are used to estimate the gravity wave momentum fluxes in the Mesosphere Lower Thermosphere (MLT) region over Trivandrum (8.5°N, 76.9°E), a low-latitude station in India. The radial velocity variances in the 82-98 km height region, which are mainly caused by gravity waves, measured by the meteor radar are used to determine the gravity wave momentum fluxes. Using a novel method proposed by Hocking (2005), altitude profiles of momentum fluxes of short period (less than 2-3 h) gravity waves are estimated for three continuous years. Seasonal variation in the gravity wave momentum fluxes showed semi annual variation with equinoctial maximum and solstitial minimum. By using estimated gravity wave momentum fluxes, an attempt is made to quantify their contribution in driving the Mesospheric Semi Annual Oscillation (MSAO). The mean flow acceleration estimated from the divergence of gravity wave momentum fluxes is compared with the observed mean flow acceleration computed from monthly mean zonal winds during six MSAO cycles over three years. This comparison reveled that the gravity wave contribution toward the westerly phase of MSAO varies from $20-60% while that toward the easterly phase varies from $30-70%. Variations are observed from cycle-to-cycle in the gravity wave forcing toward both phases of MSAO. The significance of the present study lies in estimating the gravity wave momentum fluxes in the MLT region and quantifying their contribution toward the generation of MSAO over the low-latitude for the first time.
Abstract. Tidal activity in the Mesospheric Lower Thermosphere (MLT) region over Trivandrum (8.5 • N, 77 • E) is investigated using the observations from newly installed SKiYMET Meteor Radar. The seasonal variability and vertical propagation characteristics of atmospheric tides in the MLT region are addressed in the present communication.The observations revealed that the diurnal tide is more prominent than the semi/terdiurnal components over this latitude. It is also observed that the amplitudes of meridional components are stronger than that of zonal ones. The amplitude and phase structure shows the vertical propagation of diurnal tides with vertical wavelength of ∼25 km. However, the vertical wavelength of the semidiurnal tide showed considerable variations. The vertical propagation characteristics of the terdiurnal tide showed some indications of their generating mechanisms. The observed features of tidal components are compared with Global Scale Wave Model (GSWM02) values and they showed a similar amplitude and phase structure for diurnal tides. Month-to-month variations in the tidal amplitudes have shown significant seasonal variation. The observed seasonal variation is discussed in light of the variation in tidal forcing and dissipation.
[1] Short-term tidal variabilities in the mesosphere-lower thermosphere (MLT) region using the meteor radar observations over Trivandrum (8.5°N, 77°E) during March 2006 to February 2007 are discussed. Spectral analysis of daily-averaged zonal and meridional winds revealed the presence of planetary waves, with periods ranging from $2 to 30 days in the height region of 82-98 km. The hourly zonal and meridional wind measurements are then used to estimate day-to-day diurnal tide amplitudes in the MLT region for the first time over this latitude. Spectral analysis of zonal and meridional diurnal tide amplitudes revealed tidal modulations at periods ranging from $2 to 30 days. Further, during a strong episode of tidal modulation, which is identified from the wavelet spectra, a group of subsidiary spectral components around the diurnal tide is observed. The presence of subsidiary spectral components around the diurnal tide has provided evidence for nonlinear interaction between tides and planetary waves. To substantiate this result, bispectral analysis is carried out, which showed the nonlinear tide-planetary wave interactions by revealing the presence of secondary waves exactly at the anticipated periods. As the present observational site is very near to dip equator, it is envisaged that the present results will have important implications in interpreting day-to-day variabilities in the equatorial ionosphere.Citation: Kumar, K. K., V. Deepa, M. Antonita, and G. Ramkumar (2008), Meteor radar observations of short-term tidal variabilities in the low-latitude mesosphere-lower thermosphere: Evidence for nonlinear wave-wave interactions,
[1] Continuous medium frequency radar observations of mesosphere lower thermosphere (MLT) region winds during February 2004-May 2005 (486 days) over Tirunelveli (8.7°N, 77.8°E) revealed intraseasonal oscillations (ISOs) in the 82-to 94-km height region. Two distinct oscillations with periods 50-70 and 20-40 days are noticed predominantly in zonal winds. As it is well established that these oscillations are nonstationary and localized in time, wavelet analysis has been employed to study the time evolution of these oscillations. The analysis showed that 50-to 70-day oscillation peaks during June-October and 20-to 40-day oscillation peaks during January-March in the MLT region. To trace back the origin of these oscillations, the tropospheric ISO has been studied for the same period using outgoing long-wave radiation (OLR) observations. The OLR, which is the proxy for convective activity in the lower atmosphere, around the radar site 5-10°N, 70-80°E is used for the present analysis. The wavelet analysis of OLR showed the 50-to 70-day oscillation peaking at the same time as in the MLT region. The shorter period oscillation (20-40 days), which showed its peak during January-March in the MLT region, is not observed in the OLR data during these months. However, the analysis of water vapor, which is the prime candidate for excitation of tides, showed the 20-to 40-day oscillation during the same time as in the MLT region. In the present study, the lower atmospheric convective activity through gravity wave excitation and water vapor through tidal forcing are accounted for the observed ISO in the MLT region. The significance of present results lies in showing the highly coherent oscillations in OLR and MLT region zonal winds. The coupling between the lower and middle atmospheric ISO is extensively discussed in the light of existing mechanisms.
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