Workshop Presentations - The Effects of Solar Variability on Earth's Climate - page 21

stratosphere. The La Niña type of response during solar peak years proposed by Gerald Meehl and Harry
van Loon could affect the production of planetary waves. However, Hood contended that regression
analysis of sea surface temperature data does not show this pattern.

Solar Effects Transmitted by Stratosphere-Troposphere Coupling
Joanna D. Haigh, Imperial College, London

Joanna Haigh focused on the solar effects transmitted from the stratosphere to the troposphere
through a dynamical coupling between the two layers. Solar-cycle signals in observational zonal mean
temperature data show that when the Sun is more active, warming occurs in the tropical lower
stratosphere and in vertical bands passing through the midlatitude troposphere (Figure 2.7). Consistent
with this observation is an increase in the extent of the major meridional overturning (Hadley) cells of the
tropical atmosphere and a slight shift toward the poles of the midlatitude jets. Surface air temperatures
show a pattern in the North Atlantic consistent with the positive phase of the North Atlantic Oscillation.
GCMs simulating solar influence with enhanced UV radiation show similar patterns of response, although
the magnitude depends on the changes in solar spectrum (and implied influence on stratospheric ozone).
Haigh claimed that studies with simpler models show that this pattern of response can be produced
through the effects on wave momentum and heat fluxes of changing the thermal structure around the
tropopause, and through a feedback on the mean state. She noted that recent measurements of the solar
spectrum from the SORCE satellite imply large changes in UV that would reinforce these mechanisms.

Direct Solar Forcing of the Lower Atmosphere and Ocean
Gerald A. Meehl, National Center for Atmospheric Research

Gerald Meehl showed evidence that when observed sea surface temperature data are composited
using only sunspot peak years, the tropical Pacific shows a pronounced La Niña-like pattern, with a
cooling of almost 1 C in the equatorial eastern Pacific. This result has been seen in simulations using

global coupled climate models. Diagnosis of the model results show that both bottom-up (related to air-
sea coupling) and top-down (related to stratospheric ozone) mechanisms are needed to give the correct
amplitude of the observed response (Figure 2.8). According to Meehl, the bottom-up mechanism
involves greater solar heating of the tropical and subtropical ocean in the eastern Pacific for solar
maximum, where there are relatively cloud-free conditions. The evaporated water vapor is transported to
the western Pacific by the trade winds, enhancing the convection there and thereby increasing the strength
of the Walker circulation. The enhanced surface easterlies drive a La Niña-like cold tongue in the eastern
equatorial Pacific from the increased upwelling from the cold water below the thermocline. The signal
appears in the sunspot peak years with dynamical coupled processes working on the timescale of ENSO,
and those coupled dynamics then transition the tropical Pacific to a more El Niño-like pattern in the
several years following the peak solar years. This La Niña-like pattern appears shortly after the rapid
increase in solar activity from solar minimum to solar maximum, and is usually in evidence early in the
broader solar maximum that lasts for several years. Thus, averaged over the several years of solar
maximum, the initial La Niña-like pattern is not seen as strongly, and the El Niño-like pattern is more
evident. The top-down mechanism related to stratospheric ozone also ends up strengthening tropical
convection and precipitation, with the result that the same coupled air-sea dynamics produce responses
similar to that for the bottom-up mechanism. Therefore, Meehl concluded, in the models the top-down
and bottom-up mechanisms reinforce each other and work in the same sense to give measurable signals in
sea surface temperature and precipitation in the tropics, with connections to midlatitude circulation (i.e.,
an anomalous high-pressure region in the North Pacific that extends to parts of North America).

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