Post on 28-Mar-2015
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
Conversion TSI to RF: 4 disc-area 0.7 albedo
Sunspot number (grey); Amplitude of sunspot cycle (red); Length of sunspot cycle (black); aa geomagnetic index (green) 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 & Friis- Christensen 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
Multiple regression of zonal mean T (200hPa)
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
35°S
35°N
35°S
T at 35°S
T (200hPa) regressions
Haigh (2003)
Temperature regressions
NCEP-NCAR reanalysis
1979-2000shading: <95% significance
Haigh (2003)
trend
solar
QBO
ENSO
Volc
NAO
Zonal wind regressions
NCEP-NCAR reanalysis,1979-2002
[u]
trend
solar
volcanic
ENSO
NAO
QBO
95% significance: u ~ 0.5 ms-1
solar min
solar max
Jets weaken,shift poleward
low aerosol
PinaTubo
Jets weaken,shift eq’ward
Regressed extremes of zonal wind
GCM response to stratospheric UV, O3
[T] regression:NCEP-NCAR reanalyses
Larkin et al (2000)
GCM response:HadAM3 L58
smaller amplitude
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 ] MMC [ Ψ ]
Momentum flux [ u’v’ ] Heat flux [ v’T’ ]
Stratospheric heating experiments
Experiments:
Increase stratospheric reference [ T ]
E5 : 5K * cos2φ
U5 : 5K
P10 : 10K * sin2φ
Effect is to lower and tilt reference tropopause
U5
E5
P10
Response to stratospheric heating
U5
E5
P10
[T] [u]
“volcanic” eddy flux response : U5–C
[T][u]
[u’v’] [v’T’]
“solar” eddy flux response : E5 – C
[T][u]
[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