Thermal Response of the Tropical Tropopause Region to Short-Term Solar Ultraviolet Variations


Author:  Lon L. Hood
Affiliation: Lunar and Planetary Laboratory, University of Arizona


It has been previously known that a quasi-decadal oscillation (QDO) of the tropical and subtropical lower stratosphere exists and that this long-term variation has been approximately in phase with the 11-year solar cycle during the period for which accurately calibrated data are available.  Evidence for the QDO is seen in both long-term column ozone records (e.g., TOMS/SBUV) and meteorological records (e.g., NCEP reanalysis temperature data).  However, a solar cause of the QDO has been controversial in part because of the small number (approximately 4) of solar cycles for which global calibrated data are available.  Empirical attribution of causal mechanisms has also been difficult because a variety of solar-correlated phenomena (total solar irradiance, Galactic cosmic ray flux, solar UV spectral irradiance, etc.) all vary with the solar cycle.  Here, we report correlative evidence for thermal responses of the tropical upper troposphere and lower stratosphere to solar UV variations occurring on the time scale of the solar rotation period.  While small in amplitude, the reality of the detected responses can be verified because of the large number of 27-day cycles available for analysis during two successive solar maximum periods.  Also, because short-term variations of solar UV flux, total solar irradiance, and Galactic cosmic ray flux differ significantly on the solar rotation time scale, it is possible to specifically identify the UV flux as the solar-correlated quantity that is most probably responsible for the observed responses.  It is expected that detailed studies of the observed effects of short-term solar forcing in the tropical lower stratosphere and upper troposphere will result in a better understanding of the origin of the observed QDO.