FIGURE 2.6 Dynamical mapping of persistent medieval anomalies in the Northern Hemisphere winter. SOURCE:
N.E. Graham, C.M. Ammann, D. Fleitmann, K.M. Cobb, and J. Luterbacher, Support for global climate
reorganization during the Medieval Climate Anomaly, Climate Dynamics 37:1217-1245, 2011.
remains dry. Global climate models (GCMs) currently do not reproduce the tropical features seen in
proxy records, giving instead a more uniform warming. One model was able to present an improved
spatial structure of response to medieval solar forcing when the solar flux into the Indian Ocean was
artificially enhanced, producing a small expansion of the zonal overturning circulation of the atmosphere
over the tropical Pacific Ocean (Walker cell) and inducing circumhemispheric circulations. The
mechanisms involved are complex, and it is possible that both stratospheric-tropospheric dynamical
coupling and coupled atmosphere-ocean dynamics are involved.
Climate Response to the Solar Cycle as Observed in the Stratosphere
Lon Hood, University of Arizona
Lon Hood's presentation at the workshop covered the decadal signal in measurements of ozone
mixing ratio in the upper stratosphere. These satellite measurements correlate with the UV variation
associated with the 11-year solar cycle. The cyclic response of ozone in the middle stratosphere is rather
weak, but larger again in the lower stratosphere. The WACCM3 climate model (which extends above the
stratosphere) is able to simulate the upper- and middle-stratosphere signals but not the lower-stratosphere
ones. Hood noted that it is possible that there is a change in the meridional circulation in the stratosphere,
through the interaction with planetary waves, that could bring ozone from above to the lower stratosphere.
The question is what could have produced the decadal variation in the planetary wave driving. There are
both top-down and bottom-up potential mechanisms. The top-down (actually upper to lower stratosphere)
mechanism involves the direct solar heating of the upper stratosphere, altering the circulation in such a
way as to modify the planetary propagation. The bottom-up mechanism has the TSI radiative heating
modifying the planetary wave amplitudes near the surface, which then propagate upward into the