Interannual variability of ozone and UV-B ultraviolet exposure

Zonal averages of annual and seasonal averages of ozone amounts from Nimbus 7/TOMS (1979-1992) have been examined to estimate the systematic interannual variability of UV-B (390-320 nm) exposure to solar radiation between +/-60 degrees latitude. As shown from statistical modeling, clear-sky interannual UV-B changes can be ascribed mainly to the quasi-biennial oscillation (QBO) driven by stratospheric winds. The QBO oscillations can cause interannual changes in UV-B exposure of +/-15% at 300 nm and +/-5% at 310 nm at the equator and at middle latitudes. In addition to QBO effects, there are larger interannual changes in ozone and UV-B associated with dynamical effects at higher latitudes. When UV-B attenuation from clouds is included, the general latitudinal structure of the interannual variability is maintained. At the equator the interannual variability of ozone amounts and UV exposure caused by the combination of the 2.3 year QBO and annual cycles implies that there is about a 5 year periodicity in UV-B variability caused by dynamical effects. At higher latitudes the appearance of the interannual UV-B maximum is predicted by the QBO but without the regular periodicity. The QBO effects on UV-B irradiance are larger than the long-term changes caused by the decrease in ozone amounts.