(of approximately 0.5 K) extending from the surface to the tropopause. Above this, in the lower
stratosphere, greatest warming appears in the tropics. Zonal winds in the lower atmosphere show a solar
cycle response in which the midlatitude jet-streams (and associated storm tracks) move slightly poleward
when the Sun is more active.
The observed patterns in zonal mean temperature and wind can be reproduced qualitatively in
experiments with climate models in which solar UV is increased but with surface temperatures fixed. The
amplitude of the signal is found to be enhanced if ozone concentrations in the stratosphere are allowed to
respond to the increased solar UV. The magnitude of the modeled response, however, is smaller than the
observed response. From this we conclude that UV heating of the stratosphere may make a contribution
to the solar effect on surface climate, and that the magnitude of the UV change and, importantly, its effect
on ozone, are significant in determining the magnitude.
Experiments with simplified GCMs have provided indications of the mechanisms whereby
changes in the thermal structure of the lower stratosphere may influence the atmosphere below. The
deposition of zonal momentum near the tropopause by upward-propagating synoptic-scale waves is
affected by the change in local temperature structure producing zonal accelerations and changes to the
mean meridional circulation of the troposphere. These affect the zonal wind at lower levels and thus the
background flow upon which subsequent wave propagation takes place. This provides a feedback
between the waves and mean flow anomalies that serves to reinforce the initial changes. These results
have a wider application in understanding the climate effects of other stratospheric perturbations (such as
chemical ozone depletion or the injection of volcanic aerosol) and could be important in terms of
assessing the role of human activity in past and future climate, as well as providing a good testbed for
current understanding of atmospheric dynamics.
Over the past few years the Sun has been in a state of very low activity, and measurements from
the SORCE satellite are suggesting that the solar spectrum has been behaving in an unexpected fashion.
In particular, daily measurements by the SIM instrument show a much larger (factor of four to six) decay
at near-UV wavelengths over the latter part of the most recent solar cycle than previously understood. If,
as suggested above, UV heating of the stratosphere makes a contribution to the solar effect on surface
climate, then the larger UV changes shown by SIM would imply a larger role for the stratosphere in
determining the tropospheric response to solar variability.
The Impact of Energetic Particle Precipitation on the Atmosphere
Charles H. Jackman, National Aeronautics and Space Agency Goddard Space Flight Center
Energetic precipitating particles (EPPs) include both solar particles and galactic cosmic rays,
which can influence the atmosphere. Solar particles cause impacts in the polar middle atmosphere,
and galactic cosmic rays create impacts in the lower stratosphere and troposphere.
The solar particles can cause significant constituent changes in the polar mesosphere and
stratosphere (middle atmosphere) during certain periods. Both solar protons and electrons can
influence the polar middle atmosphere through ionization and dissociation processes. Solar EPPs can
enhance HO x (H, OH, HO 2 ) through the formation of positive ions followed by complex ion chemistry
and NO x (N, NO, NO 2 ) through the dissociation of molecular nitrogen.
The solar EPP-created HO x increases can lead to ozone destruction in the mesosphere and
upper stratosphere via several catalytic loss cycles. Such middle atmospheric HO x -caused ozone loss
is rather short-lived due to the relatively short lifetime (hours) of the HO x constituents. The HO x -
caused ozone depletion of greater than 30 percent has been observed during several large solar proton
events (SPEs) in the past 40 years. HO x enhancements due to SPEs were confirmed by observations in
the past solar cycle. A number of modeling studies have been undertaken over this time period that
show predictions of enhanced HO x accompanied by decreased ozone due to energetic particles.
The solar EPP-created NO x family has a longer lifetime than the HO x family and can also lead
to catalytic ozone destruction. EPP-caused enhancements of the NO x family can affect ozone