The expansion of the Antarctic polar vortex during the 1990s, both in spatial and temporal extent into early summer, has increased the frequency of elevated UV-B episodes over sub-Antarctic populated areas (Rousseaux et al., 1999). These episodes are no longer just small pockets of ozone depleted stratospheric air coming from the break-up of the polar vortex, but include occasional excursions of the polar vortex edge over Ushuaia, Argentina and Punta Arenas, Chile. This occurred 44 times in the years 1997, 1998, and 2000 combined, with some episodes lasting three to four days. Surface measurements show average erythemal UV increases of about 70% over Ushuaia (54.47°S) since 1997, and episodic total UV-B increases of up to 80% over Punta Arenas (53.08°S) (WMO, 2007).
Diaz et al. (2003) show that Barrow, Alaska, has experienced UV-B increases related to springtime ozone depletion in March and April, but these increases are a factor of ten smaller than those observed at the southern high latitudes. Summertime low-ozone episodes in the Arctic also affect surface UV-B irradiances. These summertime events result from gas-phase chemistry involving nitrogen and hydrogen cycles, which become very efficient during the 24-hour insolation that occurs in the Arctic summer. During summer 2000, two low- ozone episodes brought about erythemal UV increases in order of 10% -15%, each lasting more than five days (WMO, 2007).
The measured amounts of UV irradiance at Palmer Station, Antarctica (64°S) and San Diego, CA (32°N), show that for all seasons, other than spring in Antarctica, there is a decrease in UV-B irradiance caused by the increased path through the atmosphere resulting in less UV-B than observed in San Diego (Fahey, 2007). The Antarctic ozone depletion that occurs each spring causes the UV-B portion of the erythemally-weighted irradiance to increase dramatically to where it exceeds even the summertime values observed in San Diego. Similar wide-area springtime low ozone amounts do not occur in the Arctic region because of the degree of meteorological wave activity in the north that leads to a weaker polar vortex and higher ozone amounts. Southern Hemisphere ozone depletion events have extended further north over Southern Africa, parts of Australia, and New Zealand.
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