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

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Presentation on theme: "Tropospheric response to Solar and Volcanic forcing Joanna Haigh, Mike Blackburn and Rebecca Day."— 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 O 3 Regressed variations in tropospheric climate Modelled response to stratospheric heating (IGCM)

3 IPCC radiative forcing

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: Wm -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 O 3 stratospheric heating downward IR flux into troposphere dynamical impacts on troposphere changes in O 3 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 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)

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

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

12 solar min solar max Jets weaken, shift poleward low aerosol PinaTubo Jets weaken, shift eqward Regressed extremes of zonal wind

13 GCM response to stratospheric UV, O 3 [T] regression: NCEP-NCAR reanalyses Larkin et al (2000) GCM response: HadAM3 L58 smaller amplitude

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 [ uv ] Heat flux [ vT ]

16 Stratospheric heating experiments Experiments: Increase stratospheric reference [ T ] E5 : 5K * cos 2 φ U5 : 5K P10 : 10K * sin 2 φ Effect is to lower and tilt reference tropopause U5 E5 P10

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

18 volcanic eddy flux response : U5–C [T][u] [uv][vT]

19 solar eddy flux response : E5 – C [T][u] [uv][vT]

20 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


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