Seasonal variabilities of low-latitude mesospheric winds

Abstract

Abstract. Observations of mesospheric winds over a period of four years with the partial re¯ection radar at Tirunelveli (8.7°N, 77.8°E), India, are presented in this study. The emphasis is on describing seasonal variabilities in mean zonal and meridional winds in the altitude region 70±98 km. The meridional winds exhibit overall transequatorial¯ow associated with dierential heating in the Northern and Southern Hemispheres. At lower altitudes (70±80 km) the mean zonal winds reveal easterly¯ow during summer and westerly¯ow during winter, as expected from a circulation driven by solar forcing. In the higher altitude regime (80±98 km) and at all altitudes during equinox periods, the mean zonal¯ow is subjected to the semi-annual oscillation (SAO). The interannual variability detected in the occurrence of SAO over Tirunelveli has also been observed in the data sets obtained from the recent UARS satellite mission. Harmonic analysis results over a period of two years indicate the presence of long-period oscillations in the mean zonal wind at speci®c harmonic periods near 240, 150 and 120 days. Results presented in this study are discussed in the context of current understanding of equatorial wave propagation.

Radar observations of long‐term variability of mesosphere and lower thermosphere winds over Tirunelveli (8.7°N, 77.8°E)

“…However, when Garcia et al [1997] examined the relationship with MF radar measured winds for the years 1990–1995, they observed such a relationship did not occur during the year 1990. Earlier, long‐term variability of mesospheric winds over Tirunelveli have been reported for the years 1993–1998 by Rajaram and Gurubaran [1998]. They noted dominant semiannual oscillation prevailing at MLT heights with an interannual variability.…”

“…Monthly mean zonal and meridional winds at MLT heights are computed to study the long‐term variation of background winds. The monthly mean wind and tidal characteristics over Tirunelveli using a few years of data (1993–1999) were earlier reported by Rajaram and Gurubaran [1998], Gurubaran and Rajaram [1999], Sridharan et al [2003], and Gurubaran et al [2005]. The stratospheric QBO winds for the years 1993–2004 up to 1 hPa are taken from the Web site http://www.pa.op.dlr.de/CCMVal/Forcings/qbo_data_ccmval/u_profile_195301-200412.html.…”

Observations of 2-4 day inertia-gravity waves from the equatorial troposphere to theFregion during the sudden stratospheric warming event of 2009

“…In this study, the horizontal component of geomagnetic field variations are used to infer planetary/gravity wave variabilities. The 1.98 MHz medium frequency radar at Tirunelveli has been installed and operated by the Indian Institute of Geomagnetism since November 1992 [ Rajaram and Gurubaran , 1998]. It provides horizontal wind information in the altitude region 68–98 km for every 2 km height interval and 2 min time interval.…”

High-frequency gravity waves observed in the low-latitude mesosphere-lower thermosphere (MLT) region and their possible relationship to lower-atmospheric convection

“…Further operational details of the radars can be found in the work of Briggs [1984] and Vincent and Lesicar [1991]. MF radar mean winds at Tirunelveli have been reported earlier [ Rajaram and Gurubaran , 1998]. In other studies, long‐term observations from Pameungpeuk and Kototabang (0.2°S, 100.3°E) have been used to examine the longitudinal variability of long‐period waves [ Sridharan et al , 2006] and terdiurnal tide [ Venkateswara Rao et al , 2011] in the MLT region.…”