The Arctic

To a good approximation the Earth is symmetric about the Equator. One might therefore expect the mechanisms that create the Antarctic ozone hole to create an analogous Arctic ozone hole. Figure 7.17 shows average March high-latitude northern hemisphere column O,. While there is a clear downward trend, with 1997 column ozone ~30% less than 1971-1972, a comparison between Figures 7.2 and 7.17 shows that the ozone loss in the north is less severe than in the southern hemisphere (note that different quantities are plotted in these figures). It should also be pointed

1975 1980 1985 1990 1995

Year

Figure 7.17 Column ozone averaged between 63°N and 90°N observed in March versus year. (After Newman et al. [270]. Figure I.) The solid line is global average tropospheric FI1 (CC(,F) abundance (pptv) (from NOAA CMDL).

1975 1980 1985 1990 1995

Year

Figure 7.17 Column ozone averaged between 63°N and 90°N observed in March versus year. (After Newman et al. [270]. Figure I.) The solid line is global average tropospheric FI1 (CC(,F) abundance (pptv) (from NOAA CMDL).

out that there are significantly higher values of column O, in the northern hemisphere in March than in the southern hemisphere in October. Figure 7.2 shows that in the 1960s and early 1970s, before the formation of the Antarctic ozone hole, typical high-latitude southern hemisphere column O, was 300-350 DU, while comparable northern hemisphere values were 450-500 DU.

The annual loss of O, in the late winter and early spring in the Arctic is due to the same mechanisms that create the Antarctic ozone hole. There are, however, important differences between the two hemispheres, and it is these differences that are responsible for less severe O, loss in the northern hemisphere.

The cooling of the lower and mid-stratosphere during the polar night causes a northern hemisphere polar vortex to be set up just as one is set up in the southern hemisphere [186,271]. However, as mentioned in Chapter 5, there are important differences in the circulation of the hemispheres due to differences in the placement of the continents in the two hemispheres. A result of these differences is that the polar vortex in the northern hemisphere is smaller and warmer than its counterpart in the southern hemisphere. Figure 7.18 shows the area covered by the 195 and 189 K contours in the northern polar vortex during the 1992-1993 winter (area

0 0

Post a comment