Figure 11: Zonal averages of the vertical partitioning index
for each hemisphere from mid-spring to mid-summer. The top four panels
show data for all years averaged in bins of 2° latitude and 15° Ls as
dots, while the curves represent the smoothed bins. The top row
illustrates the synergistically retrieved partitioning indices, while
the middle row shows the corresponding MCD prior indices. 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.
4.6 Closer look at the North Pole
The northern polar region in spring and summer is of particular interest
as the sublimation of the seasonal NPC is the main source, and thus the
main forcing, of the water cycle. Every summer, as the NPC is exposed to
sunlight and its surface temperature increases, more than one Gigaton of
water vapor is released into the atmosphere (Smith, 2002), spreading
around the entire globe as the seasons unfold. Figure 12 shows the CIA
and the vertical confinement in the form of polar plots to better
visualize the spatial distribution of the observations. Data from all
available years north of 45°N are averaged in seasonal intervals of 30°
of Ls, and on a 1°x7° latitude-longitude grid. The two top rows of
Figure 12 illustrate the CIA, and the two bottom rows show the PI, for
each group the synergy values are followed by the MCD values.
Overall, the MCD predicts higher CIAs than the synergy (as also seen in
Figure 9), with high abundances extending further equatorward, than what
is observed. The sublimation season also appears to be initiated earlier
in the MCD than what is observed, as can be seen by comparing the two
first rows of the first column (Ls=60°-90°) of Figure 12, and was also
shown in the previous section in Figure 10. At 75°N, the MCD finds
column abundances higher than 40 pr-μm, when no observations for this
time and place yield higher CIAs than 30 pr-μm. The situation is
reversed for the vertical partitioning, where the synergy indicates a
stronger near-surface confinement at all latitudes compared to the MCD.
During Ls=90°-120°, the larger sublimation peak of the MCD is likely
contributing to the overestimation of water vapor at mid-latitudes as
seen in Figure 7. This was also shown in Figure 9, where the synergy is
as much as 50% smaller than the MCD during Ls 90°-110°. For the late
summer season 120°-150° Ls, the MCD predicts a high PI confined mainly
to latitudes north of 60°, while the observations show a high PI
reaching the mid-latitudes. Overall, the MCD predicts the largest PI
poleward of 75°N, while the observations indicate that the PI remains
high for all latitudes, albeit more variable (this was further explored
in section 4.5). Still, the largest differences in the vertical
confinement are found in the mid-latitudes and not in the polar regions,
as illustrated in Figure 9.