Figure 10: Zonal averages of water vapor column abundances from
mid-spring to mid-summer for both hemispheres. The top four panels show
column abundances for all years, the dots are data points averaged in
bins of 2° latitude and 15° Ls, while the curves represent the smoothed
bins. The top row illustrates the synergistic retrievals, while the
middle row shows the corresponding MCD prior column abundances for each
hemisphere. Curves representing the same seasonal period for both
hemispheres have identical colors, with the SH Ls interval listed first.
The two bottom panels compare the synergy and the MCD averages from
Ls=255°-315° for the SH and Ls=75°-135° for the NH, covering the
sublimation season for both hemispheres.
The MCD shows a decreasing trend for all seasons in the extreme high
latitudes poleward of the CIA peak, as expected due to the polar cap
circulation known as the polar cap breeze (Haberle & Jakosky, 1990),
the Martian equivalent to the terrestrial sea breeze. The effect is
expected to be stronger in the NH where the more massive ice cap
generates a larger temperature gradient. In the retrieved synergy data,
the CIA does not always decrease poleward of the cap edge in the SH, and
most noticeably continues to increase even beyond 80°S for observations
during Ls=285°-300°. This could be due to averaging of data from
multiple years (Pankine et al., (2010) reported high interannual
variability of this behavior over the NPC), imperfect coverage of this
region and season, or perhaps a variable polar cap breeze in mid-summer
is not effectively transporting water vapor off the polar cap.
The NH is as expected far wetter than the SH. The CIA increases
monotonically from the equator, and does not remain constant across
large regions, as in the SH. Distinct maxima are visible with decreasing
abundances northward of 80° latitude for all seasonal intervals, in
agreement with the model. The overall maximum is observed at 80°N in the
Ls=105°-120° interval, same as in the SH, and reaches a peak value of 60
pr-μm. The highest column abundance obtained by the MCD is in the
interval Ls=90°-105° and reaches 83 pr-μm. The locations of the CIA
peaks are found just south of the polar cap edge, with a clear
decreasing trend for all seasons in the extreme high latitudes poleward
of the CIA maximum, as expected due to the effects of the polar cap
breeze. The sublimation onset is observed to occur later than what is
predicted from the MCD, where during Ls=60°-75°, the synergy finds a
gradually increasing latitudinal trend with a modest peak at 65°N of
just below 20 pr-μm, while the MCD already estimates a significant
maximum of 30 pr-μm at 70°N.
In the bottom two panels of Figure 10, seasonal averages of the
intervals Ls=255°-315° for the SH and Ls=75°-135° for the NH (covering
the main sublimation period for both hemispheres) are shown to provide
comparisons between the general trends in meridional CIA gradients from
the synergy and MCD. The CIA absolute values are interesting to compare,
but even more so the meridional variation. The summer sublimation
maximum in the MCD is quite easily adjusted by tuning model parameters,
while the change with latitude is subject to convection, transportation
and possible surface exchanges, and not so straightforward to modify to
obtain the desired output. In the south the trends are nearly identical,
with the synergy only yielding slightly smaller average abundances in
the 10°-30°S and 50°-70°S regions. In the north, the MCD deviates from
the synergy most significantly in two places; at 20°N and at 50°N, where
in both instances the MCD gradient distinctly increases with respect to
the synergy. The “double-hump” shape of the CIA is also much more
prominent in the MCD. The difference between the MCD and synergy is
small towards the equator for both hemispheres, which might be
indicative that the influence of the CIA sublimation peak diminishes at
lower latitudes.
Seasonal differences in the PI appear small in the MCD model compared to
observations, as can be seen for all seasons in Figure 11, where all the
curves are more or less stacked on top of each other. In our retrievals
the partitioning exhibits a wave-like behavior in both hemispheres,
oscillating around PI=0.5 in the south and around PI=0.65 in the north.
The shape of the MCD PI curves resemble those of the CIA seasonal
averages, and do not have the same wave-like quality that the synergy
finds. As the synergy yields very stable column abundances, for low/mid
latitudes for all seasons, the partitioning varies greatly, particularly
in the southern mid summer. However, the number of data points in the SH
are far fewer than for the NH, and the averages from this region should
therefore be considered somewhat less precise. This disagreement is also
visible (to a lesser extent) in the NH, indicating that the discord is
likely not purely a result of poor sampling in the south. In the NH
there is a clear tendency for the partitioning to suddenly increase
poleward of 80°N while the total water content decreases. The MCD PI on
the other hand has been steadily increasing from the mid latitudes, and
during late spring the PI even decreases north of 80°N. In the north, no
stable PI gradient is observed as the MCD suggests. The synergy finds a
highly variable PI for all latitudes and seasons, but with no clear
meridional tendency.
These differences between the MCD and synergy are highlighted in the
sublimation season averages for the PI in the two bottom panels of
Figure 11, which clearly show the observed wave-like behavior being
consistently higher than the estimated stable MCD PI. While the MCD
indicates that around 40% of the water column is kept near the surface
at all latitudes and seasons, the synergy finds that number to vary from
40-60%, with local maxima at equator, 50°S and at the pole. This trend
is very similar to what is observed in the north, albeit with a smaller
wave amplitude. The MCD PI here is not as stable as in the south, and
displays a fairly constantly increasing gradient from the mid latitudes
(PI=0.4) towards the north pole (PI=0.75). The synergy finds that the PI
never goes below 0.6, indicating that most of the column is always kept
close to the surface, but can vary rapidly. This leads to the synergy
and MCD finding similar PI values only in the north polar region.