Geographical and temporal trends in clear-sky, biologically active ultraviolet-B (UV-B, 280–315 nm) radiation were modelled from 23-yr satellite ozone records across latitudes 22°S to 35°S and longitudes16°E to 34°E in southern Africa. Modelled values were calibrated against broadband and spectral instrument-based measurements and mapped at 2.6-km2 resolution in accordance with the internationally accepted UV-index scale. Statistically significant latitudinal, longitudinal and intra-annual UV-index gradients were evident, the latitudinal gradients especially displaying large seasonal anomalies ranging from a 4.5% increase in UV-index per degree reduction in latitude during winter to a 0.5% increase in summer.
Inter-annual UV-index gradients seemingly corresponded with the 11-yr cycle of extraterrestrial solar activity in which peak UV-indices (up to 13% deviation from 23-yr averages) appeared associated with years of least solar activity, though there were exceptions. These UV-index peaks were most prominent between midwinter and early spring in July, August and September. They also displayed geographical anomalies, the largest deviations in UV-indices mostly apparent at high latitudes and low longitudes. Inter-annual UV-index gradients mostly exceed UV-index increases induced by anthropogenic emissions of ozone-depleting trace gases over the 23-yr assessment period.
The latter increases, though relatively modest (up to 9% between 1979 and 2001), have caused substantial geographical alteration in the distribution of indices across South Africa, with midday clear-sky UV-indices of magnitude 13 and greater corresponding to 72% of the country’s interior in December 2001 compared with only 2% under comparable levels of solar activity in December 1979. Such geographical shifts are expected to intensify with continuing ozone depletion, especially at the next solar cycle minimum anticipated in 2007/08
Musil, C.F., Bodeker, G.E., Scourfield, M.W. and Powrie, L.W., 2003. Geographical and temporal trends in southern African clear-sky, solar ultraviolet-B flux: 1979 to 2001. South African journal of science, 99(9/10), pp.477-483.
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