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three-dimensional spatial domains that may change with changing external or internal forcing terms.
Much of the emphasis in climate and especially paleoclimate studies has been on the global mean
response. There will, however, be significant changes as a function of height, local time, latitude,
longitude, and season. The troposphere tends to respond as a whole to changes in radiative forcing owing
to the action of moist convection, and there are coherent patterns to high-latitude response (warming) to
the movement of storm tracks and to precipitation patterns.
Tung also noted that workshop participant Gerald Meehl had shown that there is a huge response
in the equatorial Pacific to variations in solar-cycle flux, cooling of almost 1 degree, which is 10 times
what might be expected from solar heating, at the solar peak years, relative to the average climatology,
and that Meehl could reproduce this finding in global climate models with the mechanism of positive
feedback using clouds. Held pointed out that one of the fundamental questions is whether the Pacific
climate system plays a role in amplifying the response of Earth's climate system to changes in the solar
signal. He expressed his doubts that the Pacific Ocean El NiƱo-Southern Oscillation-like dynamic is
strongly involved in the response to the solar cycle. Tung summarized another mechanism discussed in
Meehl's presentation, the top-down stratosphere ozone mechanism, in which increased levels of radiation
lead to increased ozone heating and ozone production, which modifies the temperature and zonal wind in
the stratosphere, which in turn alters wave propagation. In addition, warming in the tropical lower
stratosphere changes vertical convection in the tropics, and can shift the Hadley circulation and storm
tracks. Some members of the audience and panel felt that answering the question about the effects on the
Pacific climate system may require spectrally resolved measurements of the solar irradiance rather than
just of the TSI. The role of top-down forcing is an evolving research question. Pilewskie proposed that
since this is a multidisciplinary problem, it would be beneficial to establish a cooperative program
between NASA and NSF as has been done previously for specific problems. In addition, he suggested a
standardization of data used for models as well as wide availability of data to the public. Other
participants voiced support for this suggestion as well.
A discussion of the nature of the comparison of model predictions evolved from the review of the
results of the talks. The principal issue is that when model results and observations are compared, three
effects have to be clearly delineated: (1) variation in the TSI, (2) variation in the shape and magnitude of
the spectral distribution of the solar flux, and (3) the time-dependent behavior of changes caused by
aerosol events (e.g., eruptions of volcanoes). The need for coherent and well-defined model
intercomparisons was also stressed.
Held then reviewed the results of session 2 of the workshop, summarizing four themes that he
saw as emerging from the talks: (1) there is a spatially coherent picture of past climate emerging from the
proxy-based studies; (2) there should be a broad-based study of the available proxy data to look at the
amplitude and phase of the deduced solar flux variation; (3) trying in historical studies to separate the
effects of changes in TSI from the effects of volcanoes and greenhouse gases in historical studies is
fraught with difficulty, and the community needs to be very careful in understanding the error budget
associated with this process; and (4) modeling studies are needed of the stratospheric response, the ozone
response to the solar cycle, and how those responses penetrate into the troposphere.
With respect to point 1 the spatially coherent picture emerging from proxy studies was one of
cooler temperatures at high northern latitudes, weaker monsoons, and reduced seasonal movement of the
intertropical convergence zones. Caspar Ammann felt that, when considering the correlation of the
response of different regions to changes in TSI, if the community is interested in looking at a solar
response in the system, the general response of the internal variability must be considered: What is
systematic in how the internal variability might be modulated, either in phase or in amplitude, or even in
some of its spatial expression, through the solar cycle?
Pilewskie summarized the fourth session as consisting of an investigation of the role of energetic
particles of both solar and extrasolar origin, with a contribution from Dan Lubin on the need for
understanding solar spectral variability through observations of other Sun-like stars. In his presentation,
Charles Jackman had focused mostly on solar particles and their influences on the middle atmosphere and
potentially on climate, and Jeffrey Pierce's talk described the influence of galactic cosmic rays on clouds
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