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The annual and semi-annual wind fields in low latitudes
Journal a/ Armospherrc
and Solar-Terwslrial
Pergamon PII: SOO21-9169(96)00051-7
Physics, Vol 59, No. 5, pp. 487~495, 1997 CopyrIght c; 1997 Elsevier Science Ltd Printed m Great Bntam. All rrghts reserved 1364-6826197 $17.00+0.00
The annual and semi-annual wind fields in low latitudes C. Raghava Atmospheric
Dynamics
Branch,
Reddi and Geetha
Ramkumar
Space Physics Laboratory, Vikram Trivandrum 695 022, India
(Received infinalform
Sarabhai
Space Centre,
5 March 1996; accepted 15 March 1996)
Abstract-The annual oscillations (AO) and semi-annual oscillations (SAO) of the monthly mean zonal and meridional winds in the troposphere, stratosphere and mesosphere have been studied using simultaneous data from balloonsondes, rocketsondes and Meteor Wind Radar (MWR) acquired at Trivandrum. A study is also made of the A0 and SAO of the diurnal, semi-diurnal and ter-diurnal wind oscillations in the 8OG102 km altitudes using MWR observations. The same observations have been used to study the altitude profiles of the annual mean winds, and the annual mean tidal winds. It is found that the A0 phase in the NS wind leads with increasing altitude in the stratosphere and mesosphere while SAO phase lags with altitude. The SAO of EW tidal wind amplitudes are slightly stronger than AO. For the NS wind, the A0 of the diurnal tide has the largest amplitude and the SAO of the semi-diurnal tide also has a relatively large amplitude. The results are compared with earlier reports from other low latitude stations and with the CIRA 1986 model for 8.5’N latitude. Further, they are discussed in the light of tidal dissipation in and above the mesopause (> 70 km). ‘0 1997 Elsevier Science Ltd. All rights reserved
INTRODUCTION
Leovy, 1986, 1988; Delisi and Dunkerton, 1988). Reddy and Raghava Reddi (1986) showed the longitudinal and interannual variations of SSAO. The Mesopause SAO (MSAO) was first shown by Groves (1972) using rocket and radar wind climatologies of the meteor region (80-l 10 km). Later, Hirota (1978) and Hamilton (1982) showed the MSAO exists with a peak at 80 km in low latitudes, with westerly winds during solstice and maximum easterly winds during equinox. Garcia and Clancy (1990) have shown the presence of MSAO in temperature using Solar Mesospheric Explorer (SME) satellite data. More recently, data from ground based MF and Meteor Wind Radar (MWR) at Christmas Island (2”N, 157”W) and Hawai (22”N, 160”W) indicate an MSAO near 80-85 km which exhibits a high degree ofinter-annual variability (Vincent and Lesicar, 1991; Fritts and Isler, 1992; Palo and Avery, 1993; Vincent, 1993a, b). Raghava Reddi et al. (1993a) reported A0 and SAO in temperature in the lower thermospheric region using Meteor Wind Radar (MWR) observations from Trivandrum. The SSAO is identified to be a product of atmospheric wave mean flow interaction processes (Dunkerton, 1979). Dunkerton (1982) simulated the MSAO and showed that the eastward and westward propagating inertial gravity waves can produce the required eastward and westward accelerations. Model studies of Miyahara and Wu (1989) show that the
From earlier reports (Reed, 1965, 1996; Groves, 1973; Belmont et al., 1974; Hirota, 1978; Reddy et al., 1986) based on rocketsonde and balloonsonde observations, it is known that the semi-annual oscillation (SAO), annual oscillation (AO) and quasi-biennial oscillation (QBO) account for about 90% of the zonal wind variations in the low latitude middle atmosphere. The QBO, having a maximum amplitude at about 28 km, was found to vary in the period of the oscillation (Sasi and Murthy, 1991), but the altitude variations of the amplitude and phase of the oscillation were almost independent of longitude (Reddy and Raghava Reddi, 1986). Above 35 km the equatorial middle atmosphere is characterised by the semi-annual oscillation of the zonal wind, with one peak amplitude centred at about the stratopause (w 50 km) and another at the mesopause ( z 85 km); they are 180” out of phase at the two altitudes (Hirota, 1978; Andrews et al., 1987). The stratopause SAO (SSAO) was discovered by Reed (1965, 1966) and its climatologies were studied extensively (Angel1 and Korshover, 1970; Belmont and Dartt, 1973; Hopkins, 1975) using rocketsonde winds from the MRN (Meteorological Rocket Network) stations. More recently the data from the Stratospheric and Mesospheric Sounder (SAMS) and the Limb Infrared Monitor of the Stratosphere (LIMS) have helped in understanding the dynamical processes involved in driving SSAO (Hitchman and 487
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C. Raghava Reddi and G. Ramkumar
dissipation of the westward migrating diurnal tide can generate significant mean zonal winds in the mesopause region and the semi-annual modulation of the amplitude of the tide could be related to the MSAO. Observations from Adelaide (Vincent et al., 1988) show a semi-annual variation in the strength of the propagating diurnal tide, with large amplitude in equinox; the maximum in March is stronger than the maximum in September. In this paper we report the results of SAO and A0 from ground level to 102 km, obtained from ballonsonde, rocketsonde and MWR observations at Trivandrum. Also, the A0 and SAO of the amplitudes and phases of the tidal winds are presented to evaluate the tidal wind amplitude modulations by the A0 and SAO in the zonal and meridional winds. The altitude profiles of the annual mean tidal amplitudes and the mean prevailing winds are presented. The results from this study are compared and discussed with the results from other low latitudes and the CIRA 1986 (Flemming et al., 1988) model. DATA AND ANALYSIS
The data used in this study are from the balloonsonde, M-100 rocketsonde and MWR observations, all at Trivandrum during 198551987. Monthly mean profiles of the zonal and meridional winds from balloonsonde and rocketsonde were used to compute the annual mean and the amplitude and phase profiles of A0 and SAO, in the O-80 km altitude range. The details of the analysis of rocketsonde (RS) and balloonsonde (BS) data to obtain the amplitude and phase profiles of A0 and SAO were published earlier (Reddy et al., 1986). The RS and BS data were obtained every Wednesday of the month, the former at about 1700 (Indian Standard Time, IST) and the latter about 3 hours earlier. The BS data from ground level to 20 km was combined with RS data from 21 to 80 km to obtain the profiles from ground to 80 km altitude. The diurnal variation contribution to the RS and BS profiles from the tidal winds because of the local time difference (3 h) of the RS and BS launches were assumed not to introduce discontinuities in the profiles at the transition altitude of 20-21 km. It was estimated that the errors in the computed magnitudes of the AO, SAO and annual mean wind is less than 1.5 m s-’ in the 2-35 km range, 1.5 to 2.0 m s-’ in the 35-55km range and 2-3 m ss’ above 55 km altitude. The typical phase errors are l&15” for amplitudes exceeding 10 m s-‘, 15-30” for l&5 m s-’ and 30-60” for 5-2 m s-’ amplitudes (Reddy et al., 1986). In the 81-102 km altitude region, continuous diurnal MWR data for 8-10 days each month were used.
The 12 monthly mean profiles of average wind, and amplitude and phase of the diurnal, semi-diurnal and ter-diurnal winds, were computed following the procedure described earlier in Raghava Reddi et al. (1993b). The data were spread over 1985-1987. But the monthly profiles used to represent every month of the calender year were based on the error estimates at each height of the profiles. When profiles for the same calender month were available in more than one year, the profile with the least errors and the largest altitude coverage was chosen. The profiles for August and December were for 1985; the profiles for February and October were for 1986 and the profiles for the other months were obtained in 1987. Month to month variations at all heights during the calender year did not show any abnormal variation because of the mixing of two months of 1985 and 1986 data with the data for all other months in 1987. The r.m.s errors in the monthly mean MWR wind profiles (both NS and EW winds) are less than 2 m ss’ on the average, but are relatively large at altitudes above 100 km and below 85 km. The standard deviations of the profiles were estimated to be 8810 m ss’. The phase error of the A0 and SAO computed depends on the amplitude of the oscillation and was estimated to be less than 10” for amplitudes exceeding 10 m ss’. BS, RS and MWR data for each calender month were used for the same year from the three experiments. Based on the data analysis procedures used for computing the wind profiles, the altitude resolution of the RS profile is about 5 km, whereas the resolution from balloonsonde and MWR is about 2 km, although the profiles from the three experiments are available at 1 km intervals. The 12 monthly mean values at each height in NS and EW winds were subjected to harmonic analysis to determine the amplitudes and phases of A0 and SAO along with the average of the 12 monthly values. The CIRA 1986 model values for zonal wind in the GlO5 km range for the 12 months for the latitude of 8.5”N (interpolated from values at 0” and 10”N) were also subjected to harmonic analysis to get the model A0 and SAO amplitude and phase profiles for comparison. The observational results used for comparison were obtained from published reports at Ascension Island (8”S, 14”W), Christmas Island (2”N, 157”W) and Kwajalein (9”N, 168”E) (Hirota, 1978; Vincent, 1993b; Hamilton, 1982). RESULTS
Monthly
mean winds
Annual oscillation. The A0 in the mean EW wind to the annual cycle of up to ~60 km is attributed
Atmospheric winds at low latitude Annual
mm1
wind
oscillation
0 AMPlsT”DE (m-l
)
PHASE (mm)
Fig. 1.Amplitude and phase profiles of annual oscillation in zonal wind.
solar insolation in extratropical latitudes, and the flow deflection by the Coriolis force. Above 60 km, the A0 is forced by the annual variation of the momentum fluxes transported and deposited by gravity waves, tides and other planetary scale waves. The existence of an annual cycle in the tropics is difficult to explain. The amplitude of A0 at low latitudes is small compared with the amplitude in middle and high latitudes, but it is a very significant part of the total variance in the tropics. A0 is known to exist with opposite phases in the lower stratosphere and lower mesosphere (Angel1 and Korshover, 1970) with its phase changing continuously through the mesosphere. The phase (time of westerly maximum wind due to AO) is in January in the upper mesosphere (Crane, 1979). Figure 1 shows the amplitude and phase profiles of A0 in zonal wind. Below about 6 km, the A0 amplitude decreased with altitude, and the phase (time of maximum westerly wind) is in July. These could be associated with local weather processes and may not be of global scale. Above 7 km, the A0 amplitudes show three broad peaks: one between 8 and 31 km, another in the 32-60 km range and a third between 65 and 85 km. There are sharp minima:one is at 17 km, which incidentally is the height of the tropopause over Trivandrum, and another is at 30 km, which is close to the altitude of QBO maximum (Reddy and Raghava Reddi, 1986). In the 45560 km region the A0 amplitude is nearly constant (Z 10 m s-‘) and it is known that, in this altitude region, the SAO has maximum amplitude. The broad peak, with maximum amplitude exceeding 20 m ss’ at 70 km, merges fairly smoothly with the profile from MWR above 80 km. The CIRA 1986 model shows a broad peak between 65 and 95 km altitude and does not depict most of the structures in the profile observed at Trivandrum. The maximum amplitude observed is reproduced in the
489
CIRA 1986 wind model based on satellite data, but the altitude of the peak given by CIRA 1986 is higher. The A0 amplitude profile observed at Christmas Island (Vincent, 1993b) using MF radar data shows amplitudes similar to those observed at Trivandrum in the mesopause and lower thermosphere. The A0 phase profile from CIRA 1986 and the observed profile show fair consistency (& l-2 months). The maximum westerly zonal winds occur during the northern winter month. This shows that the westerly regime observed at mid latitudes during northern winter months is extended to low latitudes. Below 90 km the phase lags with increasing altitude whereas, above 90 km, the phase leads with altitude. At Christmas Island the A0 phase leads with altitude followed by a steady value above 80 km. While the Trivandrum A0 amplitude above 70 km shows a rapid decrease from ~25 m ss’ to less than 5 m ss’, the Christmas Island profile shows an increasing trend from ~3 m s-’ to about 10 m s-’ at 90 km. The reality of these variations need to be ascertained with further data, if possible, obtained simultaneously. At 80 km the A0 zonal wind phase at Trivandrum showed a sudden jump and the amplitude which was decreasing with altitude up to 80 km, was nearly constant above 80 km. The rocket wind data used for obtaining the profiles below 80 km was from the weekly (Wednesday) rocketsonde launches at 1700, whereas the MWR data used for altitudes above 80 km were the monthly mean profiles after subtracting the contributions from tidal winds. In the MWR data, the analysis procedure is such that the contribution to the wind variance due to atmospheric waves (periods equal to or greater than 2 days) were averaged out. Thus A0 amplitude and phase profiles from MWR data would be free from any contamination due to tides and other atmospheric waves whereas the profiles below 80 km may have some contamination from the seasonal variations in the tidal and other atmospheric wave phenomena. We found that the tidal winds have seasonal variations; it is reasonable to expect atmospheric wave fluxes at 80 km also to have seasonal variations. In spite of the fact that the errors in both rocket and MWR data are a maximum at 80 km, the amplitude of the oscillation is large and significant at 80 km, and the phase jump at this altitude may be real. This is probably due to the seasonal variation in the amplitude and/or phase of the tidal winds and other atmospheric waves which are known to have significant amplitudes above 70 km. The contribution to the annual oscillation from these waves could be such that the resultant A0 could have an amplitude minimum and the associated phase shows a sudden jump. Frequent and long enough rocket
490
C. Raghava Reddi and G. Ramkumar Annual ,T”rn.
(8.5-N) ,x
meridional lnas
ISI
wind
Semi-annual
oscillation
zonal
wind oscillation
(2” NJ
Fig. 3. Amplitude
Fig. 2. Same as Fig. 1 but for meridional
wind.
data do not exist, however, to verify this conjecture quantitatively. Manson et (11.(1981) have shown that the tidal winds are large above 80 km over the mid latitude station Saskatoon and long period oscillations (AO, SAO) above 80 km studied using rocket data could be contaminated by tidal winds. Figure 2 is similar to Fig. 1 but for the meridional wind. CIRA 1986 does not give a meridional wind. The NS winds given by radar data as tabulations in the CIRA I986 model are all for the middle and high latitudes and could not be used for comparison with our observations at Trivandrum (8”N). From Fig. 2, we find the NS amplitudes are small compared with EW amplitudes below the stratopause. The NS amplitude also shows multiple peaks. The peaks at about 15 km and 45 km are observed in both NS and EW amplitudes, but the peak amplitudes in the NS wind are much less than the corresponding peak amplitudes in the EW wind. Above 60 km, the A0 amplitude shows a steady increase with altitude. Below 60 km the amplitude is less than 5 m s-‘. The steady increase of amplitude to about 18 m SK’ observed at Trivandrum was not observed at Christmas Island and, in fact, the Christmas Island amplitudes were less than 6 m ss’. The phase of the oscillation (time of southerly maximum) in the troposphere is somewhat fluctuating but occurs between September and December. The phase fluctuations could be due to relatively large errors because of the small amplitudes of the A0 in the troposphere. Barring the quasi-sinusoidal phase variation in the stratosphere, the phase of the A0 in NS wind in the troposphere and stratosphere leads at higher altitudes. In the mesopause and above, the phase remains fairly steady in October. At 80 km there is a sudden jump and this is similar to the phase jump in EW wind (Fig. 1). Just as in the case of EW wind, the radar data may be corrupted by contributions
profiles of semi-annual oscillation in zonal wind.
and phase
from the tidal and other long period atmospheric waves which attain large amplitudes above the stratopause. In a subsequent section, the diurnal phases (time of westerly and southerly maximum wind) at 80 km will be shown to be z 120” at the time (1700) of the weekly rocketsonde data. At Christmas Island, the A0 phase shows an increase in the upper mesosphere followed by a nearly constant phase in DecemberJanuary in the mesopause region. The phase of the oscillation at Christmas Island is more close to the phase from our MWR data and deviates below 80 km from our rocket data; this supports our contention that the rocket data could be contaminated by the tidal winds. The fairly smooth profiles of both amplitude and phase of A0 observed at Trivandrum add confidence to the reality of these profiles. Semi-annual oscillation. The altitude profiles of amplitude and phase of SAO in EW and NS wind are shown in Figs 3 and 4, respectively. The zonal wind profiles are compared with those from the CIRA 1986 wind model derived from satellite data for the Trivandrum latitude and with observational profiles from
Semi-annual
meridional
rind
oscillation
Fig. 4. Same as Fig. 3 but for meridional
wind.
491
Atmospheric winds at low latitude Ascension Island, Christmas Island and Kwajalein (Hirota, 1978; Vincent, 1993b; Hamilton, 1982). CIRA 1986, and the observations at Trivandrum and Kwajalein, show very close similarities up to about 60 km altitude, including the peak of SAO at about 57 km. In the upper troposphere and the lower stratosphere (17-25 km), our phase profile leads the CIRA 1986 model by 2-3 months. In the mesosphere and above there are some differences in magnitudes at different heights. CIRA 1986, Ascension Island and Christmas Island show a second amplitude peak at 75 km, 80 km and 88 km, respectively, with the peak amplitude observed at Ascension Island being equal to 30 m ss’. The peak near the stratopause and the mesopause are nearly of the same magnitude in both the CIRA 1986 model and Ascension Island. The Kwajalein profile is available over a limited region of the mesosphere and shows the increasing trend consistent with the profiles at other stations. At Trivandrum, we do not observe a prominent peak, but a nearly constant amplitude in the mesopause and above. The phase profile observed at Trivandrum and the CIRA profile agree very well in the troposphere, upper stratosphere, mesopause and above. Our profile, however, in the 65-80 km region show a lag by ~2 months from the CIRA 1986 profile and the phase jump at 80 km, the transition height from rocket data to MWR data, is noticed. The Ascension Island phase profile differs by nearly 3 months from the Trivandrum profile. The z 180” phase difference between SSAO and MSAO given by CIRA 1986 is observed at Trivandrum. The MSAO phase at Christmas Island is in December whereas for both CIRA 1986 and Trivandrum it is in February. Some of the differences between the observational profiles could be because of the different times of the data sets and are perhaps indicative of the year-to-year variability. The SAO amplitude in NS wind (Fig. 4) is very small (less than 5 m ss’) compared with the zonal wind; at Christmas Island the amplitudes of MSAO are even less than the amplitude at Trivandrum. Barring the fluctuations with altitude, the SAO phase at Trivandrum shows a continuous lag with altitude in 3&80 km; above 80 km, the phase remains fairly constant in March. The phase profiles at Christmas Island show very large fluctuations, and these are probably due to errors arising out of the very small amplitudes. The relatively smooth and continuous amplitude and phase profiles at Trivandrum show the reality and the significance of SAO in the stratosphere and above. The most significant feature of the Trivandrum SAO phase in NS wind is the steady increase of nearly one and a half cycles from 20 to 90 km.
Tidal winds The atmospheric tides play a very important role in the dynamics and energy budget of the mesosphere and lower thermosphere (> 70 km) where they grow to large amplitudes, essentially through wave saturation, wave breaking and other dissipation processes. The climatologies of the tidal winds obtained using MWR data at Trivandrum will be published elsewhere. Mentioning briefly the tidal wind characteristics at Trivandrum, we found the diurnal wind oscillation (24 h) to be stronger than the semi-diurnal (12 h) oscillation. The ter-diurnal oscillation (8 h) was found to be as strong as the 12 h oscillation. The tidal amplitudes were found to be stronger during equinoctial months than in the solstices. The tidal wind amplitude and phase profiles exhibited month-to-month variations. The amplitude and the phase profiles of the EW and NS tidal winds for the same months, as were used to compute the A0 and SAO of the monthly mean winds, were used to compute the A0 and SAO of the 24 h, 12 h and 8 h tidal wind amplitudes and phases separately for EW and NS components. The results are presented in the following subsections. A0 and SAO in EW tidal winds. The amplitude and phase profiles of mesopause-lower thermosphere A0 and SAO in zonal tidal winds are shown in Fig. 5. The SAO of the diurnal (24 h), semidiurnal (12 h) and terdiurnal (8 h) winds are of comparable magnitudes, with a general trend of increase with altitude of the 24 h wind. The phase of the SAO of the tidal components in the 82-102 km vary between January-March. The A0 amplitudes are slightly smaller than the SAO amplitudes. For the three tidal components, there is a deep minimum of amplitude at 95 km in AO. Their phase was found to vary with altitude, but the average phase over the altitudes of observation is
Zonal wind
Fig. 5. Altitude profiles of the amplitudes and phases of annual and semi-annual components of the tidal oscillations in zonal wind.
492
C. Raghava Meridional
Reddi and G. Ramkumar
wind
Annual -
Fig. 6. Same as Fig. 5 but for meridional
prevailing
Prevailing _&xurna,
_
and tidal winds Ssmi-diurnal-Ter-diurnal
_c,RA
prevailing
wind. Fig. 7. Altitude profiles of annual mean tidal amplitudes for both zonal and meridional components. The prevailing NS and EW wind profiles (mean of the 12 monthly observed mean wind profiles) is shown in triangles. The prevailing EW wind profiles from CIRA 1986 is also shown for comparison.
in August-September. Thus, taking together A0 and SAO, the diurnal tidal oscillation contributions to the zonal wind field is a maximum during AugustSeptember and a minimum during May-June. The semi-diurnal tidal contribution is also a maximum during August-September and a minimum during May-June. The magnitudes are larger at higher altitudes compared with lower altitudes for both diurnal and semi-diurnal components. For the ter-diurnal oscillation the maximum contributions are during January and February and minimum during MayJune and October-November. A0 and SAO in NS tidal winds. Figure 6 shows profiles similar to Fig. 5 but for the meridional wind. The 24 h component SAO is about 7 m s-’ whereas its A0 amplitude decreased continuously from about 15 m s-’ to about 8 m SK’. The 12 h and 8 h components have large SAO amplitudes compared with AO. The phase profiles of the three components show larger variations with altitude, but the sum of the A0 and SAO tidal winds below 100 km in general have large diurnal amplitudes during March and minimum during November-December. The semi-diurnal NS wind contribution is maximum during winter and summer and minimum during the equinoxes. Annual mean winds The average of the 12 monthly mean profiles of the monthly mean, diurnal, semi-diurnal and ter-diurnal winds for NS and EW components is shown in Fig. 7. The annual mean amplitude is largest for the 24 h wind, followed by 12 h and 8 h components successively. The annual mean winds are westerly for the EW component and southerly for the NS component; the southerly component is stronger than westerly component. The continuous increase with altitude of the
annual mean zonal wind given by CIRA observed at Trivandrum.
DISCUSSION
1986 is not
AND CONCLUSIONS
The A0 and SAO of the monthly mean winds and the amplitudes of the tidal components have been presented in this paper. The smooth and reasonably continuous profiles, even at the data transition altitude of 20 km (balloonsonde and rocketsonde), can be taken to indicate the reliability of the results. At 80 km (the transition height of rocketsonde and MWR data base), there is a significant jump in the phase and amplitudes. This is probably due to the contamination in the amplitude and phase profiles from the tidal winds in the rocket data and partly due to the relatively large errors in both rocket and MWR data at 80 km. The zonal wind A0 and SAO amplitudes and phases in the troposphere and stratosphere are consistent with the reports (Reddy et al., 1986; Reddy and Raghava Reddi, 1986) published earlier. It was reported that the A0 and SAO in zonal wind varied with longitude. Since the oscillation periods are very large, for divergence free flows, the longitudinally varying SAO and A0 in the zonal wind (Reddy and Raghava Reddi, 1986) should be associated with corresponding flow fields in the NS and perhaps, the vertical direction also. In the present study, it has been found that, though relatively small in magnitude, there are significant A0 and SAO in the NS winds also. The most significant feature noticed is that the
Atmospheric winds at low latitude phase of the A0 in the NS wind has a downward phase propagation at a rate of 334 km per month. Also, the SAO phase in the NS wind shows an upward phase propagation in 3c-90 km region at a rate of 6 7 km per month. It would be interesting to study the longitudinal variations of A0 and SAO simultaneously in EW and NS winds Rocket observations have shown the evidence for an SAO of zonal winds at the equatorial mesopause and lower thermosphere (> 80 km), which is out of phase with the stratopause SAO (Hirota, 1978; Hamilton, 1982), and the same is observed in the present study also. It has been suggested that momentum deposition associated with the breaking or dissipation of Kelvin waves, atmospheric tides and gravity waves are important in determining the mean circulation as well as forcing the MSAO (Hirota, 1978; Lindzen, 1981; Dunkerton, 1982). The present study shows a very small peak of SAO at the mesopause and, at higher levels, the amplitude in the zonal wind remained constant at 12 m ss’. In the meridional wind it decreased rapidly to insignificance. The SAO phase shows little variation above the mesopause, being in February in EW winds and March in NS winds. There seem to be longitudinal differences in the phase of MSAO. Hirota (1978) for Ascension Island (SOS) and Hamilton (1982) for Kwajalein (9”N) found westward winds to occur in March, while we observe maximum MSAO westerly amplitudes occurring in February. Our results are also not consistent with Avery et al. (1990) who found from meteor wind observations that the mean westward wind occurred at the September equinox. At Trivandrum, if we take the sum of annual mean, A0 and SAO, the winds would be maximum westerly during the September equinox, and maximum easterly during April-May for zonal winds. Strong southerly winds were observed during February-March and August-September and strong northerly winds were observed during NovemberDecember and May-June. Takahashi et al. (1995) reported strong MSAO, using airglow photometer data at Fortaleza (3.9”S, 34.4”W) and Cachoeira Paulista (22.7S, 45”W) for many years. They found temperature maxima in the equinoxes and minima in the solstices. In low latitudes large downward advection of atomic oxygen leading to corresponding increasing ozone concentration may lead to the increased temperature during equinoctial months (Garcia and Clancy, 1990). These observations suggest that the mesosphere is dynamically very active with significant vertical transport which varies latitudinally. In mesospheric models one has to take into account seasonal and latitudinal
493
variations of vertical and meridional circulations along with the seasonally and latitudinally varying vertical transport of eddy fluxes by gravity waves and tides. Vincent and Lesicar (1991) considered their observed westward drag as probably due to gravity waves and the diurnal tide. The drag exerted by the breaking (1,l) diurnal tidal mode was estimated by Lindzen (1991) to be - 16 m ss’ day-’ at the equator near 85 km altitude. However, it was shown (Raghava Reddi et al., 1993b) that the altitude variation of the amplitude and phase of the tidal modes are always more complicated than that expected for the (1,l) and (2,2) modes for the diurnal and semi-diurnal tides. It is found that, although the diurnal tide is the strongest, the semi-diurnal and ter-diurnal tides have significant amplitudes. Further, instead of the (1,l) and (2,2) modes expected to be the most significant at the equator, the diurnal (1, - 2) and higher order semidiurnal modes are more prominent. The A0 and SAO of each of the three (24 h, 12 h and 8 h) tidal amplitudes and phases do not involve a single mode. They represent the A0 and SAO of the sum of the different modes of each tidal component present simultaneously. We find that the sum of the tidal wind magnitudes during the year, computed from the amplitude profiles shows a minimum during MayJune and maximum during August-September. Both tides and the larger period A0 and SAO in the mesopause region are global phenomena. The relation between the times of maximum westerly and easterly winds, as the sum of A0 and SAO in monthly mean winds and the amplitude of the tidal winds, may not be unexpected. The role of gravity waves and other global scale atmospheric waves in the generation of A0 and SAO in the upper atmosphere is well recognised. However, actual measurements of the momentum fluxes and their altitude gradients from the different propagating waves are needed for a clearer understanding of the interrelations between the A0 and SAO and shorter period wave phenomena. Larger data bases from different longitudes at low latitudes would be required to clearly characterise the mesospheric wind fields. The most significant feature that emerged out of this study is that the A0 and SAO of monthly mean winds, and of the tidal amplitudes, are significant in the NS wind also, and that the EW wind alone cannot characterise the wind field in the middle atmosphere at low latitudes. In fact, some of the longitudinal differences (Reddy and Raghava Reddi, 1986) found in zonal wind A0 and SAO are difficult to conceive independently without corresponding signatures in the NS wind fields.
494
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Middle Atmosphere Dynamics, 489~~. Academic Press. Quasi-biennial, annual, and semi-annual zonal wind and temperature harmonic amplitudes and phases in the stratosphere and low mesosphere of the Northern Hemisphere. J. geophys. Res. 75, 543-550. A new meteor echo detection and collection system: Christmas Island mesospheric wind measurements. Radio Sci. 25, 657-669. Semi-annual variation in the zonal wind from 20 to 65 km at 80”-IO”. J. geophys. Res. 78,6373-6476. Periodic variations in stratospheric zonal wind from 20 to 65 km at 80”N to 70% Q.JI R. met Sot. 100,203211. Annual and semi-annual waves in the temperature of the mesosphere as deduced from Nimbus 6 PMR measurements. Q.JI R. met. Sot. 105, 5099520. Seasonal variations in the semi-annual oscillations. J. atmos. Sci. 45,2772-2787. On the role of the Kelvin wave in the westerly phase of the semi-annual zonal wind oscillation. /. atmos. Sci. 36, 3241. Theory of the mesopause oscillation. J. atmos. Sci. 39, 2681-2690. Monthly Mean global climatology of temperature, wind, geopotential height andpressuwfor O-120 km. NASA Tech. Memo. 100687, NASA. First observations of mesospheric dynamics with a partial reflection radar in Hawai (22”N, 16O”W). Geophys. Res. Lett. 19,4099412. Seasonal variation in equatorial mesospheric temperature observed by SME. J. atmos. Sci. 47, 1666 1673. Annual and semi-annual zonal wind components and corresponding temperature and density variations, 60-130 km. Planet. Space Sci. 20,2099-2112. Zonal wind quasi-biennial oscillations at 25-60 km altitude, 1962-69. Q. Jl R. met. Sot. 99,73-81. Rockestonde observations of the mesospheric semiannual oscillation at Kwajalein. Atmos. Ocean 20, 281-286. Equatorial waves in the upper stratosphere and mesosphere in relation to the semi-annual oscillation of the zonal wind. J. atmos. Sri. 35, 714722. Evolution of the zonal mean state in the equatorial middle atmosphere. J. atmos. Sci. 43, 3159-3176. Estimation of the Kelvin wave contribution to the semiannual oscillation. J. atmos. Sci. 45, 1462-1475. Evidence of Polar-tropical coupling in upper stratospheric zonal wind anomalies. J. atmos. Sci. 29,1076 1080. Turbulence and stress owing to gravity wave and tidal breakdown, J. geophys. Res. 86,9707-9714. Long period oscillations in mesospheric and lower thermospheric winds (6&l 10 km) at Saskatoon (52’N, 107”W, L = 4.3). J. geomog. Geoelectr. 33, 613-621. Effects of solar tides on the zonal mean circulation in the lower thermosphere: solstice condition. J. atmos. terr. Phys. 51,635-647. Mean wind and the semi-annual oscillation in the mesosphere and lower thermosphere at Christmas Island. J. geopbys. Res. 98,20,385-20,400. Annual and semi-annual temperature oscillations at the mesopause levels over Trivandrum (8.5”N, 78”E). J. geophys. Res. 98(05), 8925-8931.
Atmospheric Raghava
Reddi C., Rajeev K. and Geetha
Ramkumar
winds at low latitude 199313
Reddi C.
1986
Reddy C. A., Raghava Reddi C. and Mohankumar K. G. Reed R. J.
1986
Reed R. J.
1966
Reddy C. A. and Raghava
Sasi M. N. and Murthy Takahashi
1965
1991
B. V. K.
H., Clemesha
B. R. and Batista
P. P.
1995
Vincent
R. A.
1993a
Vincent
R. A.
1993b
Vincent
R. A. and Lesicar
Vincent
R. A.. Tsuda T. and Kato S.
D.
1991
1988
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Tidal winds in the meteor region over Trivandrum. J. utmos. terr. Phys., 55, 1219-1231. Longitudinal differences and inter-annual variations of zonal wind in the tropical stratosphere and troposphere. J. atmos. terr. Phys. 48, 108551092. Zonal wind components in the stratosphere and troposphere over India. Q.J/ R. met. Sot. 112, 81 l-823. The quasi-biennial oscillation of the atmosphere between 30 and 50 km over Ascension Island. J. atmos. Sci. 22, 331-333. Zonal wind behaviour in the equatorial stratosphere J. geophys. Rex. 71, 4223and lower mesosphere. 4233. Study of Quasi-biennial oscillation in the tropical stratosphere. J. atmos. few. Phys. 53, 1173-l 180. Predominant semi-annual oscillation of the upper mesospheric air glow intensities and temperatures in the equatorial region. J. atmos. terr. Phys. 57, 407414. Low frequency dynamics of the equatorial mesosphere, Coupling processes in the lower and middle atmospheres. E. V. Thrane et al. (eds), pp. 1255136. Kluwer, Dordrecht. Long period motions in the equatorial mesosphere. J. atmos. trrr. Phys. 55, 1067-1080. Dynamics of the equatorial middle atmosphere: First results with a new generation partial reflection radar. Grophys. Res. Lett. 18, 825-828. A comparative study of mesospheric solar tides observed at Adelaide and Kyoto. J. groph.vs. Rcs. 93, 6999708.
and Solar-Terwslrial
Pergamon PII: SOO21-9169(96)00051-7
Physics, Vol 59, No. 5, pp. 487~495, 1997 CopyrIght c; 1997 Elsevier Science Ltd Printed m Great Bntam. All rrghts reserved 1364-6826197 $17.00+0.00
The annual and semi-annual wind fields in low latitudes C. Raghava Atmospheric
Dynamics
Branch,
Reddi and Geetha
Ramkumar
Space Physics Laboratory, Vikram Trivandrum 695 022, India
(Received infinalform
Sarabhai
Space Centre,
5 March 1996; accepted 15 March 1996)
Abstract-The annual oscillations (AO) and semi-annual oscillations (SAO) of the monthly mean zonal and meridional winds in the troposphere, stratosphere and mesosphere have been studied using simultaneous data from balloonsondes, rocketsondes and Meteor Wind Radar (MWR) acquired at Trivandrum. A study is also made of the A0 and SAO of the diurnal, semi-diurnal and ter-diurnal wind oscillations in the 8OG102 km altitudes using MWR observations. The same observations have been used to study the altitude profiles of the annual mean winds, and the annual mean tidal winds. It is found that the A0 phase in the NS wind leads with increasing altitude in the stratosphere and mesosphere while SAO phase lags with altitude. The SAO of EW tidal wind amplitudes are slightly stronger than AO. For the NS wind, the A0 of the diurnal tide has the largest amplitude and the SAO of the semi-diurnal tide also has a relatively large amplitude. The results are compared with earlier reports from other low latitude stations and with the CIRA 1986 model for 8.5’N latitude. Further, they are discussed in the light of tidal dissipation in and above the mesopause (> 70 km). ‘0 1997 Elsevier Science Ltd. All rights reserved
INTRODUCTION
Leovy, 1986, 1988; Delisi and Dunkerton, 1988). Reddy and Raghava Reddi (1986) showed the longitudinal and interannual variations of SSAO. The Mesopause SAO (MSAO) was first shown by Groves (1972) using rocket and radar wind climatologies of the meteor region (80-l 10 km). Later, Hirota (1978) and Hamilton (1982) showed the MSAO exists with a peak at 80 km in low latitudes, with westerly winds during solstice and maximum easterly winds during equinox. Garcia and Clancy (1990) have shown the presence of MSAO in temperature using Solar Mesospheric Explorer (SME) satellite data. More recently, data from ground based MF and Meteor Wind Radar (MWR) at Christmas Island (2”N, 157”W) and Hawai (22”N, 160”W) indicate an MSAO near 80-85 km which exhibits a high degree ofinter-annual variability (Vincent and Lesicar, 1991; Fritts and Isler, 1992; Palo and Avery, 1993; Vincent, 1993a, b). Raghava Reddi et al. (1993a) reported A0 and SAO in temperature in the lower thermospheric region using Meteor Wind Radar (MWR) observations from Trivandrum. The SSAO is identified to be a product of atmospheric wave mean flow interaction processes (Dunkerton, 1979). Dunkerton (1982) simulated the MSAO and showed that the eastward and westward propagating inertial gravity waves can produce the required eastward and westward accelerations. Model studies of Miyahara and Wu (1989) show that the
From earlier reports (Reed, 1965, 1996; Groves, 1973; Belmont et al., 1974; Hirota, 1978; Reddy et al., 1986) based on rocketsonde and balloonsonde observations, it is known that the semi-annual oscillation (SAO), annual oscillation (AO) and quasi-biennial oscillation (QBO) account for about 90% of the zonal wind variations in the low latitude middle atmosphere. The QBO, having a maximum amplitude at about 28 km, was found to vary in the period of the oscillation (Sasi and Murthy, 1991), but the altitude variations of the amplitude and phase of the oscillation were almost independent of longitude (Reddy and Raghava Reddi, 1986). Above 35 km the equatorial middle atmosphere is characterised by the semi-annual oscillation of the zonal wind, with one peak amplitude centred at about the stratopause (w 50 km) and another at the mesopause ( z 85 km); they are 180” out of phase at the two altitudes (Hirota, 1978; Andrews et al., 1987). The stratopause SAO (SSAO) was discovered by Reed (1965, 1966) and its climatologies were studied extensively (Angel1 and Korshover, 1970; Belmont and Dartt, 1973; Hopkins, 1975) using rocketsonde winds from the MRN (Meteorological Rocket Network) stations. More recently the data from the Stratospheric and Mesospheric Sounder (SAMS) and the Limb Infrared Monitor of the Stratosphere (LIMS) have helped in understanding the dynamical processes involved in driving SSAO (Hitchman and 487
488
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dissipation of the westward migrating diurnal tide can generate significant mean zonal winds in the mesopause region and the semi-annual modulation of the amplitude of the tide could be related to the MSAO. Observations from Adelaide (Vincent et al., 1988) show a semi-annual variation in the strength of the propagating diurnal tide, with large amplitude in equinox; the maximum in March is stronger than the maximum in September. In this paper we report the results of SAO and A0 from ground level to 102 km, obtained from ballonsonde, rocketsonde and MWR observations at Trivandrum. Also, the A0 and SAO of the amplitudes and phases of the tidal winds are presented to evaluate the tidal wind amplitude modulations by the A0 and SAO in the zonal and meridional winds. The altitude profiles of the annual mean tidal amplitudes and the mean prevailing winds are presented. The results from this study are compared and discussed with the results from other low latitudes and the CIRA 1986 (Flemming et al., 1988) model. DATA AND ANALYSIS
The data used in this study are from the balloonsonde, M-100 rocketsonde and MWR observations, all at Trivandrum during 198551987. Monthly mean profiles of the zonal and meridional winds from balloonsonde and rocketsonde were used to compute the annual mean and the amplitude and phase profiles of A0 and SAO, in the O-80 km altitude range. The details of the analysis of rocketsonde (RS) and balloonsonde (BS) data to obtain the amplitude and phase profiles of A0 and SAO were published earlier (Reddy et al., 1986). The RS and BS data were obtained every Wednesday of the month, the former at about 1700 (Indian Standard Time, IST) and the latter about 3 hours earlier. The BS data from ground level to 20 km was combined with RS data from 21 to 80 km to obtain the profiles from ground to 80 km altitude. The diurnal variation contribution to the RS and BS profiles from the tidal winds because of the local time difference (3 h) of the RS and BS launches were assumed not to introduce discontinuities in the profiles at the transition altitude of 20-21 km. It was estimated that the errors in the computed magnitudes of the AO, SAO and annual mean wind is less than 1.5 m s-’ in the 2-35 km range, 1.5 to 2.0 m s-’ in the 35-55km range and 2-3 m ss’ above 55 km altitude. The typical phase errors are l&15” for amplitudes exceeding 10 m s-‘, 15-30” for l&5 m s-’ and 30-60” for 5-2 m s-’ amplitudes (Reddy et al., 1986). In the 81-102 km altitude region, continuous diurnal MWR data for 8-10 days each month were used.
The 12 monthly mean profiles of average wind, and amplitude and phase of the diurnal, semi-diurnal and ter-diurnal winds, were computed following the procedure described earlier in Raghava Reddi et al. (1993b). The data were spread over 1985-1987. But the monthly profiles used to represent every month of the calender year were based on the error estimates at each height of the profiles. When profiles for the same calender month were available in more than one year, the profile with the least errors and the largest altitude coverage was chosen. The profiles for August and December were for 1985; the profiles for February and October were for 1986 and the profiles for the other months were obtained in 1987. Month to month variations at all heights during the calender year did not show any abnormal variation because of the mixing of two months of 1985 and 1986 data with the data for all other months in 1987. The r.m.s errors in the monthly mean MWR wind profiles (both NS and EW winds) are less than 2 m ss’ on the average, but are relatively large at altitudes above 100 km and below 85 km. The standard deviations of the profiles were estimated to be 8810 m ss’. The phase error of the A0 and SAO computed depends on the amplitude of the oscillation and was estimated to be less than 10” for amplitudes exceeding 10 m ss’. BS, RS and MWR data for each calender month were used for the same year from the three experiments. Based on the data analysis procedures used for computing the wind profiles, the altitude resolution of the RS profile is about 5 km, whereas the resolution from balloonsonde and MWR is about 2 km, although the profiles from the three experiments are available at 1 km intervals. The 12 monthly mean values at each height in NS and EW winds were subjected to harmonic analysis to determine the amplitudes and phases of A0 and SAO along with the average of the 12 monthly values. The CIRA 1986 model values for zonal wind in the GlO5 km range for the 12 months for the latitude of 8.5”N (interpolated from values at 0” and 10”N) were also subjected to harmonic analysis to get the model A0 and SAO amplitude and phase profiles for comparison. The observational results used for comparison were obtained from published reports at Ascension Island (8”S, 14”W), Christmas Island (2”N, 157”W) and Kwajalein (9”N, 168”E) (Hirota, 1978; Vincent, 1993b; Hamilton, 1982). RESULTS
Monthly
mean winds
Annual oscillation. The A0 in the mean EW wind to the annual cycle of up to ~60 km is attributed
Atmospheric winds at low latitude Annual
mm1
wind
oscillation
0 AMPlsT”DE (m-l
)
PHASE (mm)
Fig. 1.Amplitude and phase profiles of annual oscillation in zonal wind.
solar insolation in extratropical latitudes, and the flow deflection by the Coriolis force. Above 60 km, the A0 is forced by the annual variation of the momentum fluxes transported and deposited by gravity waves, tides and other planetary scale waves. The existence of an annual cycle in the tropics is difficult to explain. The amplitude of A0 at low latitudes is small compared with the amplitude in middle and high latitudes, but it is a very significant part of the total variance in the tropics. A0 is known to exist with opposite phases in the lower stratosphere and lower mesosphere (Angel1 and Korshover, 1970) with its phase changing continuously through the mesosphere. The phase (time of westerly maximum wind due to AO) is in January in the upper mesosphere (Crane, 1979). Figure 1 shows the amplitude and phase profiles of A0 in zonal wind. Below about 6 km, the A0 amplitude decreased with altitude, and the phase (time of maximum westerly wind) is in July. These could be associated with local weather processes and may not be of global scale. Above 7 km, the A0 amplitudes show three broad peaks: one between 8 and 31 km, another in the 32-60 km range and a third between 65 and 85 km. There are sharp minima:one is at 17 km, which incidentally is the height of the tropopause over Trivandrum, and another is at 30 km, which is close to the altitude of QBO maximum (Reddy and Raghava Reddi, 1986). In the 45560 km region the A0 amplitude is nearly constant (Z 10 m s-‘) and it is known that, in this altitude region, the SAO has maximum amplitude. The broad peak, with maximum amplitude exceeding 20 m ss’ at 70 km, merges fairly smoothly with the profile from MWR above 80 km. The CIRA 1986 model shows a broad peak between 65 and 95 km altitude and does not depict most of the structures in the profile observed at Trivandrum. The maximum amplitude observed is reproduced in the
489
CIRA 1986 wind model based on satellite data, but the altitude of the peak given by CIRA 1986 is higher. The A0 amplitude profile observed at Christmas Island (Vincent, 1993b) using MF radar data shows amplitudes similar to those observed at Trivandrum in the mesopause and lower thermosphere. The A0 phase profile from CIRA 1986 and the observed profile show fair consistency (& l-2 months). The maximum westerly zonal winds occur during the northern winter month. This shows that the westerly regime observed at mid latitudes during northern winter months is extended to low latitudes. Below 90 km the phase lags with increasing altitude whereas, above 90 km, the phase leads with altitude. At Christmas Island the A0 phase leads with altitude followed by a steady value above 80 km. While the Trivandrum A0 amplitude above 70 km shows a rapid decrease from ~25 m ss’ to less than 5 m ss’, the Christmas Island profile shows an increasing trend from ~3 m s-’ to about 10 m s-’ at 90 km. The reality of these variations need to be ascertained with further data, if possible, obtained simultaneously. At 80 km the A0 zonal wind phase at Trivandrum showed a sudden jump and the amplitude which was decreasing with altitude up to 80 km, was nearly constant above 80 km. The rocket wind data used for obtaining the profiles below 80 km was from the weekly (Wednesday) rocketsonde launches at 1700, whereas the MWR data used for altitudes above 80 km were the monthly mean profiles after subtracting the contributions from tidal winds. In the MWR data, the analysis procedure is such that the contribution to the wind variance due to atmospheric waves (periods equal to or greater than 2 days) were averaged out. Thus A0 amplitude and phase profiles from MWR data would be free from any contamination due to tides and other atmospheric waves whereas the profiles below 80 km may have some contamination from the seasonal variations in the tidal and other atmospheric wave phenomena. We found that the tidal winds have seasonal variations; it is reasonable to expect atmospheric wave fluxes at 80 km also to have seasonal variations. In spite of the fact that the errors in both rocket and MWR data are a maximum at 80 km, the amplitude of the oscillation is large and significant at 80 km, and the phase jump at this altitude may be real. This is probably due to the seasonal variation in the amplitude and/or phase of the tidal winds and other atmospheric waves which are known to have significant amplitudes above 70 km. The contribution to the annual oscillation from these waves could be such that the resultant A0 could have an amplitude minimum and the associated phase shows a sudden jump. Frequent and long enough rocket
490
C. Raghava Reddi and G. Ramkumar Annual ,T”rn.
(8.5-N) ,x
meridional lnas
ISI
wind
Semi-annual
oscillation
zonal
wind oscillation
(2” NJ
Fig. 3. Amplitude
Fig. 2. Same as Fig. 1 but for meridional
wind.
data do not exist, however, to verify this conjecture quantitatively. Manson et (11.(1981) have shown that the tidal winds are large above 80 km over the mid latitude station Saskatoon and long period oscillations (AO, SAO) above 80 km studied using rocket data could be contaminated by tidal winds. Figure 2 is similar to Fig. 1 but for the meridional wind. CIRA 1986 does not give a meridional wind. The NS winds given by radar data as tabulations in the CIRA I986 model are all for the middle and high latitudes and could not be used for comparison with our observations at Trivandrum (8”N). From Fig. 2, we find the NS amplitudes are small compared with EW amplitudes below the stratopause. The NS amplitude also shows multiple peaks. The peaks at about 15 km and 45 km are observed in both NS and EW amplitudes, but the peak amplitudes in the NS wind are much less than the corresponding peak amplitudes in the EW wind. Above 60 km, the A0 amplitude shows a steady increase with altitude. Below 60 km the amplitude is less than 5 m s-‘. The steady increase of amplitude to about 18 m SK’ observed at Trivandrum was not observed at Christmas Island and, in fact, the Christmas Island amplitudes were less than 6 m ss’. The phase of the oscillation (time of southerly maximum) in the troposphere is somewhat fluctuating but occurs between September and December. The phase fluctuations could be due to relatively large errors because of the small amplitudes of the A0 in the troposphere. Barring the quasi-sinusoidal phase variation in the stratosphere, the phase of the A0 in NS wind in the troposphere and stratosphere leads at higher altitudes. In the mesopause and above, the phase remains fairly steady in October. At 80 km there is a sudden jump and this is similar to the phase jump in EW wind (Fig. 1). Just as in the case of EW wind, the radar data may be corrupted by contributions
profiles of semi-annual oscillation in zonal wind.
and phase
from the tidal and other long period atmospheric waves which attain large amplitudes above the stratopause. In a subsequent section, the diurnal phases (time of westerly and southerly maximum wind) at 80 km will be shown to be z 120” at the time (1700) of the weekly rocketsonde data. At Christmas Island, the A0 phase shows an increase in the upper mesosphere followed by a nearly constant phase in DecemberJanuary in the mesopause region. The phase of the oscillation at Christmas Island is more close to the phase from our MWR data and deviates below 80 km from our rocket data; this supports our contention that the rocket data could be contaminated by the tidal winds. The fairly smooth profiles of both amplitude and phase of A0 observed at Trivandrum add confidence to the reality of these profiles. Semi-annual oscillation. The altitude profiles of amplitude and phase of SAO in EW and NS wind are shown in Figs 3 and 4, respectively. The zonal wind profiles are compared with those from the CIRA 1986 wind model derived from satellite data for the Trivandrum latitude and with observational profiles from
Semi-annual
meridional
rind
oscillation
Fig. 4. Same as Fig. 3 but for meridional
wind.
491
Atmospheric winds at low latitude Ascension Island, Christmas Island and Kwajalein (Hirota, 1978; Vincent, 1993b; Hamilton, 1982). CIRA 1986, and the observations at Trivandrum and Kwajalein, show very close similarities up to about 60 km altitude, including the peak of SAO at about 57 km. In the upper troposphere and the lower stratosphere (17-25 km), our phase profile leads the CIRA 1986 model by 2-3 months. In the mesosphere and above there are some differences in magnitudes at different heights. CIRA 1986, Ascension Island and Christmas Island show a second amplitude peak at 75 km, 80 km and 88 km, respectively, with the peak amplitude observed at Ascension Island being equal to 30 m ss’. The peak near the stratopause and the mesopause are nearly of the same magnitude in both the CIRA 1986 model and Ascension Island. The Kwajalein profile is available over a limited region of the mesosphere and shows the increasing trend consistent with the profiles at other stations. At Trivandrum, we do not observe a prominent peak, but a nearly constant amplitude in the mesopause and above. The phase profile observed at Trivandrum and the CIRA profile agree very well in the troposphere, upper stratosphere, mesopause and above. Our profile, however, in the 65-80 km region show a lag by ~2 months from the CIRA 1986 profile and the phase jump at 80 km, the transition height from rocket data to MWR data, is noticed. The Ascension Island phase profile differs by nearly 3 months from the Trivandrum profile. The z 180” phase difference between SSAO and MSAO given by CIRA 1986 is observed at Trivandrum. The MSAO phase at Christmas Island is in December whereas for both CIRA 1986 and Trivandrum it is in February. Some of the differences between the observational profiles could be because of the different times of the data sets and are perhaps indicative of the year-to-year variability. The SAO amplitude in NS wind (Fig. 4) is very small (less than 5 m ss’) compared with the zonal wind; at Christmas Island the amplitudes of MSAO are even less than the amplitude at Trivandrum. Barring the fluctuations with altitude, the SAO phase at Trivandrum shows a continuous lag with altitude in 3&80 km; above 80 km, the phase remains fairly constant in March. The phase profiles at Christmas Island show very large fluctuations, and these are probably due to errors arising out of the very small amplitudes. The relatively smooth and continuous amplitude and phase profiles at Trivandrum show the reality and the significance of SAO in the stratosphere and above. The most significant feature of the Trivandrum SAO phase in NS wind is the steady increase of nearly one and a half cycles from 20 to 90 km.
Tidal winds The atmospheric tides play a very important role in the dynamics and energy budget of the mesosphere and lower thermosphere (> 70 km) where they grow to large amplitudes, essentially through wave saturation, wave breaking and other dissipation processes. The climatologies of the tidal winds obtained using MWR data at Trivandrum will be published elsewhere. Mentioning briefly the tidal wind characteristics at Trivandrum, we found the diurnal wind oscillation (24 h) to be stronger than the semi-diurnal (12 h) oscillation. The ter-diurnal oscillation (8 h) was found to be as strong as the 12 h oscillation. The tidal amplitudes were found to be stronger during equinoctial months than in the solstices. The tidal wind amplitude and phase profiles exhibited month-to-month variations. The amplitude and the phase profiles of the EW and NS tidal winds for the same months, as were used to compute the A0 and SAO of the monthly mean winds, were used to compute the A0 and SAO of the 24 h, 12 h and 8 h tidal wind amplitudes and phases separately for EW and NS components. The results are presented in the following subsections. A0 and SAO in EW tidal winds. The amplitude and phase profiles of mesopause-lower thermosphere A0 and SAO in zonal tidal winds are shown in Fig. 5. The SAO of the diurnal (24 h), semidiurnal (12 h) and terdiurnal (8 h) winds are of comparable magnitudes, with a general trend of increase with altitude of the 24 h wind. The phase of the SAO of the tidal components in the 82-102 km vary between January-March. The A0 amplitudes are slightly smaller than the SAO amplitudes. For the three tidal components, there is a deep minimum of amplitude at 95 km in AO. Their phase was found to vary with altitude, but the average phase over the altitudes of observation is
Zonal wind
Fig. 5. Altitude profiles of the amplitudes and phases of annual and semi-annual components of the tidal oscillations in zonal wind.
492
C. Raghava Meridional
Reddi and G. Ramkumar
wind
Annual -
Fig. 6. Same as Fig. 5 but for meridional
prevailing
Prevailing _&xurna,
_
and tidal winds Ssmi-diurnal-Ter-diurnal
_c,RA
prevailing
wind. Fig. 7. Altitude profiles of annual mean tidal amplitudes for both zonal and meridional components. The prevailing NS and EW wind profiles (mean of the 12 monthly observed mean wind profiles) is shown in triangles. The prevailing EW wind profiles from CIRA 1986 is also shown for comparison.
in August-September. Thus, taking together A0 and SAO, the diurnal tidal oscillation contributions to the zonal wind field is a maximum during AugustSeptember and a minimum during May-June. The semi-diurnal tidal contribution is also a maximum during August-September and a minimum during May-June. The magnitudes are larger at higher altitudes compared with lower altitudes for both diurnal and semi-diurnal components. For the ter-diurnal oscillation the maximum contributions are during January and February and minimum during MayJune and October-November. A0 and SAO in NS tidal winds. Figure 6 shows profiles similar to Fig. 5 but for the meridional wind. The 24 h component SAO is about 7 m s-’ whereas its A0 amplitude decreased continuously from about 15 m s-’ to about 8 m SK’. The 12 h and 8 h components have large SAO amplitudes compared with AO. The phase profiles of the three components show larger variations with altitude, but the sum of the A0 and SAO tidal winds below 100 km in general have large diurnal amplitudes during March and minimum during November-December. The semi-diurnal NS wind contribution is maximum during winter and summer and minimum during the equinoxes. Annual mean winds The average of the 12 monthly mean profiles of the monthly mean, diurnal, semi-diurnal and ter-diurnal winds for NS and EW components is shown in Fig. 7. The annual mean amplitude is largest for the 24 h wind, followed by 12 h and 8 h components successively. The annual mean winds are westerly for the EW component and southerly for the NS component; the southerly component is stronger than westerly component. The continuous increase with altitude of the
annual mean zonal wind given by CIRA observed at Trivandrum.
DISCUSSION
1986 is not
AND CONCLUSIONS
The A0 and SAO of the monthly mean winds and the amplitudes of the tidal components have been presented in this paper. The smooth and reasonably continuous profiles, even at the data transition altitude of 20 km (balloonsonde and rocketsonde), can be taken to indicate the reliability of the results. At 80 km (the transition height of rocketsonde and MWR data base), there is a significant jump in the phase and amplitudes. This is probably due to the contamination in the amplitude and phase profiles from the tidal winds in the rocket data and partly due to the relatively large errors in both rocket and MWR data at 80 km. The zonal wind A0 and SAO amplitudes and phases in the troposphere and stratosphere are consistent with the reports (Reddy et al., 1986; Reddy and Raghava Reddi, 1986) published earlier. It was reported that the A0 and SAO in zonal wind varied with longitude. Since the oscillation periods are very large, for divergence free flows, the longitudinally varying SAO and A0 in the zonal wind (Reddy and Raghava Reddi, 1986) should be associated with corresponding flow fields in the NS and perhaps, the vertical direction also. In the present study, it has been found that, though relatively small in magnitude, there are significant A0 and SAO in the NS winds also. The most significant feature noticed is that the
Atmospheric winds at low latitude phase of the A0 in the NS wind has a downward phase propagation at a rate of 334 km per month. Also, the SAO phase in the NS wind shows an upward phase propagation in 3c-90 km region at a rate of 6 7 km per month. It would be interesting to study the longitudinal variations of A0 and SAO simultaneously in EW and NS winds Rocket observations have shown the evidence for an SAO of zonal winds at the equatorial mesopause and lower thermosphere (> 80 km), which is out of phase with the stratopause SAO (Hirota, 1978; Hamilton, 1982), and the same is observed in the present study also. It has been suggested that momentum deposition associated with the breaking or dissipation of Kelvin waves, atmospheric tides and gravity waves are important in determining the mean circulation as well as forcing the MSAO (Hirota, 1978; Lindzen, 1981; Dunkerton, 1982). The present study shows a very small peak of SAO at the mesopause and, at higher levels, the amplitude in the zonal wind remained constant at 12 m ss’. In the meridional wind it decreased rapidly to insignificance. The SAO phase shows little variation above the mesopause, being in February in EW winds and March in NS winds. There seem to be longitudinal differences in the phase of MSAO. Hirota (1978) for Ascension Island (SOS) and Hamilton (1982) for Kwajalein (9”N) found westward winds to occur in March, while we observe maximum MSAO westerly amplitudes occurring in February. Our results are also not consistent with Avery et al. (1990) who found from meteor wind observations that the mean westward wind occurred at the September equinox. At Trivandrum, if we take the sum of annual mean, A0 and SAO, the winds would be maximum westerly during the September equinox, and maximum easterly during April-May for zonal winds. Strong southerly winds were observed during February-March and August-September and strong northerly winds were observed during NovemberDecember and May-June. Takahashi et al. (1995) reported strong MSAO, using airglow photometer data at Fortaleza (3.9”S, 34.4”W) and Cachoeira Paulista (22.7S, 45”W) for many years. They found temperature maxima in the equinoxes and minima in the solstices. In low latitudes large downward advection of atomic oxygen leading to corresponding increasing ozone concentration may lead to the increased temperature during equinoctial months (Garcia and Clancy, 1990). These observations suggest that the mesosphere is dynamically very active with significant vertical transport which varies latitudinally. In mesospheric models one has to take into account seasonal and latitudinal
493
variations of vertical and meridional circulations along with the seasonally and latitudinally varying vertical transport of eddy fluxes by gravity waves and tides. Vincent and Lesicar (1991) considered their observed westward drag as probably due to gravity waves and the diurnal tide. The drag exerted by the breaking (1,l) diurnal tidal mode was estimated by Lindzen (1991) to be - 16 m ss’ day-’ at the equator near 85 km altitude. However, it was shown (Raghava Reddi et al., 1993b) that the altitude variation of the amplitude and phase of the tidal modes are always more complicated than that expected for the (1,l) and (2,2) modes for the diurnal and semi-diurnal tides. It is found that, although the diurnal tide is the strongest, the semi-diurnal and ter-diurnal tides have significant amplitudes. Further, instead of the (1,l) and (2,2) modes expected to be the most significant at the equator, the diurnal (1, - 2) and higher order semidiurnal modes are more prominent. The A0 and SAO of each of the three (24 h, 12 h and 8 h) tidal amplitudes and phases do not involve a single mode. They represent the A0 and SAO of the sum of the different modes of each tidal component present simultaneously. We find that the sum of the tidal wind magnitudes during the year, computed from the amplitude profiles shows a minimum during MayJune and maximum during August-September. Both tides and the larger period A0 and SAO in the mesopause region are global phenomena. The relation between the times of maximum westerly and easterly winds, as the sum of A0 and SAO in monthly mean winds and the amplitude of the tidal winds, may not be unexpected. The role of gravity waves and other global scale atmospheric waves in the generation of A0 and SAO in the upper atmosphere is well recognised. However, actual measurements of the momentum fluxes and their altitude gradients from the different propagating waves are needed for a clearer understanding of the interrelations between the A0 and SAO and shorter period wave phenomena. Larger data bases from different longitudes at low latitudes would be required to clearly characterise the mesospheric wind fields. The most significant feature that emerged out of this study is that the A0 and SAO of monthly mean winds, and of the tidal amplitudes, are significant in the NS wind also, and that the EW wind alone cannot characterise the wind field in the middle atmosphere at low latitudes. In fact, some of the longitudinal differences (Reddy and Raghava Reddi, 1986) found in zonal wind A0 and SAO are difficult to conceive independently without corresponding signatures in the NS wind fields.
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