Solar Influence on Stratosphere-Troposphere Dynamical Coupling Mike Blackburn (1), Joanna D. Haigh (2), Isla Simpson (2), Sarah Sparrow (1,2) (1) Department.

Solar Influence on Stratosphere-Troposphere

Dynamical Coupling

Mike Blackburn(1), Joanna D. Haigh(2), Isla Simpson(2), Sarah Sparrow(1,2)

(1) Department of Meteorology, University of Reading, UK

(2) Space and Atmospheric Physics, Imperial College London, UK

Earth Simulator Center, 12 November 2007

Outline

• Introduction - influence of the 11-year solar cycle on climate

• Observed atmospheric variability - regressions

• Model experiments to investigate how the tropospheric response over the 11-year cycle could be produced by a dynamical response to stratospheric heating

equilibrium response to heating

spin-up ensembles - mechanisms

• Comparison of two different stratospheric heating perturbation cases

• Relationship to internal annular variability

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 albedoSunspot number (grey)

Amplitude of sunspot cycle (red)Length of sunspot cycle (black)aa geomagnetic index (green) IPCC TAR

http://www.grida.no/

Proposed Amplification Mechanisms

• 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

Solar index regressions using reanalysis data

Crooks & Gray (2005)ECMWF reanalyses 1979-2001 (ERA-40)

Observed stratospheric temperature signal

solar max - solar min

Temperature changes over the 11-year cycle

• Non-uniform.• Increase of ~1K in equatorial stratosphere, decreasing

towards the poles.• Banded increase in temperature in mid-latitudes.

Figure: Haigh (2003)

Multiple regression analysis of NCEP/NCAR reanalysis, 1979-2000

Circulation changes over the 11-year cycle

• Weakening and poleward shift of the mid-latitude jets.• Weakening and expansion of the Hadley cells.• Poleward shift of the Ferrell cells.

Haigh and Blackburn (2006)

Multiple regression analysis of NCEP/NCAR reanalysis, DJF, 1979-2002

GCM simulation (UGAMP GCM)

(a) Control run

(b) Difference between solar maximum and solar minimum:

Total solar irradiance Stratospheric ozone

Similar response found using the Met Office model

GCM response to solar irradiance & ozone (DJF)

Haigh, Science (1996); QJRMS (1999)

The Hypothesis

Are the tropospheric changes observed over the 11-year solar cycle a response to perturbations in the tropical (lower) stratosphere, which are a response to enhanced UV absorption at solar maximum?

Investigate using idealised stratospheric heating experiments in a simplified atmospheric GCM:

Can we reproduce the tropospheric response?

What (dynamical) mechanisms are involved?

Simplified GCM - “dynamical core” model

Based on University of Reading primitive equation model: (1)

• Spectral dynamics: T42 L15• No orography• Newtonian cooling – idealised equinoctial radiative-

convective equilibrium temperatures Te(lat,height) (2)

• Boundary layer Rayleigh friction

Experiments:

1. Equilibrium response to perturbations to stratospheric Te(Haigh, Blackburn & Day, J.Clim., 2005)

2. Spin-up ensembles200 x 50-day run

(1) Hoskins & Simmons (1975)

(2) Held & Suarez (1994)

Comparing Uniform and Equatorial Heating

5K

0K

5K

Equatorial heating (5K) (E5)

Uniform heating (5K) (U5)

Weakening and poleward jet shift?

How does the tropospheric response depend on the heating distribution?

(b)

(c)

(a)

Simplified GCM equilibrium response

zonal wind (ms-1)

Run C

U5 - C E5 - C

Vertically-integrated budget of zonal momentum

0

1 s

ZONAL E S

p

DDYu dp Cg t

C

• Haigh et al (2005) - Equatorial heating gave a similar tropospheric response to that seen over the Solar cycle

• Coherent displacement of the jet and storm-track

• How does this arise?

• Spin-up ensemble for the equatorial heating case:

– 200, 50-day runs

Ensemble spin-up Experiments:

5K 0K4.5K0.5K

Change in temperature over the spin-up

Control

Equilibrium (Equatorial heating (5K) – Control)

09

2029

4049

Change in zonal wind over the spin-upEquilibrium (Equatorial heating (5K) – Control)

Control 09

2029

4049

increases

increases

decreases

decreases

Changes in eddy momentum fluxes are in the right sense to drive meridional circulation changes.

y

vuvf

t

u

]''[][

][

y

vu

''

][v

][v

y

vu

''

Mean meridional circulation

Horizontal Eddy Momentum Flux [u’v’]

Anomalous meridional circulations are accompanied by zonal wind accelerations in the troposphere:

y

vuvf

t

u

]''[][

][

increases

increases

decreases

decreases

Mean meridional circulation

Zonal mean zonal wind [u]

][v

][u

][v

][u

Comparison with zonally symmetric model.

• Eddy forcing remains fixed at its value of the control run.• Heating perturbation applied and the model run as before.

• Not much response in the troposphere, particularly at mid/high latitudes. it is altered eddy momentum fluxes that are important in driving the

tropospheric circulation changes.

Full 3D model No change in Eddy fluxes

[mmc]

[u]

What’s causing the change in eddy momentum fluxes?

E-P Flux

]''[

]''[

vF

vuF

p

Refractive Index

222

2

2cosa

NH

f

a

k

cu

qn y

C=8ms-1

Days 0 to 9 of the spin-up:

Change in E-P Flux and

Change in

222

2

2cosa

NH

f

a

k

cu

qn y

''vu

Change in : cu

qy

a) Only changing yq

b) Only changing u

Days 40 to 49 of the spin-up:

Change in E-P Flux and

Change in

222

2

2cosa

NH

f

a

k

cu

qn y

''vu

Change in : cu

qy

a) Only changing yq

b) Only changing u

Contributions to the change in PV gradient(days 0 9):

pp

pyyy

u

T

p

R

fuq

2

yyu

u

Meridional Curvature

Third term (only changing )

Third term (only changing )

Total change in PV gradient

u

Outline of mechanism:

Altered vertical temperature gradients Zonal wind accelerations

stratosphere/tropopause

Change in horizontal eddy momentum flux

Changes in mean meridional circulation

Zonal wind accelerations in the troposphere.

Altered horizontal temperature gradients

Comparing Uniform and Equatorial Heating:

5K

0K

5K

Equatorial heating (5K) (E5)

Uniform heating (5K) (U5)

Weakening and poleward jet shift.

Weakening and equatorward jet shift.

yq

''vu

E-P Flux

E5 (days 0 9 ) U5 (days 0 9 )

E5 (days 40 49 ) U5 (days 40 49 )

u

E-P flux and

n2

Conclusions (1)

• The tropospheric response to increased Solar activity could be produced by a dynamical response to increased heating in the stratosphere.

• Changes in eddy momentum flux are important in driving circulation changes in the troposphere.

• Feedback with changing zonal wind in the troposphere influencing eddy propagation.

• Change in vertical temperature gradient around the tropopause and its localisation in latitude is important in determining the direction of the jet shift.

Relationship with internal annular variability

• Internal Variability– Empirical Orthogonal Functions (EOFs)– Phase space trajectories– Vertically integrated zonal momentum budget– EP Flux and zonal wind anomalies

→Dynamical mechanisms

Equilibrium Response

• U5: Jets weakened and shifted equatorwards.

• E5: Jets weakened and shifted polewards.

Control Run U5 - Control E5 - Control

Latitude (equator to pole) →

Hei

ght

→

Haigh et al (2005)

Leading Modes of Variability

EOF 1 (51.25%) EOF 2 (18.62%)

Latitude (equator to pole) →

Hei

ght

→

• Mean state differences from idealised forcing experiments project strongly onto the leading modes of variability in the control run.

Projections of Mean State Differences

The signal of the experiments can be viewed as displacements in principal component (PC) phase space.

Mean state differences project most strongly onto EOF1 and EOF2.

PC

2 A

mpl

itude

→

EOF Number →

Am

plitu

de (

ms-1

) →

U5-Control

E5-Control

PC1 Amplitude →

Phase Space

Pole

ward

Eq

uato

rward

Broader, Weaker

Narrower, Stronger

Internal Variability: Phase Space Trajectories

• At low frequencies circulation is anticlockwise with a timescale of ~46 days.

• At high frequencies circulation is clockwise with a timescale of ~ 7 days.

Arrow×2

Arrow×½

Unfiltered

Periods Longer than 30 Days

Low Pass Filter

Periods Shorter than 21 Days

High Pass Filter

PC1 →P

C2

→

Zonal Wind Evolution: Low Frequency

• Jet strengthens and moves polewards.• New subtropical jet grows forming double jet structure.• Poleward jet collapses and merges with the new

subtropical jet.

Zonally-averaged zonal momentum equation:

Integrated through depth of atmosphere:

or:

Vertically Integrated Zonal Momentum Budget

22

1cos

cos

uuv u f v F

t a p

220 0 0

1cos

cos

s s sp p pu dp u v u v dp F dp

t a

0

1 sp

ZONAL EDDY Su dp C Cg t

Vertically Integrated Momentum Budget: Low Frequency

0

1 sp

ZONAL EDDY Su dp C Cg t

Vertically Integrated Momentum Budget: High Frequency

0

1 sp

ZONAL EDDY Su dp C Cg t

Phase Space View of Momentum Budget

• Surface stress points slightly in advance of the origin in phase space.

• Eddies change behaviour at high and low frequencies.

0

1 sp

ZONAL EDDY Su dp C Cg t

PC1 →

PC

2 →

PC1 →

PC

2 →

Low Pass High Pass

EP Flux Anomalies: Low Frequency

Low PC1 Composite High PC1 Composite

• EP Flux anomalies reinforce current state.

• Subtle differences between the wind anomalies and the EP Flux cause phase space circulation

EP Flux Anomalies: High Frequency

Low PC1 Composite High PC1 Composite

Less LC1 More LC1More LC2 Less LC2

longitude

lati

tud

e

Thorncroft et al (1993)

Conclusions (2)

• Tropospheric response to stratospheric temperature changes project strongly onto dominant modes of annular variability.

• Distinct difference in behaviour at high and low frequencies:– Low frequency: poleward migration (quasi-equilibrium)– High frequency: equatorward migration (strongly evolving)

• Eddies drive the phase space trajectory at high and low frequencies:– Eddies are balanced more strongly by surface stress at low

frequencies leading to a slower circulation– High frequency eddy anomalies reflect past baroclinicity; feedback

understood in terms of LC1/LC2 behaviour

- Thank you -

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)