3.1 Seasonal variations
Figures 4 and 5 show the seasonal variations of temperature, water vapor and saturation ratio in both hemispheres, extracted from our dataset. The map exhibits the well-known imprint of seasons on temperature and water that far exceeds variations due to latitudinal sampling. The first strong increase of water vapor in the atmosphere corresponds to the global dust storm of 2018 that begun shortly after Ls=193° and which was studied in detail in (Aoki et al., 2019, Fedorova et al., 2020, Belyaev et al., 2021). Both hemispheres show a sharp temperature rise of up to 220 K below 60 km (Fig.4B, 5B). The water mixing ratio in the northern hemisphere exceeds 100 ppmv below 80 km and stays at 50 ppmv up to 100 km (Fig.4C). Such a strong increase in water was never observed again after in MY34 and later during the entire MY35. The second strong increase in water at high altitudes corresponds to Ls=240°-300° and is related to perihelion near southern summer solstice when the Hadley circulation cell brings water from the wetter Southern Hemisphere to the Northern Hemisphere (Houben et al., 1997; Richardson and Wilson, 2002a, 2002b; Montmessin et al., 2007). At that season, southern water exceeds 50 ppmv at all altitudes below 100 km and 200 ppmv below 60 km. In contrast, in the Northern Hemisphere two layers of water were observed: (1) a lower layer containing more than 100 ppmv from the lowest observable altitude up to 40-50 km followed by a sharp decrease to 10-20 ppmv between 50 and 70 km, (2) a higher layer marked by an increase up to 30-50 ppmv from 70 to >90 km. This water layer observed both in MY34 and 35 in the upper atmosphere illustrates the strength of water transport during that part of the year.
A regional “C-storm” occurred in MY34 between Ls 315° and 330°. This type of dust event was studied in detail, combining observations and model to illuminate the water escape in these particular climatic conditions (Stone et al., 2020; Chaffin et al., 2021; Holmes et al., 2021b). The intensity of this event varies from year to year (Kass et al., 2016). Another C-storm in ACS data occurred in MY35 although a little earlier (~10°) in the season where water in the Southern Hemisphere was carried up to 80 km.
Around aphelion, the equatorial region meets its coldest time, leading to a strong decrease of the hygropause and water lines were not observed above 60 km. In the Northern Hemisphere, a water maximum of 200 ppmv was observed below 20-30 km, with hygropause altitude growing from high to low latitudes. Above these altitudes, water sharply decreases down to 1-5 ppmv, which was observed both in MY35 and in MY36 and was well correlated with the decrease in temperature from 200 to 160K (fig.4B,C). In the Southern Hemisphere, water vapor was also observed up to 60 km, but the typical values rarely exceeded 1-3 ppmv except close to the equator where values were found to reach 20 ppm (fig.5C). At that season (Ls=30° to 150°) in the middle latitudes, the atmosphere near 30-40 km is characterized by a warm layer associated with a water abundance of 3-5 ppm. This layer is produced by transport from the Northern Hemisphere. In the cold low atmosphere the temperature does not exceed 160 K and water remains mostly below 1 ppm. Yet water lines are still visible , and a high supersaturation was confidently inferred.