Seasonal variation of Martian middle atmosphere tides observed by the Mars Climate Sounder

The increased local time coverage retrieved from the Mars Climate Sounder on board the Mars Reconnaissance Orbiter enables the direct extraction of thermal tides in the Mars middle atmosphere. Using temperature profiles from Mars years 30 to 32, we studied the latitudinal and seasonal variations in the tides and stationary planetary waves with zonal wave numbers s=1-3. The amplitude of the migrating diurnal tide exhibits strong semiannual variations in both the equatorial region and the middle latitudes of Southern Hemisphere. The migrating semidiurnal tide (SW2) shows clear semiannual variations in the equatorial region and the middle latitudes of Northern Hemisphere but an annual variation in the Southern Hemisphere. The spatial and temporal correlations between the SW2 amplitude and the density-scaled opacity of both the water ice and dust in the equatorial region may provide a possible explanation for the tidal forcing of SW2. Three Kelvin modes with zonal wave numbers 1-3 (DE1-DE3) have significant seasonal variations in the equatorial region. DE1 appears to have a semiannual variation, whereas DE2 and DE3 have clear annual variations. Herein, for the first time, we have extracted the westward propagating diurnal tide with s=2 and 3 and semidiurnal tide with s=1 in the Mars middle atmosphere using observational data. All three waves have asymmetric latitudinal distributions, which should correspond to their possible excitation source, i.e., nonlinear interactions between stationary planetary waves and migrating tides.