Radar observations of the 3.5-day ultra-fast Kelvin wave in the low-latitude mesopause region

Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1241 – 1250

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Radar observations of the 3.5-day ultra-fast Kelvin wave in the low-latitude mesopause region S. Sridharana , S. Gurubarana; ∗ , R. Rajarama; b a Indian

Institute of Geomagnetism, Equatorial Geophysical Research Laboratory, Krishnapuram, Tirunelveli 627 011, India b Indian Institute of Geomagnetism, Colaba, Mumbai 400 005, India

Abstract The medium frequency (MF) radar observations of the 3.5-day ultra-fast Kelvin (UFK) wave in the mesopause region (84 –98 km) over Tirunelveli (8:7◦ N; 77:8◦ E), India for a period of nearly 3 years are reported in the present work. The UFK wave reveals semi-annual variability at heights (∼85 km) where the mesopause semi-annual oscillation (MSAO) in the mean wind peaks. Large-amplitude wave events preferentially occur during the westward :ow regimes of the background wind. Mean eastward winds and their shears are qualitatively shown to be associated with bursts of waves with moderate amplitudes. Vertical wavelength estimates agree with earlier estimates based on satellite temperature retrievals for a wavenumber 1 ultra-fast c 2002 Elsevier Science Ltd. All rights reserved. Kelvin wave.  Keywords: MF radar; Kelvin waves; Semi-annual oscillation; Eastward acceleration; Vertical momentum :ux; Body force per unit mass

1. Introduction Kelvin waves are equatorially trapped eastward propagating waves transporting eastward momentum in the middle atmosphere. They are forced by the latent heat release in large-scale convective clusters in the tropical troposphere (Holton, 1972). In the absence of mean wind shear, the Kelvin waves are expected to have a Gaussian structure with maximum amplitudes in zonal wind, temperature, vertical velocity and pressure at the equator whose amplitudes decay with latitude exponentially. The linear theory predicts no meridional perturbation associated with the wave (Andrews et al., 1987). Matsuno (1966), adopting a numerical approach, showed that this mode is similar to an oceanic Kelvin wave. The shallow water gravity waves in a bounded ocean would propagate parallel to a coastline with no velocity component normal to the coastal boundary (Holton and Lindzen, 1968). Kelvin waves are broadly classiAed into three distinct frequency bands: the slow, fast and ultra-fast modes. The slow ∗ Corresponding author. Tel.: +91-462-579465; fax: +91-462573305. E-mail address: [email protected] (S. Gurubaran).

Kelvin wave was Arst identiAed in the lower stratosphere from radiosonde balloon observations (Wallace and Kousky, 1968). This wave is characterized by wavenumber 1 with an eastward phase speed of 20 –40 ms−1 , wave period of 10 –20 days and vertical wavelength of 10 km. Fast Kelvin waves with a speed of 50 –80 ms−1 were subsequently observed in the upper stratosphere with rocketsondes (Hirota, 1978). They are eastward propagating zonal wavenumber 1 waves with a period near 10 days and a vertical wavelength of 20 km. Satellite remote sensing measurements conArmed the presence of ‘fast’ (FK) and ‘ultrafast’ (UFK) Kelvin waves, the latter having zonal phase speeds exceeding 100 ms−1 and wave period of 3– 4 days (Salby et al., 1984, Lieberman and Riggin, 1997). Recent ground-based radar measurements of the winds in the equatorial mesosphere and lower thermosphere (MLT) region provide further evidences for the presence of UFK waves (Riggin et al., 1997; Kovalam et al., 1999; Yoshida et al., 1999). For more than three decades, the role of equatorial Kelvin waves in producing the zonal mean eastward :ow associated with the stratospheric quasi-biennial oscillation (QBO) and the stratopause and mesopause semi-annual oscillations (SSAO and MSAO, respectively) in the equatorial mean

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S. Sridharan et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1241 – 1250

zonal :ow is being debated (Lindzen and Holton, 1968; Dunkerton, 1979; Hitchman and Leovy, 1988; Hamilton and Mahlmann, 1988; Takahashi and Boville, 1992, to state a few). Though earlier model simulations suggested the possible role of Kelvin waves (Dunkerton, 1979, for example) in the long-term dynamical variability of the middle atmosphere, later studies (Hitchman and Leovy, 1988, for example) provided indications for the Kelvin waves to be apparently too weak to fully drive the eastward phase of the SSAO. Because of the lack of observational evidences for the presence of these waves in the stratosphere and mesosphere in the past, it has not been possible to establish whether the Kelvin waves do provide the requisite momentum forcing for the observed long-period oscillations in the middle atmospheric wind Aeld. Recently, there have been a few studies, both satellite and ground-based, that revealed the characteristics of the Kelvin waves, in particular, the ultra-fast mode, in the upper middle atmosphere (Lieberman and Riggin, 1997; Kovalam et al., 1999; Yoshida et al., 1999). Lieberman and Riggin (1997) used the HRDI data obtained from the UARS mission and concluded that the 3-day Kelvin waves are an important source of eastward momentum in the lower thermosphere but they are likely to be ineNective in the MSAO region near 85 km. The results of Kovalam et al. (1999) based on the medium frequency (MF) radar data obtained from Christmas Island in the Central PaciAc and Pontianak in Indonesia, suggest that the role of Kelvin waves in the mesospheric momentum budget is small. Yoshida et al. (1999), from their meteor wind radar and radiosonde observations in Indonesia, observed a pronounced seasonal variation of the 3-day Kelvin wave. Considering that the wave amplitudes in the Indonesian sector are enhanced twice a year, they suggest a possible interaction between the UFK waves and MSAO and=or SSAO. The present work makes use of the wind observations obtained from the MF radar operated at Tirunelveli (8:7◦ N; 77:8◦ E) in India. The characteristics of the observed UFK waves in the mesopause region over Tirunelveli are examined and the results are discussed in the context of the present understanding of the behaviour of the UFK in the upper middle atmosphere. We will show that the large-amplitude Kelvin waves undergo semi-annual variability at heights where the mesopause SAO is dominant in the mean zonal wind, the preferential occurrence being during the westward phase of the MSAO. Examples are presented to show that the UFK wave events sometimes coincide with regimes of eastward acceleration and time-mean eastward :ow of the background wind suggesting wave momentum deposition.

mode. The system details, mode of operation and the method of wind determination, were described earlier by Vincent and Lesicar (1991) and they are not repeated here. Important results on mean winds and tidal climatologies in the altitude region 84 –98 km were reported earlier (Rajaram and Gurubaran, 1998; Gurubaran and Rajaram, 1999). Results on the variabilities of the mesospheric quasi-2-day planetary wave and their eNects on the equatorial ionospheric current system were presented in a recent work by the authors (Gurubaran et al., 2001). The present study on the 3.5-day UFK wave is based on data collected over a period of nearly 3 years (November 1994 –September 1997). For want of continuous observations during both day and night, the analysis is restricted to altitudes above 80 km. Hourly time series for three groups of heights (data averaged over 82–86, 88–92 and 94 –98 km heights) are generated from the raw measurements. The short gaps of a few hours in the data set are linearly interpolated. The data are subjected to both the standard Fourier technique and the time-domain Altering with Anite impulse response (FIR) Alters (Press et al., 1992). Standard Fourier techniques are capable of yielding the temporal variability of the wave energy. In this approach the power spectral densities (PSD) are calculated for overlapping time segments of data of length 1024 points (hours). Integrating the PSD at frequencies between 0.25 and 0:33 day−1 , which corresponds to the period range 3– 4 days, a description of the spectral behaviour with time is thus made. To compute the wave characteristics, namely, the amplitude, phase and the period, the data after passing through the time-domain Alter are subjected to harmonic analysis with period varied from 72 to 96 h. The wave parameters are determined in the least squares sense.

3. Results In Fig. 1 we present the hourly mean zonal wind data for the height group 88–92 km observed during 6 –28 March

6-28 March 1996 (88-92 km)

Tirunelveli

40

Zonal wind (m/s)

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20 0 -20 -40 -60 Observed Filtered (3-4d)

-80 -100 6

2. Observations and data analysis The wind data utilized in the present work were acquired by the MF radar operated at Tirunelveli in the spaced antenna

9

12

15

18

21

24

27

Day number

Fig. 1. Hourly mean zonal wind data for the height group 88–92 km observed during 6 –28 March 1996 (curves joining the squares) and 3– 4 day band-pass Altered output (smooth, continuous, curve).

S. Sridharan et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1241 – 1250

Power spectra for 10 April - 22 May 1996 14 82-86 km 88-92 km 94-98 km

5.3d

Zonal wind amplitude (m/s)

12 10 3.6d 8 6 4 2 0 0.000

0.005

0.010

0.015

Frequency (hour)

(a)

0.020

0.025

-1

Power spectra for 10 April - 22 May 1996 14

Meridional wind amplitude (m/s)

1996. Curves joining the squares that represent the data points are the observed time series and the smooth, continuous, curve represents the band-pass Altered output. The pass-band chosen was between 3 and 4 days. Large variations in the wind with alternating westward and eastward motions indicate the presence of a wave oscillation with a period of ∼3 days. Though the hourly wind plotted in the Agure suggests this wave to last only a few cycles, the data after having passed through the band-pass Alter reveal oscillations of ∼10 ms−1 amplitude all through the 22-day period of observation. It may be noted that there are quite a few occasions during the 3-year period of interest wherein we notice a pronounced oscillation with a period of ∼3 days. Further evidence for the presence of the near 3-day oscillation in the mesopause region over Tirunelveli is presented in the form of power spectra in Figs. 2a and b. Data for the period 10 April–22 May 1996 were used for this purpose. Peaks at 3.6 days are clearly present in the zonal wind spectra (Fig. 2a) but with variable amplitude for the three height groups. There are peaks near 40 days and at ∼5:3 days. The spectra for the meridional wind (Fig. 2b) do not show any resemblance to the zonal wind spectra. The peaks noticed in the zonal wind spectra appear to be diminished in the meridional wind spectra. The 40-day peak, prominently seen in the zonal wind, falls in the time scales associated with the intraseasonal oscillations (Eckermann et al., 1997, for example). The 5.3-day peak probably corresponds to the 5-day planetary wave as discussed by Wu et al. (1994). With reference to the Figs. 3 and 4 of Wu et al. (1994), it may be noted that the meridional component associated with this wave has amplitudes that are negligible at the equator and therefore is not detectable at the latitude of Tirunelveli. There is only a broad peak near a period of 2 days in the meridional wind spectra, which may correspond to the quasi-2-day planetary wave. The observation that the activity near 3 days in the meridional wind is negligible agrees with the feature expected for a Kelvin wave. Clear examples for the 3.5-day wave propagation in the mesopause region over Tirunelveli are presented in Fig. 3. The time segments considered are for the months of March 1996 and July 1997. All heights between 84 and 98 km at steps of 2 km were considered for this plot. The March event has been more intense with amplitudes of the oscillation exceeding 10 ms−1 . Individual phases, for example, the wave crests as shown by the dashed lines joining them, reveal downward propagation. The average period for these events is estimated to be 3.4 days and the vertical wavelength 49 km. Though the latter agrees with the result of Salby et al. (1984) derived from the Nimbus-7 LIMS data sets, it is less than that estimated by Kovalam et al. (1999). Vertical wavelength estimates from Pontianak range from 70 to 80 km. In Table 1 the 3.5-day wave characteristics for six large-amplitude (¿ 8 ms−1 ) wave events representing 4-day time segments are presented. The estimated wave

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10 8 6 4 2 0 0.000

(b)

82-86 km 88-92 km 94-98 km

12

0.005

0.010

0.015

Frequency (hour)

0.020

0.025

-1

Fig. 2. (a) Zonal wind spectra for the period 10 April–22 May 1996. (b) Same as (a) but for meridional wind.

parameters are for the altitude 90 km. The number of events considered is limited because of smaller wave amplitudes at many altitudes on other occasions. Four events are during the spring of 1996 and 1997 and two are during the summer of the same years. For these events the period of the wave lies in the range 3.4 –3.7 days. The vertical wavelength is estimated by the change of phase with height. Though in general this parameter lies in the range 30 –60 km, the average being 43 km, there has been one occasion during 9 –12 April 1997 (not shown in Table 1) for which the estimated vertical wavelength is ¿100 km.

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3.5-day Kelvin wave over Tirunelveli

3.5d wave propagation in zonal wind (84-98 km) 80

Meridional wind (82-86 km) 2 -2

Variance (m s )

10 m/s

68

70

72

74

76

78

80

82

Day (1996)

40 Zonal wind (82-86 km)

0 N

F

M

A

N

F

1995

A

N

F

1996

M

A

1997

Fig. 4. Temporal variation of the UFK waves in zonal (bottom panel) and meridional (top panel) winds in the ‘area-preserving’ variance-content form for the height group 82–86 km.

10 m/s

196

M

198

200

202

204

206

208

210

Day (1997)

Fig. 3. 3.5-day wave propagation in zonal wind in the mesopause region (84 –98 km) over Tirunelveli for the months of March 1996 (top panel) and July 1997 (bottom panel).

The temporal variation of the UFK waves in zonal and meridional winds for the height group 82–86 km is depicted in Fig. 4. The results are depicted in the ‘area-preserving’ variance-content form. Peaks of wave activity in the zonal wind (bottom panel) are noticed in July–September 1995, March–April 1996, September–October 1996, May–June 1997 and August–September 1997. The temporal variation of the 3.5-day wave activity in the meridional wind is depicted in the top panel of Fig. 4. As expected, the wave energy is small at most of the times except during the months of August and September in 1996 when the activity has nearly the same strength as in zonal wind. Riggin et al. (1997) also observed that the 3-day Kelvin

wave in the zonal wind over Christmas Island was sometimes accompanied by a large meridional component (for example, during February 1993). This indicates that the observed Kelvin wave, on certain occasions, has dispersion characteristics that diNer from a classical Kelvin wave. In order to establish any possible relation with the mean zonal wind, we present in Fig. 5 the comparison for the temporal variation of the 3.5-day wave energy (top panel) with that of the mean wind (bottom panel). Each plot in the top panel is shifted by 40 m2 s−2 . The mean zonal wind reveals the semi-annual oscillation; time-mean eastward motions are observed during solstices and the time-mean westward :ow occurs in spring and autumn. The duration over which the wind is eastward is much shorter than the duration over which the westward :ow is observed. The MSAO weakens above 90 km and there is a time-mean westward :ow at all times above 94 km. We further notice that the westward velocities in spring undergo interannual variability. An important feature to be noticed is that the transition from westward to eastward :ow and vice versa occurs Arst at highest altitudes; lower the altitude, later it occurs. This indicates absorption and momentum deposition of upward propagating waves. All these features were reported earlier for the same location (Rajaram and Gurubaran, 1998).

Table 1 3.5-day Kelvin wave characteristics for selected wave events Day and month

Year

3.5-day wave amplitude at 90 km (m=s)

Period (days)

Vertical wavelength (km)

11–14 March 18–21 March 27–30 April 2–5 June 12–15 March 23–26 July

1996 1996 1996 1996 1997 1997

15.7 16.8 10.6 10.8 10.8 9.0

3.4 3.6 3.7 3.6 3.7 3.5

37.9 56.9 44.3 31.2 40.2 48.7

S. Sridharan et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1241 – 1250

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3.5d wave and mean wind (zonal) 120

2 -2

Variance (m s )

94-98 km

80 88-92 km 40 82-86 km 0 N

F

M

A

N

F

M

A

N

F

M

A

N

F

M A 1995

N

F

M A 1996

N

F

M A 1997

Mean wind (m/s)

20

0

-20

-40

-60

Fig. 5. Comparison for the temporal variation of the 3.5-day wave energy (top panel) with that of the mean wind (bottom panel). Each plot in the top panel is shifted by 40 m2 s−2 .

The UFK wave energy in the lower layer (82 –86 km) tends to appear preferentially during the westward phase of the MSAO. Peaks during September 1995, March 1996, October 1996 and September 1997 coincide with the peak westward zonal :ow. On one occasion during July 1995, the wave is enhanced in the westward shear regions. Yoshida et al. (1999) have observed a similar behaviour for the UFK wave over Indonesia. It may also be noted that the observed Kelvin wave amplitudes are relatively smaller at times of intense mean westward motions in the zonal :ow, for example, during the spring months of 1995 and 1997. Though the conditions for propagation are favourable during these times, the presence of these waves is essentially governed by the strength of the wave excitation processes at lower altitudes. The peaks in the wave activity at intermediate altitudes and above are clearly evident during November–December 1995, April–May 1996 and March 1997. The spring event

in 1996 occurs when the westward :ow turns eastward. The 3.5-day Kelvin wave could have possibly contributed to the eastward acceleration of the mean :ow around this period at higher altitudes as these shear regions propagate down to and below the mesopause region. It may be noted that enhancements of the UFK wave were reported for the same period from the other equatorial sites in the Central PaciAc and Indonesia (Kovalam et al., 1999). Over Pontianak, zonal eastward acceleration as large as 1 ms−1 day−1 was estimated due to the momentum deposition of the Kelvin wave during April–May 1996. As wave amplitudes diminish towards the end of June, the mean wind over Tirunelveli turns westward. In March 1997 the peak occurs when the mean winds were westward. During November–December 1995, the :ow has been (weak) westward above 86 km and (weak) eastward at lower altitudes. Around this time, the wave amplitudes are larger at higher heights (in the height groups 88–92 and 94 –98 km).

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S. Sridharan et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1241 – 1250

Fig. 6. Height-time variation of the diurnal tide amplitude (bottom panel), 3.5-day Kelvin wave amplitude (middle panel) and the 10-day mean zonal wind (top panel) for November 1995 –January 1996.

These features are further examined by following the evolution of the mean zonal :ow during these speciAc periods. Figs. 6 and 7 depict the height-time variation of the 3.5-day Kelvin wave amplitude (middle panel) and the 10-day mean zonal wind (top panel). The dissipation of the (1,1) diurnal tide is expected to contribute to the westward acceleration of the mean zonal :ow (Lindzen, 1981, for example). Taking into account its contributions, the observed height-time variation of the amplitude of the diurnal tide in the zonal wind is plotted in the bottom panel of Figs. 6 and 7. Fig. 6 is for November 1995 –January 1996 whereas Fig. 7 represents the behaviour during May–July 1995. In the top panel the shaded portions represent westward winds and the unshaded portions of the plots represent the eastward :ow regime. The enhanced wave activity centred at day number 320 (in the third week of November 1995) occurred when the mean winds were westward. At 90 km the :ow veloci-

ties are observed to change from −18 ms−1 (westward) to 4 ms−1 (eastward) between day numbers 324 and 345. This eastward acceleration of the mean zonal wind is accompanied by Kelvin wave activity with amplitudes in the range 6 –10 ms−1 . There is another burst of wave activity noticed between day numbers 370 and 380 (5 –15 January). Associated with this event, an eastward acceleration and a time-mean eastward :ow are observed below 90 km. The diurnal tide, for this period, shows enhanced amplitudes centred at day numbers 320, 350 and 370. Except around day number 370 when the amplitude had a maximum at intermediate heights (∼90 km), the other events are characterized by an increase of amplitude with height throughout the sampling region. Whether these enhancements in diurnal tide amplitude are capable of providing the necessary westward momentum forcing on the mean zonal wind is to be examined.

S. Sridharan et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1241 – 1250

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Fig. 7. Same as Fig. 6 but during May–July 1995.

In the next example presented in Fig. 7 we notice time-mean eastward :ow at altitudes below 92 km up to day number 180, except for a brief transition to westward motion around day number 152. Eastward :ow velocities up to 12 ms−1 are observed during the months of May and June. The occurrence of time-mean eastward motions coincides with moderate Kelvin wave activity (of amplitudes in the range 6 –9 ms−1 ). The weak eastward :ow (0 –5 ms−1 ) observed during the end of May and beginning of June (between day numbers 145 and 160) occurs when the Kelvin wave has negligible amplitude. The diurnal tide activity is enhanced between day numbers 170 and 190; amplitude lies in the range 15 –25 ms−1 during this period. The descent of the westward wind regime that begins at 98 km near day number 166 indicates upward wave propagation and momentum deposition. The analysis does not convincingly show that this regime of time-mean westward :ow is caused by the damping of the observed diurnal tide.

In the analysis to be described below, the drag force provided by the UFK waves and the time-mean eastward acceleration generated by this force for the same observation periods discussed above are computed, and the role of the Kelvin wave in the mesospheric momentum budget over the observing site is ascertained. 3.1. Estimation of eastward acceleration rate It is known that dissipating waves exert a body force on the atmosphere through their convergence of the vertical :ux of horizontal momentum which may be written as u w , where   denotes time average,  is the background atmospheric mass density, and u and w are the zonal and vertical wind :uctuations associated with the wave. Since MF radars do not measure reliable vertical velocities, Kovalam et al. (1999) used the polarization relation (r = w =u ) and the dispersion relation for the Kelvin wave as obtained by

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1.0

20

0.5

10

0.0

0

-0.5

-10

-1.0

Mean wind (m/s)

-1

-1

Acceleration (ms day )

Acceleration inferred from 3.5-day Kelvin wave at 90 km Mean wind at 90 km

-20 310

320

330

340

350

360

370

380

390

400 20

0.5

10

0.0

0

-0.5 -1.0 120

-10

Mean wind (m/s)

-1

-1

Acceleration (ms day )

Day number (November 1995 - January 1996) 1.0

-20 130

140

150

160

170

180

190

200

210

Day number (May 1995 - July 1995)

Fig. 8. The body force per unit mass computed over the height range 86 –94 km (curves joining the circles) and the mean wind at 90 km (solid curve) for November 1995 –January 1996 (top panel) and May–July 1995 (bottom panel). The scale corresponding to the mean wind at 90 km is shown on the right side ordinate.

Body force per unit mass contributed by 3.5-day Kelvin wave 1.0 90 km

Acceleration (m/s/day)

Riggin et al. (1997) to compute the eastward acceleration. We used the same approach as of Kovalam et al. (1999) to compute the vertical momentum :ux and the body force per unit mass. The zonal and the vertical wavenumbers are taken to correspond to the wavelengths 40 000 km (wavenumber 1) and 55 km, respectively. The body force per unit mass computed over the height range 86 –94 km for the selected periods in Figs. 6 and 7 is shown in Fig. 8. For comparison, the time-variation of the mean zonal wind at 90 km is plotted. The top panel is for the period November 1995 –January 1996 and the bottom panel is for May 1995 –July 1995. The acceleration inferred from Kelvin wave activity is eastward at nearly all times with the largest eastward acceleration (∼0:5 ms−1 day−1 ) observed near day numbers 325, 340, 170 and 190. These events correspond to the wave bursts with amplitudes of 8–10 ms−1 identiAed in Figs. 6 and 7. The large westward wind near day number 325 decelerates and this eastward acceleration could very well be contributed by the large Kelvin wave drag imposed around this time. In spite of intense diurnal tide activity noticed at 90 km around day number 370, the eastward acceleration enforced by the Kelvin wave might be suQcient to produce time-mean eastward wind during the month of January. In the month of July (between day numbers 180 and 200), the eastward forcing provided by the Kelvin wave has not been suQcient enough to produce net eastward motion of the background atmosphere. It is likely that the diurnal tide

0.5

0.0

-0.5

-1.0 J

F

M

A

M

J

J

A

S

O

N

D

Month (1996)

Fig. 9. Body force per unit mass contributed by 3.5 day Kelvin wave at 90 km for the year 1996.

variation with height and time is such that it generates the required momentum :ux convergence and causes the observed westward acceleration and mean westward :ow between day numbers 180 and 200. Diurnal tide amplitudes reaching 20 ms−1 are indeed observed around this period (refer to Fig. 7). The eNect of the body force due to the Kelvin wave driving appears to be diminished during this observation period. In order to compare the zonal :ow changes associated with the Kelvin wave driving over Tirunelveli with those over the other equatorial site, Pontianak, we depict in Fig. 9

S. Sridharan et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 1241 – 1250

the result at 90 km for the year 1996. For this analysis, the amplitude of the Kelvin wave is computed from the least squares harmonics subjected to 30-day data window at steps of 15 days. As noted in Fig. 9, except during the months of August, October and November, the acceleration is mostly positive (eastward) throughout the year. Largest body force per unit mass and hence the largest eastward acceleration (∼0:5 ms−1 day−1 ) contributed by the Kelvin wave is observed during March. In contrast to this, the largest acceleration (∼1 ms−1 day−1 ) over Pontianak was observed during May (refer to Fig. 10 of Kovalam et al., 1999). 4. Discussion MF radar observations of the 3.5-day UFK waves in the mesopause region (82–98 km) over Tirunelveli are presented in this work. Clear descending phase structures provide evidence for the upward propagating wave at periods between 3.2 and 3.7 days. Largest amplitudes of more than 15 ms−1 were observed during March–April in 1996. The estimated vertical wavelength mostly lies in the range 30 –60 km. The average vertical wavelength for the six selected events presented in Table 1 is 43 km which is close to the value derived from the data sets obtained from an earlier satellite mission (Salby et al., 1984). Temperature retrievals from the Nimbus 7 LIMS satellite radiances yield a UFK wave with phase speeds in the range of 115 –135 ms−1 and a vertical wavelength of 41 km, traversing up to the lower mesosphere. The long-term variability of the UFK waves is examined in the present work. The UFK wave energy at lower altitudes (82–86 km) shows a semi-annual variation with a distinct relationship to the MSAO. The enhancement of wave amplitude tends to occur preferentially during the westward phase of the MSAO and at times during the peak westward :ow of the background wind (for example, the intense wave activity during the autumn of 1995 and during both westward phases in 1996). This behaviour of the UFK wave observed over Tirunelveli is similar to that reported for the Indonesian sector (Yoshida et al., 1999). As the SAO amplitude decreases above 90 km, the semi-annual variation of the UFK wave amplitude disappears. Rather, at these heights, peaks representing intense wave activity recur at time intervals shorter than 6 months as noted in Fig. 5. The semi-annual variation of the UFK waves and its relationship to the MSAO=SSAO need to be examined. The out-of-phase nature of the MSAO and the SSAO is well recognized (Hirota, 1978; Hamilton, 1982; Burrage et al., 1996; Garcia et al., 1997). The Kelvin wave that is observed at 90 km and above has to be ‘swift’ enough to propagate unattenuated in the eastward shear regions of the SSAO. Since the intrinsic phase speeds of the 3.5-day Kelvin waves are large, they are expected to traverse the entire middle atmosphere. There is a time-mean eastward :ow at heights near 110 km as evidenced in recent satellite measurements and the momentum deposition of these UFK waves is con-

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sidered important in this process (Lieberman and Riggin, 1997). The long-term behaviour of the UFK waves at these heights, however, is yet to be documented. Yoshida et al. (1999) made an attempt to conArm the role of UFK waves in causing the MSAO eastward motions similar to the role played by the FK waves in the generation of SSAO eastward :ow. They could only suggest a possible relationship between the UFK wave variability and the MSAO, as the mechanism is largely unknown. Kovalam et al. (1999), from their radar observations at Christmas Island and Pontianak, concluded that the role of UFK waves in providing momentum to the background wind at heights near 85 km where the MSAO peaks is small. In the present work net eastward motion is sometimes observed to coincide with the UFK wave bursts. Though the amplitudes are moderate (6 –10 ms−1 when compared to 12–15 ms−1 during large-amplitude wave events that occur preferentially when the mean wind is westward), the time-mean eastward :ow at these instants of time could very well be caused by the dynamical processes associated with the propagation of the UFK wave. As shown in one of the examples presented in this work, the eNect of the Kelvin wave drag does not look obvious on many other occasions due to the persistent diurnal tide driving that dominates at these latitudes to produce a net westward acceleration. Comparison of the computed Kelvin wave body force per unit mass over a period of a year with other low latitude sites yields signiAcant longitudinal diNerences. The estimated acceleration over Tirunelveli is one-half the value estimated for Pontianak. Kovalam et al. (1999) earlier noted that the Kelvin waves have larger amplitudes in the Indonesian sector than in the central PaciAc. The seasonal maximum in the estimated wave drag over Tirunelveli occurs during the month of March as against May over the Indonesian sector. 5. Conclusion The present work leads to this scenario: the UFK waves undergo semi-annual variation at heights where the mesopause SAO in the mean zonal wind peaks. As suggested by Yoshida et al. (1999), this variability is likely to be caused by the interaction of the wave with the mean wind in the underlying stratopause SAO regime. Large-amplitude UFK waves occur preferentially when the mean winds blow westward and are observed up to the highest altitude (98 km) sampled by the MF radar. These waves can induce the time-mean eastward :ow at altitudes above 105 km as revealed by the satellite data sets. Similar to the earlier reports, the present work does not favour a dominant role for the Kelvin waves in the driving of the low-latitude mesopause region. Acknowledgements The authors thank Professor Gordon Shepherd for inviting one of them (S.G.) to present this work at the PSMOS

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workshop. S.S. thanks the Director, Indian Institute of Geomagnetism, Mumbai, for oNering a research scholarship. Technical support for running the MF radar system is provided by K. Unnikrishnan Nair. This work is supported by the Department of Science and Technology, Government of India.

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