The effect of the ozone hole is quite pronounced in measurements from the South Pole (Figs. 3.4 and 3.5). There is a strong asymmetry between spring and summer. The daily maximum and 90th percentile peak at the end of November, shortly before the time when the annual ozone hole typically starts to disintegrate (Fig 3.4(a)). The maximum UV Index ever observed was 4.0 and was measured on 30 November 1998.
There is a striking difference between recent measurements and the estimate for historical data. During October and November, recent measurements from the years 1991 - 2007 are on average 55% - 85% larger than in the past (red line in Fig. 3.4(b)). The difference for the 90th percentile is about 95%. Recent data peak at the end of November, but the peak is absent in the historical estimate. Past UV Indices for January and February are 10% - 20% lower than contemporary data. By comparing GMD Dobson total ozone data of the years 1963 -1980 with data of the years 1991 - 2005, we found that the increase in UV radiation for these two months can also be attributed to changes in total ozone. Compared to the earlier period, monthly average total ozone for the years 1991 - 2005 are lower by the following percentages: January: 12%; February: 7%; October: 48%; November: 35%; and December: 19%. Differences for all months are statistically significant.
Figure 3.5 indicates that contemporary measurements taken before the solstice are larger than UV Indices measured after the mid-summer mark (i.e., ratios are larger than one). Not surprisingly, the difference increases with the time from the solstice. On 13 October (70-day mark), the average and 90th percentile are larger by 82% and 102% respectively, than on the conjugate day of 2 March. Maximum UV Indices measured on the two days differ by 140% (grey line in Fig. 3.5). This pattern is very different from the situation prevalent before the development of the ozone hole. Prior to the 1980s, the reconstructed UV Index was smaller before the solstice: the thin red line in Fig. 3.5 is smaller than one up to day 50 from the solstice, and close to one thereafter.
Figures 3.4(a) and 3.5 also indicate that the 10th percentile is lower in spring than summer, both for recent and historical data. This characteristic is a consequence of the natural annual cycle of ozone concentrations caused by the Brewer-Dobson circulation (Holton et al., 1995). This phenomenon leads to a poleward transport of ozone from the tropics during the winter and early spring, resulting in an ozone maximum in spring and a minimum in autumn. During times in the spring when either the South Pole is outside the perimeter of the ozone hole or the ozone hole has already closed for the year, total ozone tends to be larger than in summer. Such situations lead to lower UV Indices in spring relative to summer. In contrast, when the ozone hole is over the South Pole, ozone concentrations are smaller than during the summer, explaining the annual cycle of the 90th percentile.
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