Stratosphere-Troposphere Dynamical Coupling and its relationship to Annular Variability of the Troposphere Michael Blackburn (1), Joanna D. Haigh (2),

Stratosphere-Troposphere Dynamical Coupling and its relationship to

Annular Variability of the Troposphere

Michael Blackburn(1), Joanna D. Haigh(2), Isla Simpson(2,3), Sarah Sparrow(1,2)

(1) NCAS-Climate, Department of Meteorology, University of Reading, UK

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

(3) Department of Physics, University of Toronto, Canada.

NCAS Staff Meeting 11 November 2009

Outline

• Motivation – response to climate forcings

• Idealised stratospheric heating experiments

equilibrium response

spin-up ensembles – mechanisms

unforced annular variability

• Dependence on tropospheric climatological basic state

Climate Change: annular response

Lorenz & DeWeaver (2007)

IPCC AR4 models

2080-2099 minus 1980-1999

A2 scenario (“business as usual”)

Zonal mean zonal wind 850hPa zonal wind

Temperature change

Yin (2005); Miller et al (2006);

© Imperial College LondonPage 4

Polar cap T trend (2001-2050) DJF u trend (2001-2050)

Comparing IPCC models with and without ozone recovery:

With recovery With recoveryWithout recovery Without recovery

21st century Ozone Recovery

Son et al (2008), Science

Solar index regressions using reanalysis data

Frame & Gray (2009)

ECMWF reanalyses 1979-2001 (ERA-40)

Observed stratospheric temperature signal

solar max - solar min

Crooks & Gray (2005);

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

Simplified GCM - “dynamical core” model

Control run zonal wind

Control run temperature

Relaxation Temperature

Based on Hoskins & Simmons (1975) primitive equation model• Spectral dynamics: T42 L15

• Newtonian relaxation (Held-Suarez)

• Boundary layer friction (Rayleigh drag, σ > 0.7)

• No orography / forcing of planetary waves

Idealised stratospheric heating

• Heating perturbations can be applied to the stratosphere by changing the relaxation temperature profile

P10 Polar heating (10K)

5K0K

5K 0K

E5 Equatorial heating (5K) U5 Uniform heating (5K)

10K

• Applied 3 different

heating perturbations

Haigh et al (2005)

Equilibrium ResponseZonal mean Temperature

Zonal mean zonal wind

Control zonal wind

E5 U5 P10

E5 U5 P10

E5 case gives a similar response in the troposphere to that seen over the solar cycle

• Poleward or equatorward shift of tropospheric jet dependent on stratospheric heating distribution

• 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

Simpson et al (2009)First recall storm-track diagnostics....

• Flux of wave activity in latitude-height plane

• Conserved following eddy group velocity (assumptions)

• Components proportional to eddy heat + momentum fluxes

• E-P flux divergence quantifies eddy forcing of mean state

Eliassen-Palm flux

Eliassen & Palm (1961); Charney & Drazin (1961); Andrews & McIntyre (1976); Edmon et al (1980)

Eddy-feedback processes

Ensemble spin-up response to stratospheric heating distributions in the idealised model (Simpson et al, 2009)

Tropopause [qy] trigger

|][|

][~2

cu

qn y

Refraction feedback amplifies tropospheric anomalies

Baroclinicity feedback moves wave source

t

uF

.

E-P Flux, days 0 to 9 E-P Flux, days 20 to 29 E-P Flux, days 40 to 49

u, days 20 to 29 u, days 40 to 49Heating: δT_ref

zFz

u

Response to forcing projects onto leading annular modes

(2D phase space projection)

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

Latitude (equator to pole) →

Hei

ght

→Leading Modes of Variability

Sparrow et al (2009)

Po

lew

ard

Eq

ua

torw

ard

Narrower, Stronger

Broader, Weaker

PC1 Amplitude

PC

2 A

mpl

itude

E5U5

C

• Poleward jet migration

• Driven by upper-level momentum / EP-fluxes

Low frequency variability - dynamics

Sparrow et al (2009)

Low frequency phase-space circulation

• Positive eddy feedback

• Leads to long timescales of variability

• Similarity to forced response

High-PC1 composite

E5 dependence on tropospheric basic state

• Equilibrium experiments with modified tropospheric reference temperature

• Stronger response to stratospheric forcing for lower latitude jets

• Indicative of stronger eddy feedback (despite weaker eddies in control)

TR1 TR2 TR3 TR4

Decreasing baroclinicity Increasing baroclinicity

TR5

TR

u

E5δu

NOTE: THERE IS 1 BLANK BOX

HIDING TEXT ON THE RIGHT

Control climatology E5 response

Possible causes of sensitivity

• Under investigation….

• Not due to proximity of stratospheric heating to jet (equatorial and polar heating responses scale similarly)

• Timing of spin-up response should indicate refraction versus baroclinic mechanism:

- apparently conflicting evidence

- projection of eddy forcing onto wind response varies

- responses differ only after ~50 days

• Are different responses related to unforced variability?

Relationship to unforced internal variability

• Find strongest response to forcing for lower latitude jets

• How is this related to the unforced internal variability?

• Fluctuation-Dissipation Theorem (FDT) predicts a stronger response for longer timescales of internal variability

• Due to stronger internal (eddy) feedbacks, maintaining the leading mode(s) of variability against dampingNOTE: THERE IS 1

BLANK BOX HIDING PLOTS ON

THE RIGHT FDT references: Leith (1971); Ring & Plumb (2008) etc

© Imperial College LondonPage 21

Timescales of variability

• 1-point correlation maps of zonal wind anomalies wrt peak easterly response at 200hPa

• Mid-latitude jets: short timescale; propagating

• Low latitude jets: long timescale; stationary

Annular variability in TR3 control

• Evidence for 2 types of natural variability:

poleward propagating anomalies – short timescale

persistent stationary anomalies – long timescale

• Persistent behaviour dominates for lower latitude jets

• Propagating behaviour dominates for higher latitude jets

• Need to separate and characterise these distinct “modes” of variability

Conclusions

• Mechanism identified by which stratospheric change displaces the tropospheric jet and storm-track.

• Relevant to the tropospheric response to all stratospheric climate forcings.

• Dynamical mechanism is related to the slowest modes of annular variability.

• Forced response and variability are both driven by storm-track transient eddy feedback.

• Strength of eddy feedback depends on the latitude / width of the jet:

- GCMs need realistic variability for correct forced response (FDT)

- 2 types of annular variability in sGCM - under investigation

- Questions? -

NCAS Staff Meeting 11 November 2009

Idealised GCM: annular response

Lorenz & DeWeaver (2007)

Zonal wind response to localised heating 150hPa deep, 20° wide latitude

Phase Space View of Momentum Budget

• Eddies change behaviour at high and low frequencies and jet migration changes direction.

• At low frequencies it is unclear what drives the poleward migration.

0

1 sp

ZONAL EDDY Su dp C Cg t

PC1 →

PC

2 →

PC1 →

PC

2 →

Low Pass High Pass

Empirical Mode Decomposition: Phase SpaceMode 1 Mode 2

Mode 4

Mode 3

Mode 6Mode 5

Tc = 4.96 ± 0.05 days Tc = 8.0 ± 0.3 days Tc = 20.3 ± 0.8 days

Tc = 39 ± 2 days Tc = 78 ± 5 days Tc = 198 ± 19 days

Transformed Eulerian Mean Momentum Budget

High Frequencies: • Eddies drive equatorward

migration.• Eddies out of phase with

winds near the surface.

Intermediate Frequencies:• Eddies drive poleward

migration.• Residual circulation drives

jet migration at lower levels.

• Eddies in phase with the winds near the surface.

][][

][cos][cos

][][ *

**

Fp

uwu

a

vvf

Fcos

1][

adt

ud––+ ω

Eddy feedback processes

Refractive Index determined by wind anomalies

|][|

][~2

cu

qn y

Eddies propagate towards high refractive index

Resulting EP-flux divergence drives zonal wind changes (phase offset)

Eddy source lags baroclinicity (zonal wind anomalies) by 2-4 days

Latitude Latitude Latitude Latitude

Hei

ght

Hei

ght

Hei

ght

Hei

ght

Latitude

Hei

ght

Latitude

Hei

ght

Latitude

Hei

ght

LatitudeH

eigh

t

Hig

h F

requ

ency

Low

Fre

quen

cy

Conclusions

• Annular variability at different timescales in a Newtonian forced AGCM:

– Equatorward migration of anomalies at high frequencies

– Poleward migration at low frequencies

• For all timescales the jet migration is driven by the eddies at upper levels and conveyed to lower levels by the residual circulation.

• Evidence for two feedback processes:

• Eddy source responds to low-level baroclinicity, with lag 2-4 days:

– High frequency flow is so strongly eddy driven that wind anomalies almost out of phase with wave source.

– Low frequency wind anomalies and eddy source are almost in phase.

• Wind anomalies dominate refractive index, leading to positive eddy feedback via EP-flux divergence.

• Direction of propagation from relative phases of wave source/sink and wave refraction.