3.3 Perihelion season
Figure 7 shows the same latitudinal variations of temperature, water
vapor, saturation ratio and aerosol extinction as Figure 6, except for
the perihelion season from Ls=180 to 360°, in 30° Ls bins for MY34 and
35.
The Martian year 34 was characterized by the GDS that began at Ls=193°
and vanished around Ls=240°. This event generated a strong interannual
variability between MY34 and MY35 from Ls 180 to 240°. During MY35,
water is distributed from 60°S to 45°N with values of 200 ppmv found in
the South up to 60 km, above which water decreases down to 30 ppmv at 80
km. In the North, these high abundances are only seen up to 40 km. The
atmospheric layer where most of the decrease occurs is well correlated
with aerosol extinction and temperature. This period of time
corresponded to the onset of large supersaturations. Close to the poles,
the 10 to 30 ppmv water layers located at 40-70 km appear correlated
with warm temperatures and are likely the result of advection. Below 40
km, temperatures decrease, and water falls to <1 ppm, still
exhibiting some slight supersaturation. As reported by other studies
(Neary et al.,
2020, Heavens et al.), the MY34 GDS warmed the atmosphere as captured
by ACS in the North (Fig. 7A) where the temperature below 60 km is 30-40
K warmer than in MY35. In the southern hemisphere the region between
10°S-60°S was observed before the GDS whereas the region between
60°S-90°S could only be observed after it started (Fig. 3). This period
is characterized by water observed up to 80 km with values of 200 ppmv,
which sharply decrease to 30-50 ppmv above 100 km (Belyaev et al.,
2021). At this season, water begins to be supersaturated only above
70-80 km where clouds actually form (Fedorova et al., 2020; Luginin et
al., 2020). At Ls=210°-240° the atmosphere is much warmer than in MY35.
Water mixing ratios up to 100 ppmv were observed at high altitudes, that
is above 70 km in high southern latitudes and above 60 km in high
northern latitudes. In both cases, water is again observed to fall
rapidly down to a mixing ratio of 30 ppm. At these high altitudes, water
is supersaturated, yet aerosols are present up to 100 km, indicating
that cloud or aerosol presence was not sufficient to relax the
atmosphere to saturation. In MY35 between 45°S-60°N, 100-200 ppmv of
water prevails below 40-50 km with a decrease to 5-30 ppmv above. Again,
water is strongly supersaturated up to 60 km even in the presence of
aerosols. At 70-50°S, the water maximum is pushed higher, up to 80-100
km, which may actually be the consequence of the set-up of the
solstitial circulation
(Richardson and
Wilson, 2002a, 2002b; Montmessin et al., 2007).
During the southern summer solstice Ls=240-270° and 270-300° the water
distribution is very similar for two years with an asymmetry between
hemispheres also observed by SPICAM/MEx (Fedorova et al., 2021). In the
mid-to-high southern latitudes 200 ppmv are observed up to 60-70 km.
This water top gradually decreases to 40 km in the northern hemisphere
at 60°N. At these altitudes clouds are observed for both years from 60°S
to 60°N. At 30-70°S above 70 km the water abundance sharply decreases to
50-70 ppmv and reaches 100 km and higher. In the northern hemisphere
H2O decreases to 10-30 ppmv between 40 and 80 km and
then increases again, revealing the transport of water from the southern
to the northern hemisphere at high altitudes. Water is strongly
supersaturated during this season for both years. But saturation is much
higher for MY35 compared to MY34 and begins above 40 km near the equator
and above 70 km at high latitudes . Moving to the northern vernal
equinox, circulation is shaping a nearly symmetric picture that is
observed both years. The regional dust storm “C” happened between
Ls=315° and 330°, whose effect can be grasped in the Ls=300-330°
interval in the form of a rise of water mixing ratio and aerosol
extinctions up to 70-80 km in the 45°S-70°S region. In both years,
supersaturation was observed at high altitudes with water mixing ratio
about 30 ppmv. At Ls=330-360° in high southern polar latitudes below 20
km the supersaturation reappears despite the sub-ppmv concentration. A
simple interpretation points to the local cooling of the polar
atmosphere, as water is still present and condensation is unable to keep
up with the fast temperature decrease.