Long-term variability of mean winds in the mesosphere and lower thermosphere at low latitudes

Abstract

Long-term variations of monthly mean zonal and meridional winds in the Mesosphere and Lower Thermosphere (MLT) at low-latitudes are analyzed using four medium frequency (MF) radars and three meteor radars located in the Asia-Oceania region. Radar data taken at close-by latitudes are appended to construct long-term data sets. With this, we have long-term data from five distinct latitudes within ±22° (viz., 22°N, ∼9°N, 0–2°N, 6–7°S and 21°S). The data length varies at different latitudes and spans a maximum of two decades during 1990–2010. The zonal winds show semiannual oscillation (SAO) at all locations with westward (eastward) winds during equinoxes (solstices). The month height pattern of SAO is similar within ±9° and is different at ±22°. The westward winds in the March equinox were enhanced every two or three years during 1990–2002. We define this phenomenon as Mesospheric Quasi-Biennial Enhancement (MQBE). Such signature is not clear after 2002. The meridional winds show annual oscillation (AO), with northward and southward winds during the December and June solstices, respectively. However, the timing at which the wind direction changes does not coincide at all latitudes. The amplitude of the AO is enhanced after 2004 and 2008 at ∼9°N and ∼7°S, respectively. Orthogonal components of SAO and AO are detected with persistent phase relation, which suggests that the zonal and meridional winds are coupled. The meridional winds show long-term trends at latitudes of ∼9°N and ∼6–7°S, but not at other latitudes. The zonal winds do not show significant long-term trends.