The 6.5‐day wave in the mesosphere and lower thermosphere: Evidence for baroclinic/barotropic instability

Abstract

[1] A westward propagating zonal wave number 1 wave with a period near 6.5 days was a prominent feature in the mesosphere and lower thermosphere (MLT) during the 1994 equinoxes. The meridional structure of the wave in the upper stratosphere and the MLT is consistent with the 5-day wave structure predicted by normal mode theory. However, the amplitude increases sharply above 80 km, where the wave exhibits a highly organized baroclinic circulation. The eddy fluxes and the background state suggest that the wave is amplified by instability of the mesospheric winds.

Mean winds, temperatures and the 16- and 5-day planetary waves in the mesosphere and lower thermosphere over Bear Lake Observatory (42° N, 111° W)

“…There is also an oscillation of period ∼16 days and amplitudes of order ∼15 m s −1 and 10 K in December and January, similarly marked on the figure. These periods are consistent with those reported for the "5-day wave" and the "16-day wave", respectively (e.g., Espy and Witt, 1996;Espy et al, 1997;Luo et al, 2002b;Lieberman et al, 2003;Riggin et al, 2006;Day and Mitchell, 2010a,b;Day et al, 2011).…”

“…Salby (1981a) suggested on theoretical grounds that the 16-and 5-day planetary waves are manifestations of the gravest symmetrical wavenumber 1, westward-travelling Rossby wave. The periods of the 16-and 5-day wave has, in fact, been observed to be between about 12-20 days and 4-7 day, respectively (e.g., Williams and Avery, 1992;Luo et al, 2000Luo et al, , 2002aLieberman et al, 2003;Riggin et al, 2006;Belova et al, 2008;Day and Mitchell, 2010a,b). The 16-day wave has been reported to have wind amplitudes of up to about ∼15 m s −1 and temperature amplitudes reaching…”

The 5-8-Day Kelvin and Rossby Waves in the Tropics as Revealed by Ground and Satellite-Based Observations

“…The amplitude increase and phase decrease with increasing altitude appear to be continuous from the lower stratosphere to MLT heights. There have been several observations and theoretical studies on planetary scale waves of having periods of between five and eight days in the mesosphere and lower thermosphere (Wu et al 1994;Meyer and Forbes 1997;Kovalam et al 1999;Talaat et al 2001Talaat et al , 2002Lieberman et al 2003;Liu et al 2004). As in the present study, these waves have, in general, been identified as symmetric Rossby waves.…”

“…This theory is supported by the High Resolution Doppler Imager (HRDI) observational analysis of Lieberman et al (2003). Alternatively, using HRDI observations of winds, temperature and atomic oxygen, Talaat et al (2001) reported a different picture of the 6.5-day wave as an internal, forced oscillation, propagating from the lower atmosphere and having an amplitude growth rate that exceeds that of normal modes below 80 km.…”

“…However, as the wave shows distinct phase variation with height and the change of the 5-day wave period due to Doppler shift would be less than 0.5 days (Geisler and Dickinson 1979), Meyer and Forbes (1997) proposed that the wave could be generated at middle and high latitudes due to baroclinic/barotropic instabilities, which could act as an in situ wave source for the planetary wave. Lieberman et al (2003) provided observational support for the wave as an unstable wave and suggested that the wave was amplified by baroclinic instability. Recent model simulations by Liu et al (2004) have demonstrated the importance of the atmospheric zonal wind in the propagation and amplification or decay of the 6.5-day wave.…”