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.