Figure 7.18 Area covered by temperatures less than 195 and 189 K at 465 K potential temperature (-20 km) in the northern hemisphere. Temperatures are from the UKMO assimilation system.

covered by the 185 K contour is negligible). Comparison with Figure 7.4 shows that the chlorine activation (~195 K) and ice-water frost point (~189 K) thresholds cover a far smaller area in the northern than in the southern hemisphere.

Additionally, meteorological conditions of the Arctic exhibit significant year-to-year variability [272-274]. This variability is far greater in most vortex metrics— size, temperature, wind speed at vortex edge, etc.—than is seen in the southern hemisphere.

The descent of air in the northern polar vortex is plotted in Figure 7.19. Comparison with Figure 7.7 shows that total descent during the winter and early

Figure 7.19 Potential temperature versus lime for air parcels initialized at various potential temperatures in the Arctic polar vortex in 1988-1989. The lines show the evolution of potential temperature of individual parcels. (After Rosenfield et at. 1224).)

spring is much greater in the northern polar vortex than in the southern polar vortex. This is consistent with higher vortex temperatures and therefore higher rates of radiative cooling in the north. Of special interest is the fact that descent in the vortex occurs throughout the northern hemisphere winter and into spring. In the southern hemisphere, descent has essentially stopped by mid-winter. As we will discuss later in this chapter, this makes it much more difficult to quantify O, loss in the northern polar vortex.

7.2.2 Polar processing

Polar stratospheric clouds Temperatures below the 196 K chlorine activation threshold occur every year in the Arctic, leading to chlorine activation throughout the polar vortex [249,275 j. The time period over which these temperatures exist, however, is generally shorter than in the southern hemisphere (compare Figures 7.18 and 7.4).

Denitrification The cold temperatures of the wintertime northern polar vortex lead to denitrification [252,255], just as they do in the southern hemisphere. However, the warmer temperatures of the northern hemisphere vortex result in far less denitrification than in the southern hemisphere. Figure 7.20 shows a time series of HNO,

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