3.2 Aphelion season
Figure 6 presents the latitudinal variations of temperature, water
vapor, saturation ratio, aerosols, and H2O ice
extinction from Ls=0° to 180° for MY35 and MY36. Shortly after the
northern vernal equinox (Ls=0°-30°), temperature, and water are
distributed almost symmetrically about the equator, with a warm region
of 180K between 30°S to 40°N from the surface up to 40 km (Fig.6A). The
picture is very close for both years, despite the difference in latitude
coverage. In the mid-to-high latitudes, warm layers of about 180 K were
observed in both hemispheres at 40-60 km with temperature minima near
the poles at low altitudes named “polar vortex”
(Heavens et
al., 2011). Water vapor also shows a maximum at 40 km with a mixing
ratio of 50 ppmv between 30°S to 40°N, which then sharply decreases to
3-5 ppmv between 40 and 60 km (Fig. 6B). In the mid-latitudes, the
prominent 10 ppmv branch of water at 40-60 km that is visible in both
years is a consequence of the transport of water from the equatorial
region to high latitudes. Below 20 km poleward of 60° in both
hemispheres, water abundances decrease abruptly to the sub-ppm level.
Downwelling water starts to be strongly supersaturated (fig.6C) with
clouds’ formation (fig.6D) in this polar region in MY35 and MY36 in the
South and only in MY35 in the North . In the North, saturation ratios of
only <3 were detected. The difference in this region between
MY35 and 36 is related to the temporal coverage of occultation. In MY35
occultation measurements occurred during the Ls=0°-10° timeframe, but in
MY36 they occurred between 20 and 30° at a time when the atmosphere is
becoming warmer (Fig. 6A). The second supersaturated area is located at
low-to-mid latitudes. There, the lowest supersaturated altitude is found
around 40 km near the equator and at 55-60 km at 40°S and 45°N of
latitude. This saturation layer is located above the cloud formation
region that is lofted below 40 km. The cloud level marks a sharp
decrease in water from 50 ppmv to 3-5 ppm, as a result of condensation.
From equinox to solstice, the ~200 K near-surface
temperature maximum shifts to the middle (Ls=30°-60°) and high
(Ls=60°-90° and 90°-120°) northern latitudes (Fig. 6A). The water
maximum also shifts from 50-80 ppmv below 20 km at latitudes of 20-50°N,
with an increase to the pole to 100-200 ppmv below 20 km at 30-70°N
(Fig. 6B). In these areas, water is always far from saturation. In the
Southern Hemisphere, at Ls=30°-60°, 30-50 ppmv of water is still
observed near the surface in the mid-latitudes. At higher altitude
(~40 km) of the high latitudes, a drier layer of water
of about 3 ppmv persists and is associated with large supersaturation
above 50 km where almost no aerosol is observed (Fig.6 C, D). In the
southern polar latitudes below 20 km, water also begins to be
supersaturated despite the drier sub-ppm conditions 1. At Ls=60-90° and
90-120° from middle southern to middle northern latitudes the water
decreases above the cloud region at 35-40 km from 30-50 ppmv to
<1-2 ppmv . Once again, water above the cloud level layer is
found to be highly supersaturated (Fig. 6 A-C).
Closer to autumn equinox, the Ls=120°-150° and 150°-180° intervals show
a shift of the water peak to the equator again, following the onset of
the equinoctial circulation pattern, characterized by two cells facing
each other around the equator. However, no strong supersaturation was
observed below 20 km in the northern polar region due to warmer
temperatures compared to the southern polar region.