Measurements of solar UV irradiance performed during the last 18 years at six high-latitude locations and San Diego have revealed large differences of the sites' UV climates. The ozone hole has a large effect on the UV Index at the three Antarctic sites, and to a lesser extent at Ushuaia. UV Indices measured at South Pole during the ozone hole period are on average 20% - 80% larger than measurements at comparable solar elevations during summer months. When the ozone hole passed over Palmer Station late in the year, the UV Index was as high as 14.8 and exceeded the maximum UV Index of 12.0 observed at San Diego. The maximum UV Index at Ushuaia was 11.5, which is comparable with summer-time measurements at San Diego. UV Indices at the two Arctic sites Barrow and Summit are lower than at southern-hemisphere sites as ozone columns are generally larger in the northern hemisphere, and ozone depletion is less severe.
A comparison of UV levels at the network sites revealed that differences between sites depend greatly on the data product used. Average noontime UV Indices at San Diego during summer are considerably larger than at Antarctic sites under ozone-hole conditions, but the difference disappears when daily doses are compared. This contradicts the common notion that UV levels at high latitudes are small because of small solar elevations.
Reconstructions of historical UV Indices based on long-term ozone records and climatological cloud and aerosol patterns indicate that contemporary UV Indices measured during the ozone hole period at Antarctic sites are on average 30% - 85% larger than estimates for the past. These reconstructions were based on the assumption that cloud, albedo, and aerosol conditions have not changed over the last 40 years. Analysis of pyranometer data from South Pole and Barrow indicated that this assumption is justified. Similar long-term observations are not available from other UVSIMN locations, and estimates of historical UV irradiance at those sites are therefore more uncertain. Clearly, the reconstruction of UV levels from proxy data has a larger uncertainty than actual measurements. Operation of the UVSIMN is expected to continue, providing the opportunity to assess future developments of high-latitude UV climate more accurately than in the past. These measurements will help document changes in UV levels due to the expected recovery of the ozone layer (WMO, 2007) and the impact of climate change, which will likely modify stratospheric temperatures; ozone (column and profile); surface albedo (e.g., due to changes in the timing of snow melt (Stone et al., 2002)); clouds (frequency and optical properties), aerosols (e.g., changes in Arctic haze (Bodhaine and Dutton, 1993)), and atmospheric circulation patterns (Knudsen and Anderson, 2001).
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