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Solar Influence on Stratosphere-Troposphere Dynamical Coupling

Isla Simpson, Joanna D. Haigh, Space and Atmospheric Physics, Imperial College London

Mike Blackburn, Department of Meteorology, University of Reading

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• Introduction to the Solar influence on climate over the 11-year cycle.

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

• Comparison of two different stratospheric heating perturbation cases.

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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

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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.

Figure: Haigh and Blackburn (2006)

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

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Model Experiments:

• Simplified General Circulation Model (Reading IGCM2.2)– T42L15– Newtonian Relaxation– No Orography

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

• Spin-up ensemble:– 200, 50-day runs

5K 0K4.5K0.5K

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Change in temperature over the spin-up

Control

Equilibrium (Equatorial heating (5K) – Control)

09

2029

4049

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

Control 09

2029

4049

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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’]

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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

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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]

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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

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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

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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

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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

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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

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Comparing Uniform and Equatorial Heating:

5K

0K

5K

Equatorial heating (5K) (E5)

Uniform heating (5K) (U5)

Weakening and polewards jet shift.

Weakening and equatorwards jet shift.

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yq

''vu

E-P Flux

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

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

u

E-P flux and

n2

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Conclusions

• 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.

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+ = control

* = E5

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pp

pyyy

u

T

p

R

fuq

2

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Equilibrium (Equatorial heating (5K) – Control)

Control

Change in mean meridional circulation over the spin-up

09

2029

4049

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Stratospheric and Tropopause level accelerations:

• Consistent with altered temperature gradients.• Thermal wind balance:

][][ T

z

u

Zonal mean Temperature Zonal-mean zonal wind

Increase Decreased Decreased Increased

][T

][T

z

u

][

z

u

][

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Momentum Balance:

y

vuvf

t

u

]''[][

][

Acceleration of zonal mean zonal wind Coriolis force on

meridional wind

Convergence of horizontal eddy momentum flux

[u] days 20-29 - control [v] days 20-29 - control

•[u] and [v] terms don’t balance changes in eddy momentum fluxes must be important.

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E5

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U5