Free and forced modes in the Martian atmosphere

Abstract

Short‐period, apparently global oscillations are clearly present in the multiyear time series of surface pressure variations recorded at the two Viking Lander sites on Mars. Previous work has emphasized the contributions of solar‐driven thermal tides to the surface pressure field. Recent analyses of single‐station data on Earth and on Mars suggest that the planetary normal modes (i.e., free modes of planetary scale) may also contribute to the short‐term, seasonally modulated variability in surface pressure. In this paper the periods of free modes in the Martian atmosphere are computed, with due consideration to the short atmospheric radiative damping time and to the seasonal and interannual variation of globally representative temperatures, both of which differ by an order of magnitude from their terrestrial counterparts. The likelihood of atmospheric resonance (i.e., free modes) or of efficient excitation and possibly resonant amplification of forced modes is then discussed. In summary, a thermally forced diurnal Kelvin wave still seems the best explanation for the short‐period, transient mode which appears just prior to or during the episodic great dust storms on Mars. Resonant modes seem most likely to occur during northern spring and summer, as previously suggested by Tillman. However, the limited atmospheric temperature data appear to have the wrong seasonal trend for resonance alone to account for the transient surface pressure oscillations observed in mid to late northern summer at the Viking Landers. One possibility is that during this period, unobserved changes in the atmospheric static stability near the surface have temporarily amplified the resonant modes or have moved the resonant frequency through the frequencies of the solar thermotidal forcing.