MF radar observations of 6.5-day wave in the equatorial mesosphere and lower thermosphere

Abstract

Wind measurements made during January 1995 through December 1997 using Partial Reflection Radar located at Tirunelveli (8.7°N, 77.8°E), a tropical station in India, have been used to monitor the long-period planetary wave activity in mesosphere and lower thermosphere. Lomb-scargle periodogram analysis and wavelet techniques are used to study the temporal development of these oscillations and their vertical structure. The observed amplitude and power spectrum show a broad peak near 5–7 days, which corresponds to 6.5-day wave reported earlier. The 6.5-day wave is observed to be an equinoctial phenomenon when background wind is westward, being strong during April/May and September/October months. This shows possible effect of semi annual oscillation on this wave. The 6.5-day oscillation is particularly strong in the zonal wind component, and seems to be present almost continuously in the 84–Full-size image (<1 K) height region. The wave activity is found to maximize during the weak-quasi biennial oscillation year. However, this observed feature need to be examined with a much longer dataset and we have initiated some efforts towards this direction. Mean climatology of these variations observed during three years (1995–1997) has been presented and found that there exists a strong interannual variability. The observed wave activity and its possible source mechanisms are compared and discussed with the available literature.