2:45 PM - 3:00 PM
[AAS10-05] Seasonal variation mechanisms of the quasi-2-day wave and 4-day wave based on long-term reanalysis data for the whole middle atmosphere
Keywords:middle atmosphere, normal mode, quasi-2-day wave, 4-day wave
The normal modes for the atmosphere were derived using the classical tidal theory, which assumes a resting isothermal atmosphere. The presence of several normal modes in the real atmosphere with structures slightly different from the classical theory, likely owing to the background condition, was reported by observations (e.g., Hirota and Hirooka, 1984). The quasi-2-day wave (QTDW), corresponding to the zonal wavenumber 3 Rossby-gravity mode, has large amplitudes in the summer mesosphere and small amplitudes in the winter mesosphere (e.g., Wu et al., 1996). Several amplification mechanisms were proposed for the QTDW amplification (e.g., Plumb, 1983; Salby, 1981) but have not been fully understood. Yamazaki et al. (2021) reported that the amplitude of the 4-day wave (4DW), corresponding to the zonal wavenumber 2 first symmetric Rossby mode, is large in the seasons slightly before and after the summer solstice in the mesosphere and lower thermosphere. The purpose of this study is to elucidate the climatology and mechanisms of seasonal variations of QTDW and 4DW in the whole middle atmosphere.
Reanalysis data covering the whole middle atmosphere produced by the data assimilation system JAGUAR-DAS (Koshin et al., 2020; 2022) over 16 years from September 2004 to August 2020, were used. The zonal wavenumber-frequency power spectra were calculated. Isolated spectral peaks corresponding to QTDW and 4DW were detected in the mesosphere and extracted by bandpass filters.
The geopotential height (GPH) amplitude of RG3 is maximized near 30N and 30S in the mesosphere during the solstitial seasons. The maximum amplitude is larger in the summer hemisphere than in the winter hemisphere as is consistent with previous studies. The latitudes of the GPH amplitude maxima accord well with those in the classical theory. In the summer mesosphere, there are the regions where the meridional gradient of the quasi-geostrophic potential vorticity (qy) is negative owing to the strong easterly jet. In the negative qy region near the critical line of QTDW, the positive EP flux divergence (EPFD) due to QTDW is strong. Upward EP fluxes originating from the positive EPFD region is observed. These features suggest that QTDW is excited and/or amplified by barotropic and/or baroclinic (BT/BC) instability in the summer mesosphere.
The GPH amplitude of 4DW is large in the mesosphere slightly before and after the summer solstice as is consistent with the previous study. The seasonal variation mechanism of 4DW was investigated. Different amplification mechanisms were suggested before and after the summer solstice, which are attributed to the different structures of the summer easterly jet during each season. There are many similarities between the amplification mechanisms of 4DW after the summer solstitial period and QTDW during the summer solstitial period. The reason why 4DW GPH amplitude is small in the mesosphere near the solstice was also examined. This is likely because the critical line of 4DW is not located in the negative qy regions in the summer mesosphere, and hence 4DW is not effectively enhanced by BT/BC instability.
Reanalysis data covering the whole middle atmosphere produced by the data assimilation system JAGUAR-DAS (Koshin et al., 2020; 2022) over 16 years from September 2004 to August 2020, were used. The zonal wavenumber-frequency power spectra were calculated. Isolated spectral peaks corresponding to QTDW and 4DW were detected in the mesosphere and extracted by bandpass filters.
The geopotential height (GPH) amplitude of RG3 is maximized near 30N and 30S in the mesosphere during the solstitial seasons. The maximum amplitude is larger in the summer hemisphere than in the winter hemisphere as is consistent with previous studies. The latitudes of the GPH amplitude maxima accord well with those in the classical theory. In the summer mesosphere, there are the regions where the meridional gradient of the quasi-geostrophic potential vorticity (qy) is negative owing to the strong easterly jet. In the negative qy region near the critical line of QTDW, the positive EP flux divergence (EPFD) due to QTDW is strong. Upward EP fluxes originating from the positive EPFD region is observed. These features suggest that QTDW is excited and/or amplified by barotropic and/or baroclinic (BT/BC) instability in the summer mesosphere.
The GPH amplitude of 4DW is large in the mesosphere slightly before and after the summer solstice as is consistent with the previous study. The seasonal variation mechanism of 4DW was investigated. Different amplification mechanisms were suggested before and after the summer solstice, which are attributed to the different structures of the summer easterly jet during each season. There are many similarities between the amplification mechanisms of 4DW after the summer solstitial period and QTDW during the summer solstitial period. The reason why 4DW GPH amplitude is small in the mesosphere near the solstice was also examined. This is likely because the critical line of 4DW is not located in the negative qy regions in the summer mesosphere, and hence 4DW is not effectively enhanced by BT/BC instability.