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.