McMurdo Station

McMurdo is affected by the ozone hole from September until early December (Fig. 3.6(a)). The maximum UV Index was 7.6 and occurred in November; UV Indices measured after the solstice were below 5.5. The 90th percentile also exhibits a distinct maximum in late November. The average UV Index is fairly symmetric within ± 40 days about the solstice, but the 10th percentile is lower in the spring than in the summer. This feature is again a consequence of the Brewer- Dobson circulation. Historical UV Indices for McMurdo were estimated from total ozone measured by TOMS on NASA's Nimbus-7 satellite (McPeters and Labow, 1996). Measurements began in 1978, when the ozone hole had already started to develop. Model calculations were based on the average ozone column measured between 1978 and 1981. The UV irradiance derived from this calculation is therefore likely already larger than UV intensities prevailing in the 1960s. Due to the short reference period, it was not possible to calculate a range of historical UV Indices caused by variations in ozone during the pre-ozone-hole period. The grey range in Fig. 3.6(a) reflects variation by cloudiness only. Cloud effects are considerably larger compared to the South Pole due to optically thicker clouds at the Antarctic coast and lower surface albedo, ranging from 0.84 during winter and early spring to about 0.74 in summer (Bernhard et al., 2006b). Contemporary measurements are significantly above the historical estimate for all months. Figure 3.7(a) shows that the average UV Index for October and November is 30% - 60% larger now than it was historically. The 90th percentile is higher by 70% -110%. The increase in the ratio of contemporary to historical data during September reflects the gradual photochemical loss in stratospheric ozone concentrations as the ozone

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Figure 3.6 Same as Fig. 3.4(a), but for McMurdo (Panel (a)), Palmer (Panel (b)), and Ushuaia (Panel (c))

JASONDJ F M A M .1 Austral month

Figure 3.6 Same as Fig. 3.4(a), but for McMurdo (Panel (a)), Palmer (Panel (b)), and Ushuaia (Panel (c))

(a)
(b)

Month

Figure 3.7 Same as Fig. 3.4(b), but for McMurdo (Panel (a)), Palmer (Panel (b)), and Ushuaia (Panel (c))

Month

Figure 3.7 Same as Fig. 3.4(b), but for McMurdo (Panel (a)), Palmer (Panel (b)), and Ushuaia (Panel (c))

hole forms at the end of winter. For January through March, differences are on the order of 8% -15%. Increased UV levels for these months are a consequence of lower total ozone values during the 1980s and 1990s, also observed for months not directly affected by the ozone hole (WMO, 2007). Approximately the same increase in the UV Index of 5% -15% was observed at all austral network sites during summer months (compare Figs. 3.4(b) and 3.7(a), (b), (c)).

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