Effects of solar UV variability on the stratosphere

Previously thought to produce only relatively minor changes in ozone concentration, radiative heating, and zonal circulation in the upper stratosphere, solar ultraviolet (UV) variations at wavelengths near 200 nm are increasingly recognized as a significant source of decadal variability throughout the stratosphere. On the time scale of the 27-day solar rotation period, UV variations produce a stratospheric ozone response at low latitudes that agrees approximately with current photochemical model predictions. In addition, statistical studies suggest an unmodeled dynamical component of the 27-day response that extends to the low and middle stratosphere. On the time scale of the 11-year solar cycle, the ozone response derived from available data is characterized by a strong maximum in the upper stratosphere, a negligible response in the middle stratosphere, and a second strong maximum in the tropical lower stratosphere. The 11-year temperature response derived from NCEP/CPC data is characterized by a similar altitude dependence. However, in the middle and upper stratosphere, disagreements exist between analyses of alternate temperature data sets and further work is needed to establish more accurately the 11-year temperature response. In the lower stratosphere, in contrast to most model predictions, relatively large-amplitude, apparent solar cycle variations of geopotential height, ozone, and temperature are observed primarily at tropical and subtropical latitudes. As shown by the original work of Labitzke and van Loon [1988], additional large responses can be detected in the polar winter lower stratosphere if the data are separated according to the phase of the equatorial quasi-biennial wind oscillation. A possible explanation for the unexpectedly large lower stratospheric responses indicated by observational studies is that solar UV forcing in the upper stratosphere may influence the selection of preferred internal circulation modes in the winter stratosphere.