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Tropospheric response to Solar and Volcanic forcing

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Presentation on theme: "Tropospheric response to Solar and Volcanic forcing"— Presentation transcript:

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 experiments
U5 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

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