Abstracts Prepared by Workshop Speakers - The Effects of Solar Variability on Earth's Climate - page 42

shift in its distribution will have effects on the vertical temperature profile of Earth's atmosphere, and
therefore on various important properties, such as the chemical composition, temperature gradients, and
steering of waves through refraction or reflection. Strong spectral changes in solar output might therefore
be a possible cause of mean circulation changes. The strongest effects would be expected above the
lower atmosphere, but because of vertical coupling, some effects down toward the surface are possible.
This is where the second factor potentially could be helping to substantiate such a solar
modulation of the climate system. Paleoclimate records have long been used to suggest a solar influence
on climate. The problem generally has been that a unifying framework was lacking as to how to interpret
that influence. Very often, the interpretation of a solar influence in the paleoclimate archives was simply
based on statistical analyses, most commonly through quasi-identification of certain frequencies of
variability close to “known solar bands.” Criticism of picking and choosing of records, as well as the lack
of process understanding that would have helped explain mechanistically a solar influence on the records,
separated the paleo community on the Sun-climate connection question. However, as researchers gain
more insight into the various paleoclimate records, it becomes possible to interpret the various time series
within a quantitative geophysical framework, a framework held together by dynamical processes, not
mere correlations. With this approach, new ways to reconstruct multivariate climate fields have emerged.
These approaches allow for a more flexible and comprehensive inclusion of different climate signals that
include seasonally dependent temperature, and moisture, as well as links to regional or large-scale
dynamics such as atmospheric wave structure, coastal upwelling, and even ocean overturning.
Based on such a system-level interpretation of past climate, it becomes possible to analyze both
temporal and spatial changes in light of different climate drivers. This topic offers a fruitful environment
for scientific investigations across the solar physics, climate dynamics, paleoclimate, and climate
modeling disciplines. Although not conclusive, a solar influence on climate can be postulated more
robustly in the arena of indirect effects on large-scale circulation rather than through direct irradiance
alone. At the same time, such a multiscale approach might offer an important evaluation of climate
models in their ability to reproduce changes in variability that are ultimately going to be responsible for
regional climate, just as they have in the past.
Climate Response to the Solar Cycle as Observed in the Stratosphere
Lon L. Hood, Lunar and Planetary Laboratory, University of Arizona
Multiple linear regression (MLR) analyses of satellite-derived stratospheric ozone and
temperature records indicate the existence of significant responses to 11-year solar forcing primarily at
tropical and subtropical latitudes. The observed 11-year variation of ozone and temperature in the
tropical upper stratosphere is attributable to direct photochemical and radiative forcing by solar irradiance
at UV wavelengths, which is mainly responsible for the production of ozone in the stratosphere. In
addition, a significant 11-year variation of ozone and temperature is observed in the tropical and
subtropical lower stratosphere that has a dominantly dynamical origin and is currently not well
understood. The lower stratospheric ozone variation is the principal contributor to the solar-cycle
variation of total (column) ozone. At higher latitudes in the polar upper stratosphere and lower
mesosphere, solar and magnetospheric energetic particle precipitation produces detectable interannual and
decadal changes in ozone, especially in the Southern Hemisphere. Finally, in the polar lower
stratosphere, a nonlinear response to 11-year solar forcing of temperature and geopotential height is
observed with a sign that depends on the phase of the equatorial quasi-biennial wind oscillation (QBO).
The origin of the observed tropical and subtropical lower stratospheric response to 11-year solar
forcing is a topic of current research and has implications for understanding of solar-induced climate
change in the troposphere. Two end-member mechanisms can be identified. First, it is possible that
direct solar (mainly UV) forcing in the upper stratosphere perturbs stratospheric circulation in such a way
as to modify planetary wave propagation and decelerate the mean meridional (Brewer-Dobson)
circulation (BDC) near solar maxima. This “top-down” mechanism would then result in an 11-year