Seasonal and Interannual Variability of Mesospheric Semidiurnal Tides at mid- and High-Latitudes: Influence of Quasi Biennial Oscillation?

Abstract:

The semi-diurnal tides (SDT) are the dominant component of the dynamical variations in the mid-and high-latitude mesosphere. These tides are known to influence the energy and momentum budget of the ionosphere thermosphere system. The seasonal and climatological studies of these tides are being made since long using systematic radio sounding measurements. Here in this study, meteor radar based horizontal wind observations during 2003-2014 from a high-latitude station, Andenes (69^oN, 16^oE) and during 2008-2014 from a mid-latitude station, Juliusruh (54^oN, 13^oE) are used to study the behavior of mesospheric SDTs. It has been observed that the amplitudes of SDTs are enhanced in both the stations during September equinox in all the years and after the March equinox in some years. Some earlier studies from low-latitudes, where quasi biennial oscillation (QBO) has maximum influence, showed SDT amplitude enhancement in both March and September equinox in all the years. Previous studies attributed such enhancements to be due to changes in the refractive properties of the medium arising from wind and temperature variations. In this study it has been observed that these enhancements in September equinox are below/above mean level for those years in which the low-latitude QBO wind at 50 hPa is westward/eastward (QBOe/QBOw). It is conjectured that a Holton-Tan type mechanism, wherein the equatorial Kelvin waves influence the high- and mid-latitude tidal and planetary waves, is responsible for lower values of the SDT amplitudes during September equinox of the QBOe phase cases. These results will be discussed with supporting evidence from reanalysis data and WACCM model simulations.