Simultaneous observations of the quasi 2-day wave climatology over the low and equatorial latitudes in the mesosphere lower thermosphere

Simultaneous observations of the mesosphere lower thermosphere (MLT) winds in the 80–100 km height region over a low-latitude station Thumba (8.5°N, 77°E) and an equatorial station Kototabang (0.2°S, 100.3°E) during 2006–2012 are used to study the climatology of mean winds and the quasi 2-day waves (QTDW). The 7 year mean annual cycle of the zonal and meridional winds and their interannual variability over both the observational sites are discussed in details. A remarkable agreement is noticed in the annual cycle of zonal winds over both the sites, whereas some discrepancies are observed in the meridional winds. The hourly meridional winds are analyzed to extract the day-to-day variability of the QTDW and their monthly mean. The climatology of the QTDW in the meridional winds over both the sites shows very good agreement, with three peaks in wave activity during austral summer, boreal summer and in the month of October. It is noted that the peak observed in the month of October is not very distant from the boreal summer peak. These three peaks are consistently observed in all years, with strong interannual variability. In general, the austral summer QTDW activity is the strongest among the three over both the observational sites. Apart from discussing the mean and interannual variability of the QTDW over both the sites, instantaneous occurrence of the QTDW over both the sites is further analyzed to infer their zonal wavenumbers. The wavelet analysis is employed to extract time evolution of the QTDW simultaneously over both the sites and the same is used to infer the zonal wavenumbers. The results from the case studies show that the observed QTDW during austral summer (January 2012), boreal summer (July–August 2008) and October month (October 2006) are westward propagating waves with zonal wavenumbers 3, 4 and 2, respectively. Further analysis using simultaneous space based observations are essential for generalizing the observed seasonal variations in the zonal wavenumbers. The significance of the present results lies in using the simultaneous observations of the MLT winds over the low and equatorial latitudes to bring out the climatology, interannual variability and the zonal wavenumbers of the QTDW. The result discussed in the present study adds to the current understanding of the QTDW, especially by inferring the zonal wavenumbers using ground based observations.