[P-PS09] 火星と火星衛星

[PPS09-16] Global free modes in the Martian atmosphere revealed using the EMARS reanalysis data

*阿隅 杏珠¹、関戸 大登¹、佐藤 薫¹ (1.東京大学　理学系研究科　地球惑星科学専攻)

キーワード：火星大気、再解析データ、自由振動

Normal modes are free oscillations like the eigenmodes of a musical instrument. In the real atmosphere, normal modes are sporadically excited, each having a distinct global structure depending on the zonal wavenumber and frequency. Theoretically, it is interpreted as eigenmodes of the Laplace tidal equation derived from a linearized primitive equation. We examined the normal modes in the Martian atmosphere mainly composed of CO₂ using a recently available reanalysis dataset, EMARS. First, the equivalent depth was calculated by solving the vertical structure equation with the global-mean vertical temperature profile from EMARS. The obtained equivalent depth H is 13.8km, which is slightly longer than the 13.7 km obtained assuming the isothermal resting atmosphere at 200 K (Hamilton & Galcia, 1986). Next, we separated the geopotential height fluctuations into equatorially symmetric and antisymmetric components and obtained zonal wavenumber-frequency spectra for four Mars years of MY29-32. The spectra for higher frequencies than 1sol^-1 were analyzed and compared with the theory with H=13.8 km. Results showed that besides the diurnal and its higher harmonic components corresponding to thermal tides, many isolated peaks were detected. These peaks are located at the theoretical eigen frequencies for respective zonal wavenumbers of normal modes, although the discrepancy from the theory tends to be larger at higher frequencies. Similar to the Earth atmosphere, for the equatorially symmetric modes, gravity modes and Kelvin modes are detected, whereas for the equatorially antisymmetric modes, gravity modes and Rossby-gravity modes are observed. There are two notable and unique features for Martian normal modes: One is that Kelvin modes have frequencies close to the diurnal frequency and its higher harmonic. Second, the spectral widths of most peaks are wider compared with those of the Earth. Those broader spectral peaks in the Martian atmosphere may reflect strong seasonal variation of the mean winds. The mean zonal winds on Mars are several times stronger than on Earth. In addition, the seasonal change in the mean wind is more significant on Mars than on Earth, owing to low thermal inertia with no oceans and the larger eccentricity. Such strong mean winds with significant seasonal variation should cause broaden spectral width of the peak even for the normal modes with higher eigen frequencies than 1sol^-1 by the Doppler shift.