Download Solar & Volcanic Impacts on Tropospheric Climate

Tropospheric response to
Solar and Volcanic forcing
Joanna Haigh, Mike Blackburn and Rebecca Day
Outline
• Climate change context
• Observed solar variability
• Amplification of the solar signal – stratospheric O3
• Regressed variations in tropospheric climate
• Modelled response to stratospheric heating (IGCM)
IPCC radiative forcing
Natural causes of climate change
Explosive
volcanoes
Solar activity
Observations
of total solar
irradiance
>2 solar cycles
Absolute values
uncertain
~0.08% (1.1Wm-2)
variation
C. Frölich, PWDOC
http://www.pmodwrc.ch/
Reconstruction using solar indices
Extrapolate an index
which correlates with
TSI over the observed
period
Several indices!
IPCC: change in
radiative forcing since
1750: 0.3  0.2Wm-2
Sunspot number (grey);
Amplitude of sunspot cycle (red);
Length of sunspot cycle (black);
aa geomagnetic index (green)
Conversion TSI to RF:
4 disc-area 0.7 albedo
IPCC TAR
http://www.grida.no/
Amplification of Solar Forcing
• Solar UV and impact on stratospheric O3 (Haigh 1994)
- solar cycle variation ~7% at 200nm (cf 0.08% in TSI)
 absorption by O3  stratospheric heating
 downward IR flux into troposphere
 dynamical impacts on troposphere
 changes in O3
• Modulation of low-level cloud
cover (Svensmark & FriisChristensen 1997)
- assumed mechanism involving
galactic cosmic rays
Dynamical Correlations
• 30hPa geopotential height (Labitzke & van Loon, 1997)
- 4 solar cycles, 10.7cm solar radio-flux
•200hPa subtropical temperature (Haigh, 2003)
- 1979-2000 multiple regression
T (200hPa)
regressions
Multiple regression of
zonal mean T (200hPa)
35°N
35°S
NCEP-NCAR reanalysis
-
solar variability (red)
volcanic aerosol (green)
QBO (cyan)
NAO (blue)
ENSO (black)
trend (straight black line)
amplitude/phase of annual
& semi-annual cycles
Haigh (2003)
35°S
T at 35°S
Temperature
regressions
NCEP-NCAR reanalysis
1979-2000
shading: <95% significance
trend
ENSO
solar
Volc
QBO
NAO
Haigh (2003)
Zonal wind
regressions
NCEP-NCAR reanalysis,
1979-2002
QBO
[u]
volcanic
trend
ENSO
solar
95% significance: u ~ 0.5 ms-1
NAO
Regressed extremes of zonal wind
solar min
solar max
Jets weaken,
shift poleward
low aerosol
PinaTubo
Jets weaken,
shift eq’ward
GCM response to stratospheric UV, O3
[T] regression:
NCEP-NCAR
reanalyses
GCM response:
HadAM3 L58
smaller amplitude
Larkin et al (2000)
Idealised GCM experiments
IGCM, Held-Suarez forcing:
Newtonian heating; Rayleigh friction (PBL)
 Modify reference state in lower stratosphere
Reference state [ T ]
Climate average [ T ]
Control climate
Zonal wind [ u ]
Momentum flux [ u’v’ ]
MMC [ Ψ ]
Heat flux [ v’T’ ]
Stratospheric heating experiments
Experiments:
Increase stratospheric
reference [ T ]
E5 : 5K * cos2φ
U5 : 5K
U5
E5
P10 : 10K * sin2φ
Effect is to lower and tilt
reference tropopause
P10
Response to stratospheric heating
[T]
[u]
U5
E5
P10
“volcanic” eddy flux response : U5–C
[u]
[T]
[u’v’]
[v’T’]
“solar” eddy flux response : E5 – C
[u]
[T]
[u’v’]
[v’T’]
Conclusions
• Modelled responses agree with analysis regressions
• Suggests that dynamical eddy feedbacks dominate
over moist feedbacks in troposphere
Future work
• Causality chain from ensemble spin-up experiments
• Zonally symmetric model to separate eddy feedbacks
from zonally symmetric processes