The role of tides in Mars’ atmosphere Alison F.C. Bridger a Robert M. Haberle b James Schaeffer b DPS, Pasadena, October 2000 a Dept. of Meteorology, San Jose State University, San Jose CA b NASA Ames Research Center, Moffett Field, CA

10/26/2000DPS, Pasadena, October Introduction §Two annual simulations of the NASA Ames Mars General Circulation Model (MGCM) are conducted…one with and one without tides. §We seek to characterize the impact the tides have on the circulation of the atmosphere as it varies through the year. §In particular, we look at the zonally-averaged fields of etc. §We also look at stationary eddy components, and at how these are impacted by the presence/absence of tides. §Each run has the following characteristics: 7.5  latitude by 9  longitude grid, 30 levels (  coordinates), full physics, dust loading assumed with  =0.3 dust opacity. §Results are examined at every 30  of L s (12 “seasons”).

10/26/2000DPS, Pasadena, October Surface pressure tides §In the nominal simulation (with tides), there is full diurnally-varying heating. §Tides are switched off in the second simulation by the use of diurnally- averaged heating. §As a check, we compute the amplitude and phase of the diurnal (D) and semidiurnal (SD) tide in the surface pressure field at the model’s VL1 gridpoint. l Since we have observations here (from Viking), this provides a convenient check on the model tides in the nominal case. §Figure 1 shows the amplitudes of the D and SD tides.

10/26/2000DPS, Pasadena, October Surface pressure tides (2) §With amplitudes computed from five sol-composited data (right panels), we see - as expected - minimal D and SD amplitudes at VL1 throughout the year. §With amplitudes computed on a sol-by-sol basis (left panels), we see surprisingly large amplitudes in the northern winter season (centered on L s 270  ) in the “no tides” simulation (red curves). §In the nominal simulation (blue curves), amplitudes are only weakly modified by going from analysis of single sol data to analysis of composited data (with the largest changes noted in the D tide). §However, in the “no tides” run, there is a large change in computed amplitudes.

10/26/2000DPS, Pasadena, October Surface pressure tides (3) §Together, these results suggest that the frequencies of the dominant modes in northern winter (i.e., of the baroclinic waves) are different in the presence/absence of tides. §It would appear that power from higher frequency baroclinic waves is being aliased into the diurnal signal in the “no tides” case, but not in the “tides” case. This, in turn suggests that the frequency of these waves may be modified by tides. §This is in line with the results of Collins et al (1996, JGR), who discussed the competing roles of higher- and lower-frequency modes in GCM simulations, in some of which tides were excluded. They noted that the period of the highest frequency mode in their simulations decreased somewhat from 2.6 days to 2.2 days when tides were excluded.

10/26/2000DPS, Pasadena, October Zonally-averaged fields §Figure 2 shows fields for the equinox and solstice seasons in the two runs (left panels = nominal, right = no tides), and Figs. 3 and 4 are the same but for and. §For reference, Fig. 5 shows the amplitudes of the D and SD tides in the temperature field at these four seasons. SD amplitudes increase with altitude in the model and peak around equinox. The D tide has a more complicated structure, but it too has maximum amplitudes at equinox. §Westerly jets are weakened and easterly jets are strengthened by the presence of tides. This is true in all 12 seasons examined, so the tides appear to exert a westward drag on the zonally averaged flow throughout the year at all latitudes and altitudes.

10/26/2000DPS, Pasadena, October Zonally-averaged fields (2) §While surface temperatures are comparable in the two runs, the upper atmosphere (above c mb/50 km) is O(10 K) cooler in the presence of tides, and thus the atmosphere is less stable (statically). §In midwinter, a tongue of warm air extends poleward and upward (L s 90  in the southern hemisphere, L s 270  in the north). This feature appears less distinct in the absence of tides. It also develops sooner in the run with tides (not shown). §Streamfunction fields at solstice are quite similar, but with tides the equinox circulations are more intense and extend higher into the atmosphere (especially at L s 360  ).

10/26/2000DPS, Pasadena, October Stationary waves §Figure 6 shows the amplitudes of stationary geopotential wave one, and Fig. 7 shows the corresponding temperature perturbations. §In the absence of tides, southern winter wave one amplitudes are much stronger at most seasons, although at L s 90  the effect is minimal (at L s 30  for example the amplitude is at least doubled (not shown)). §Large wave one amplitudes persist further into spring in the southern hemisphere without tides, so that the L s 180  distributions between the two run are dramatically different. §Profound changes take place in the eddy fields between L s 180  and 210  and between L s 360  and 30  (not shown), and these changes appear enhanced without tides. In fact, without tides, the southern wave one dominates its northern counterpart for seven of the 12 seasons (compared to six seasons when tides are included).

10/26/2000DPS, Pasadena, October Stationary waves (2) §By contrast, northern winter wave one amplitudes are only moderately affected by tides. §Peak amplitudes occur at lower elevations when tides are present. §Corresponding to this, we see enhanced temperature perturbations at L s 90  in the run with tides (Fig. 7). §Further, since the southern wave one persists well into spring, a large amplitude temperature wave one appears at L s 180  when tides are not present. §Stationary wave one midwinter phase distributions appear essentially unchanged in the two cases (not shown), and indicate a gradual westward tilt with height.

10/26/2000DPS, Pasadena, October Stationary waves (3) §The large-amplitude geopotential wave one at L s 180  in the no tides case shows a stronger westward tilt with height than is seen at midwinter, presumably indicative of increased eddy fluxes. The wave in the “tides” case is nearly barotropic (not shown). §Finally, geopotential stationary wave two is modestly stronger in both midwinter hemispheres of the “no tides” case. Wave two temperature perturbations also have higher amplitudes. However, wave two amplitudes are still generally smaller than wave one (e.g., up to O(20 K) for wave one vs up to O(5 K) for wave two).

10/26/2000DPS, Pasadena, October Summary §The sensitivity of the annual circulation of Mars’ atmosphere to the presence of tides is examined via simulations with a comprehensive MGCM. §Tides impose a westward drag on the zonally-averaged zonal wind field throughout the year. §They also cool and destabilize the upper atmosphere. In equinox seasons (when tidal amplitudes are largest), this allows a stronger and deeper overturning meridional circulation to develop (compared to that with no tides). §We note that the strength of this circulation also increases with elevated dust levels (during the appropriate season), and at those times tidal amplitudes also increase. It will be interesting to determine the extent to which tides contribute to this expansion in dusty times.

10/26/2000DPS, Pasadena, October Summary (2) §Stationary waves are modified by tides. In particular, the amplitude of the southern hemisphere winter wave one would be much larger in the absence of tides. §The surface pressure tidal analysis also suggests that baroclinic waves (i.e., eastward-travelling mid-latitude cyclone-scale waves) are modified by tides (at a minimum, their frequencies appear to be modified, as noted previously by Collins et al). §Changes in simulated stationary wave structures are presumably a response to altered background winds…refractive index profiles (not shown) for the two runs are consistent with this conclusion. §The mechanism(s) by which the tides modify these various aspects of the circulation will be further examined.