scostep 2010 (stp12) - Leibniz-Institut für Atmosphärenphysik an der ...

STP12 Abstracts
Berlin, 12 - 16 July 2010
SCOSTEP Symposium 2010
Solar influences on climate through stratosphere-troposphere coupling
Haigh Joanna
Imperial College London
This presentation will consider two different aspects to how solar activity may influence the
climate of the lower atmosphere.
The first aspect concerns the dynamical coupling processes whereby changes in tropospheric
circulation and temperature are produced in response to solar heating of the stratosphere. A
simplified general circulation model has been used to investigate the chain of
causality. Spinup ensemble experiments have been performed to examine the evolution of the
tropospheric circulation in response to heating perturbations. The study focuses on a
perturbation which is largest in the tropical lower stratosphere, as found previously to be the
solar 11-year cycle signal in observational data, but the mechanisms discussed have wider
relevance for the impact of stratospheric perturbations on the troposphere. The results
demonstrate the importance of changing eddy momentum fluxes in driving the tropospheric
response; it is apparent that feedback between the tropospheric eddy fluxes and tropospheric
circulation changes is required to produce the full model response. The latitudinal distribution
of the stratospheric heating determines the direction of displacement of the tropospheric jets
and storm tracks: heating of lower latitudes produces poleward shifts, as observed in analyses
of the response of zonal winds to solar cycle variability.
The second aspect concerns solar radiative forcing of climate and its dependence on the
spectrum of solar irradiance variability. A 2D (latitude-height) radiative-photochemicaltransport
model is used to investigate the response of the atmosphere to changes in solar
spectral irradiance between 2004 and 2007. The results obtained using spectra measured by
SORCE/SIM are compared with those obtained using data from an established multiparameter
model of spectral irradiance. The latter produces a broad structure of increased
ozone (2004 relative to 2007) with maximum values of around 0.8% near 40km altitude while
the SIM data produce a peak increase of over 2% in low latitudes around 35km along with
significant decreases above 45km. This unanticipated structure is also detected in the profile
of ozone change in contemporaneous measurements of ozone from the MLS instrument on
EOS-Aura. The decreases are due to enhanced destruction by the HOx which results from
increased O( 1 D) from O2 photolysis. Taking into account the stratospheric effects it is found
that with the SIM data solar radiative forcing at the tropopause is out of phase with solar
activity. If this type of spectral variability has occurred over previous solar cycles, or on
longer timescales, an entire revision would be necessary of the attribution of causes to
observed variations in temperature throughout the atmosphere.