Abstract
Abstract. The influence of the sudden stratospheric warming (SSW) on a quasi-2-day wave (QTDW) with westward zonal wave number 3 (W3) is investigated using the Thermosphere–Ionosphere–Mesosphere Electrodynamics General Circulation Model (TIME-GCM). The summer easterly jet below 90 km is strengthened during an SSW, which results in a larger refractive index and thus more favorable conditions for the propagation of W3. In the winter hemisphere, the Eliassen–Palm (EP) flux diagnostics indicate that the strong instabilities at middle and high latitudes in the mesopause region are important for the amplification of W3, which is weakened during SSW periods due to the deceleration or even reversal of the winter westerly winds. Nonlinear interactions between the W3 and the wave number 1 stationary planetary wave produce QTDW with westward zonal wave number 2 (W2). The meridional wind perturbations of the W2 peak in the equatorial region, while the zonal wind and temperature components maximize at middle latitudes. The EP flux diagnostics indicate that the W2 is capable of propagating upward in both winter and summer hemispheres, whereas the propagation of W3 is mostly confined to the summer hemisphere. This characteristic is likely due to the fact that the phase speed of W2 is larger, and therefore its waveguide has a broader latitudinal extension. The larger phase speed also makes W2 less vulnerable to dissipation and critical layer filtering by the background wind when propagating upward.
Highlights
The westward quasi-2-day wave (QTDW) is a predominant phenomenon in the mesosphere and lower thermosphere region in the summer hemisphere with zonal wave numbers 2, 3, and 4
Our results show that the summer easterlies in the stratosphere and lower mesosphere are strengthened during stratospheric warming (SSW) periods, which results in a larger waveguide and more favorable background conditions for the propagation of wave number 3 (W3)
The westward wave number 3 QTDW was simulated by specifying geopotential height perturbations of 1000 m at the lower model boundary (∼ 30 km) for both the standard W3 run and the SSW runs
Summary
The westward quasi-2-day wave (QTDW) is a predominant phenomenon in the mesosphere and lower thermosphere region in the summer hemisphere with zonal wave numbers 2, 3, and 4. Tunbridge et al (2011) studied the zonal wave numbers of the summertime QTDW with satellite temperature observations from 2004 to 2009 They found that the W2 is amplified mainly during January in the Southern Hemisphere with a maximum amplitude at middle latitudes, which always coincides with the temporal variations of the W3. The horizontal wind observations from the HRDI instrument onboard the UARS satellite showed that the meridional wind perturbations of the W2 maximize in the equatorial region at the mesopause (Riggin et al, 2004) This W2 was suggested to be excited in situ at high altitude, which has little direct connection with the 2-day activities at lower altitudes. Anomalous 2-day wave activities with zonal wave number 2 were observed in the Aura/MLS temperature and line-of-sight wind (Limpasuvan and Wu, 2009), which was suggested to be an unstable mode induced by the strong summer easterly jet during January 2006.
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