1. Tropospheric response to Solar and Volcanic forcing
Joanna Haigh, Mike Blackburn and Rebecca Day

2. 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)

3. IPCC radiative forcing
IPCC radiative forcing slide, from CLIVAR website

4. Natural causes of climate change
Explosive volcanoes
Solar activity

5. Observations of total solar irradiance
>2 solar cycles
Absolute values uncertain
~0.08% (1.1Wm-2) variation
C. Frölich, PWDOC

6. 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

7. 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

8. Dynamical Correlations
30hPa geopotential height (Labitzke & van Loon, 1997)
- 4 solar cycles, 10.7cm solar radio-flux
200hPa subtropical temperature (Haigh, 2003)
multiple regression

9. T (200hPa) regressions 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°S
T at 35°S
Haigh (2003)

10. Temperature regressions
NCEP-NCAR reanalysis
shading: <95% significance
trend
ENSO
solar
Volc
QBO
NAO
Haigh (2003)

11. Zonal wind regressions
NCEP-NCAR reanalysis,
volcanic
ENSO
NAO
QBO
[u]
trend
solar
95% significance: u ~ 0.5 ms-1

12. Regressed extremes of zonal wind
solar min
solar max
Jets weaken,
shift poleward
low aerosol
PinaTubo
Jets weaken,
shift eq’ward

13. GCM response to stratospheric UV, O3
[T] regression:
NCEP-NCAR reanalyses
GCM response:
HadAM3 L58
smaller amplitude
Larkin et al (2000)

14. Idealised GCM experiments
IGCM, Held-Suarez forcing:
Newtonian heating; Rayleigh friction (PBL)
 Modify reference state in lower stratosphere
Reference state [ T ]
Climate average [ T ]

15. Control climate Zonal wind [ u ] MMC [ Ψ ] Momentum flux [ u’v’ ]
Heat flux [ v’T’ ]

16. Stratospheric heating experimentsU5
Experiments:
Increase stratospheric reference [ T ]
E5 : 5K * cos2φ
U5 : 5K
P10 : 10K * sin2φ
Effect is to lower and tilt reference tropopause E5
P10

17. Response to stratospheric heating
[u] U5 E5
P10

18. “volcanic” eddy flux response : U5–C
[T]
[u’v’]
[v’T’]

19. “solar” eddy flux response : E5 – C
[T]
[u’v’]
[v’T’]

20. Conclusions Future work
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