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The annual cycle of zonally averaged column Ar mass mixing ratio (qAr) is shown in

Figure 5.3. The averaged mass mixing ratio of Ar agrees with what was measured by the VLs [Owen et al., 1977]. As expected, enrichment of Ar in terms of its mass mixing ratio is seen

during the growing phase of CO2 frost. After the winter solstices, the seasonal caps start to

retreat and qAr decreases accordingly due to the sublimation of CO2 ice, which dilutes the

non-condensables. As soon as the polar caps are completely gone, terminating the supply of pure CO2 at the surface, further decrease of qAr also halts. Instead, qAr increases because of

the mixing with air from lower latitudes. The annual process is also evident in Figure 5.4, which shows selected latitudinal cross sections derived from Figure 5.3.

We notice that the qAr cycles at different latitudinal bands transit in phase steadily. In

the tropical regions, the cycles at opposite hemispheres do not diverge a lot in phase. Starting from the equator, the higher peak moves to the later part of a year as the sampling location moves to the north; and vice versa, the higher peak moves to the earlier part of a year as the sampling location moves to the south. For instance, in Figure 5.4, the thin blue curve (7.5°N) and the thick dashed blue curve (7.5°S) are very close to each other. The thin red curve (82.5°N) and the thick dashed red curve (82.5°S) are out of phase by roughly 180 degree, which is simply due to the timing of seasons at different poles.

In the equatorial regions, the qAr cycle has an obvious negative correlation with the

surface pressure cycle (see Figure 5.2). The troughs/peaks in the qAr cycles coincide with the

peaks/troughs in the surface pressure cycles. The lower troughs of qAr are evident at the

period when the higher peaks of the surface pressure present. Notice in a hydrostatic model atmosphere, surface pressure is simply the mass of air in a column times the gravity divided

by the surface area. Therefore, this negative correlation between the qAr cycle and the surface

pressure cycle suggests that the total Ar mass in low latitudes is relatively constant.

The peak value of qAr near the south pole is much higher than that near the north pole.

This dichotomy of enrichment between two different poles is the most obvious in Figures 5.3 and 5.4. This dichotomy has been reported by observation teams and simulated using other

GCMs [Nelli et al., 2007; Sprague et al., 2007]. The cause is thought to be the different

behavior of eddy activities at different hemispheres. Stronger pole-ward transient eddies near the south polar region during souther winters help build up the high enrichment. On the other hand, during northern winters, strong stationary waves, transient eddies, and the zonal averaged transports tend to cancel each other. As a result, the total transport in the north pole is very stable and not much enhancement of the Ar can be built up during that period of time [Nelli et al., 2007].

The zonally averaged column Ar mass per unit area (mAr) annual variation is shown

in Figure 5.5. In general, mAr is larger in the northern hemisphere because of the different

atmosphere thickness resulting from the topography. The Hellas basin introduces an increase of Ar mass between 30°S and 60°S. In the tropical region (between 30°N and 30°S for example), the column Ar mass does not change much in a Martian year. At higher latitudes

(60°N/S or more), we see enrichment of mAr in the beginning of the winter seasons, which

indicates transportation of Ar from the lower latitudes to the polar regions. This additional amount of Ar, combined with the decrease of total air mass is responsible for the increase of

qAr. Similarly, a deficit of Ar mass can be found when the surface CO2 frost starts to

sublimate. It suggests an equator-ward flow transporting Ar away from the polar regions. When the sublimation is over, the meridional mixing brings Ar back to the poles and the

cycle starts over. Notice the enrichment dichotomy is less obvious when viewed as total column mass. It is offset by the huge atmospheric mass difference because of the huge elevation difference between the two hemispheres.

We represent our simulation in terms of the enhancement factors (EFs) and compare with calibrated data from GRS [Sprague et al., 2004; Sprague et al., 2007] in Figure 5.6. The EF is essentially the column mass mixing ratio normalized by the reference value given by the VLs. It tells us the relative enrichment of non-condensable gas as to the “standard” VL

value near the equator (the definition of EFs was provided by Nelli et al. [2007]). The

comparison of MarsWRF results and GRS data are presented in Figure 5.6. A similar annual pattern in the southern polar region (black dashed curve in Figure 5.6) can be identified. Both curves dwell above average between Ls 30° to 150°; and the small dip at Ls 70° is evident. The peak EF in MarsWRF outputs, however, is much smaller compared to the GRS record. The maximum EF is almost 10 times as much as the minimum value for the GRS records, while the difference is about 2 to 3 folds in the MarsWRF simulations. The peak EF of GRS is 6 while MarsWRF predicts 1.6 near the south pole, even lower than what has been reported

by the ARC MGCM [Nelli et al., 2007].

This mismatch is seemingly a result of excessively fast meridional mixing of Ar in the simulation for the southern hemisphere, which dilutes the qAr faster than it actually occurs.

The increase of qAr after the complete sublimation of the southern seasonal CO2 cap is much

more abrupt than what the observations show. It suggests that the modeled polar vortex may have disappeared too rapidly, or may have not been strong enough, allowing excessive meridional mixing. In addition, the seasonal variation we found in the northern polar region is not obvious in the data. Even though our absolute values are within the detection error of

the GRS measurement, it seems at least plausible from the data that there may be different mechanisms controlling the dynamics of Ar in different hemispheres.