Time Spectral Analysis of Midlatitude Disturbances in the Martian Atmosphere

Abstract

The nature of the synoptic period variations in the Viking 2 pressure, wind and temperature data is investigated, using time-spectral and cross-spectral analysis, for selected portions of the Mars fall, winter and spring seasons. Estimates of the phase relationships between the highly coherent pressure, wind and temperature oscillations are obtained, and are very similar to those expected for baroclinic waves, and to these obtained from terrestrial surface data. Phase speeds and zonal wavenumbers are inferred by interpreting the pressure and meridional wind variations in terms of eastward traveling, quasi-geostrophic waves. The calculated phase speeds are on the order of 5?15 m s?1, consistent with the baroclinic wave interpretation, while the wavenumbers of the two dominant fall and spring periodicities are approximately 2 and 4 (the smaller value corresponding to the longer period of 6?8 Mars solar days or sols, and the larger to a 3-sol wave). These wavelengths are in general agreement with those expected for baroclinic waves in the martian atmosphere, on the basis of linear baroclinic instability theory and the results of numerical experiments. Only the pressure data from Lander 1 are examined: the synoptic period variations are reduced in amplitude and slightly frequency shifted relative to those at Lander 2. Additionally, some atmospheric opacity data obtained by the Lander 2 cameras is studied and seems to show evidence of synoptic period fluctuations, possibly due to advection of the north polar hood clouds over the Lander 2 site. Possible connections between changes in the atmospheric thermal state and circulation, associated with the global-scale dust storms, and the wave characteristics are discussed, as well as the regularity of the waves.