time course of quasi-2day variations in the meridional wind during the interval 15 July-15 August 1991 (c) Seasonal course of the amplitudes of quasi-2-day variations in the zonai and meridional wind during 199 1~
I275
WAVE
VARIABILIITY OF TIDES
BERNARD (198 1) showed that variations with periods of a few days are often observed in the tidal parameters. He suggested that long-period variations are related to variations of the tidal excitation source, or of propagating conditions in the middle atmosphere. That is why the next problem of interest in the present paper is the study of the planetary wave (PW) changes of the above-mentioned tides. We obtained first the time series of daily tidal amplitudes and phases and then calculated the spectra. Figure 6 presents the amplitude spectra obtained far the whole investigated time interval {I January 1991-.I6 June 1992) for the different tidal amplitudes. These results show that all amplitudes of the zonal or meridional tides possess almost the same PW fluctuations. However, the only peaks that are considered consistently significant (close to 99”/,) are 9 days (with weaker peaks of 2X-30 days) for the zontal tidal amplitudes ; and 40 days (with weaker peaks around 8 days) for the meridional amplitudes. Other clusters of peaks (often similar for the different tides and components) are of lower significance and are not consistently seen for al1 tidal periods. The most pronounced peaks in the amplitude spectra of the tidal phases (not shown in a figure) are l&20,50 (zonal) and 38.-4Odays (meridional). An interesting result is that the strongest variations in the zonal tidal amplitudes and phases do not coincide. The time series of the meridional tidal amplitudes and phases have well-marked fluctuations around 40 days. The planetary wave var~abi~jty of the tidal amplitudes and phases is stronger during winter and equino& months. Figure 7 describes an example of PW activity, observed in the amplitudes of zonal diurnal and semidiurnal tides in the winter of 1991/1992 (1 October 1991-31 March 1992). The amplitude spectra of Fig. 7(a) show that the strongest waves have mean periods of about 9.5-l 0 days, followed by the 24-30-
D.
1276
PANCHEVA
and
PL.
MUKHTAROV
a0
a0
m 0
.
’
loo
Pe%d (da;)
COB :
.p 3
________________I
____
s 0.0
Period
(days)
Fig. 6. Amplitude spectra, obtained for the whole time interval (January 1991LJune 1992) for the different tidal amplitudes in zonal and meridional wind (solid lines mark 99% probability of presence and dashed ones 75%).
1277
Variabiiity of mesospheric dynamics
the possible mechanisms of this strato-mesospheric interaction could be related to the fact that the interaction between the variations of the mean zonal circulation and the quasi-stationary planetary waves (Hi or HZ) leads to the generation of travelling planetary ~u~tuations (HIK~TA. 1971) manifesting themselves in the modulation of tidal amplitudes with planetary wave periods. TEITELBAUMand VIAL (1991) experimentally and theoretically explained this phenomenon as a result of non-linear interaction between tides and planetary waves.
PERIOD (hours)
(a) 6.
CONCLUSIONS
On the basis of a best fit model, containing all subharmonics of a solar day, with variable amplitudes and phases (MUKHTAROVand PANCHEVA,1992), the characteristics of the prevailing wind and tides over Bulgaria, for the time interval January 1991-June 1992 are obtained. DAYS (1.Oct.‘9l-31 .Mar.‘92)
(b)
-
HI
30hF3
6ON
(4 Fig. 7(a). Amplitude spectra of zonal sem~diurna~ (solid) and diurnal (dashed) amplitudes for the time interval I October 1991-31 March 1992. (b) The instantaneousamplitudesof9IO-day fluctuations in zonal semidiurnal (solid) and diurnal (dashed)
amplitudes. (c) The instantaneous amplitudes the height wave H 1 at 30 hPa pressure level.
of
day fluctuations. Figure 7(b) shows the instantaneous amplitudes of 9%IO-day waves, obtained by the complex demodulation method (BLOOMFIELD, 1976). Substantial amplification of the investigated waves is observed during the middle of January and in the first half of February, when the height-wave Hl in the stratosphere shows intensification also [Fig. 7(c), NAUJOKATet al., 19921. This result suggests an interaction between tides and planetery waves. One of
l While the prevailing meridional wind does not show a clear seasonal course, the zonal one is definitely eastward during the summer, with a mean velocity of about lOm/s and weakly westward during equinoctial months, with the speed not more than - 2 m/s. l The Yambol mean semidiurnal tide possesses a speed maximum of about 13.5m/s, which occurs around 06 LT in the direction southwest-northeast (60’ from the cast-west direction), with clockwise direction of rotation. The mean speed maximum of the diurnal tide is about 12 m/s, at the mean angle of 47 The direction of rotation depends on the season : clockwise during the winter and equinoctal months and counter-clockwise during the summer. l On the basis of IO-day averaged time segments, by using the best fit 4-component mode!, the main characteristics of the quasi-2-day variations are found. The strongest increase of these w-aves is observed during mid-summer and the secondary one during winter. The amplification of the quasi-2-day waves in the meridional component leads that in the zonal one by about 10 days. l The PW changes of the tidal amplitudes and phases for the whole investigated time interval (January 199 I-June 1992) are studied. The most pronounced ~uctuations observed in the tidal amplitudes are 8-10,28-30 and around 40 days, while in the tidal phases they are 18-20 and 4C50 days. The 30 day fluctuations of the tidal characteristics could probably be connected with the variations of solar flux with a period of about 27 days. l An attempt is made to study the influence of the winter stratospheric warming on the tidal variability.
1278
D.
PANCHEVA and PL. MUKHTAROV
Substantial amplification of the 9-l 0 day variations in the zonal diurnal and semidiurnal tides during the winter of 1991/1992 is observed when the height-wave
H 1 at the 30 hPa pressure level shows intensification also. This result suggests an interaction between tides and planetary waves.
REFERENCES BERNARD R. BL~~MFIELD P.
1981 1976
J. atmos. terr. Phys. 43, 663. Fourier Anulysis of Time Series:
an Introduction.
J.
Wiley, New York. HIROTA 1. KOPE~KY M. and KUKLIN G LYSENKO Y., MIHAJLIK P., PETROV B., S~ASOV H. and MANEV P. MUKHTAROV P. and PANCHEVA D. NAUJOKAT B., LABITZKE K., LENSCHOW R., PETZOLDT K. and WOHLFART R.-C. TE~TELBAUMH. and VIAL F. VIAL F., FORBESJ. and MIYAHARA S.
1971
J. met. Sot. Japun 49,439.
1971 1988
Issled. geomagn. aeronom. Russian). Bul9. Ge0ph.w. J. 14, 34.
1992 1992
C. r. Acad. bu!q. Sci. 45, N 11. Beilage zur Berliner Wetterkarte,
1991 1991
J. geophys. Rex 96, 14,169 J. geophys. Res. 96, 1215.
Fiz.
Solntsa
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