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