Workshop Presentations - The Effects of Solar Variability on Earth's Climate - page 19

MECHANISMS FOR SUN-CLIMATE CONNECTIONS
Mechanisms proposed to explain Earth's climate response to solar variability can be grouped into
three broad categories involving the response to variations in total solar irradiance, UV irradiance, and
corpuscular radiation. The following talks at the workshop outlined the current rationale for considering
how these stimuli might lead to significant responses by the climate system.
Issues in Climate Science Underlying Sun/Climate Research
Isaac M. Held, National Oceanic and Atmospheric Administration
Geophysical Fluid Dynamics Laboratory
In his presentation Isaac Held asserted that the response of the climate to radiative heating—
whether it comes from greenhouse gases trapping heat, stratospheric aerosols from volcanic eruptions or
aerosols of various origin reflecting sunlight back to space, or finally variable TSI heating—involves both
the troposphere and the ocean. The surface and the troposphere are intimately coupled through fast
radiative-convective adjustments so that they respond as a whole, with part of the heat input going into
the ocean. The ocean heat uptake and later slow release back to the atmosphere are the factors
responsible for the difference between the transient response of the climate to radiative forcing as
compared to the equilibrium climate (some 40-70 percent of the adjustment is achieved on a timescale on
the order of 4 years, whereas equilibration takes centuries). This transient behavior can be demonstrated
using a simple two-box model of the mixed layer and deep ocean, and it applies to all radiative forcings,
such as to the Mount Pinatubo volcanic aerosols, as well as for the response to the 11-year solar cycle.
On stratosphere-troposphere coupling, there is recent observational evidence that in the Southern
Hemisphere the surface westerlies (and the storm track) have shifted poleward by a few degrees due
possibly to the ozone hole over the South Pole in the stratosphere.
Held summarized work on this issue, focusing on a potential mechanism that employs the fact
that cooling in the polar stratosphere associated with the loss of ozone increases the horizontal
temperature gradient near the tropopause. Strengthening the horizontal temperature gradient alters in turn
the fluxes of angular momentum by midlatitude eddies. The angular momentum budget of the
troposphere controls the surface westerlies. This mechanism could work with volcanic aerosol warming
or greenhouse gas cooling of the stratosphere, as well as for the solar warming of the lower stratosphere
through UV absorption by ozone. Held noted that it is more difficult for perturbations to the middle
stratosphere to engage this kind of mechanism. Other dynamics would be needed to communicate signals
from the middle to the lower stratospheric regions capable of influencing Earth's angular momentum
budget significantly.
Indirect Climate Effects of the Sun Through Modulation of the Mean Circulation Structure
Caspar Ammann, National Center for Atmospheric Research
Caspar Amman emphasized the indirect climate effects of the Sun. He suggested that solar
heating appears to be plausible as a radiative driver for the medieval warm period (approximately A.D.
900-1250) (Figure 2.6). When Earth's radiative balance is altered, as in the case of a change in the solar-
cycle forcing, not all locations are affected equally. He argued that although the global mean temperature
change may be small, regional signatures in moisture, pressure, and temperature offer a consistent picture
as revealed by proxy records. The equatorial central Pacific is generally colder, the runoff from rivers in
Peru is reduced, and drier conditions affect the western United States. The western tropical Pacific is
warmer, with a high-pressure system in the northwestern Pacific steering storm tracks further north,
bringing moisture to Alaska and warming the interior of northern continents. The storm tracks drift to
northern Europe, with moisture deposited in the northern part of Scandinavia although the Mediterranean