Influence of the stratosphere on surface winter climate Adam Scaife, Jeff Knight, Anders Moberg, Lisa Alexander, Chris Folland and Sarah Ineson. CLIVAR

Influence of the stratosphere on surface winter climate

Adam Scaife, Jeff Knight, Anders Moberg,

Lisa Alexander, Chris Folland and Sarah Ineson.

CLIVAR Climate of the 20th Century workshop, 2007.

European winter climate and the NAO

Stratospheric perturbation

experiments:

Mean and Extremes

Trop-strat vs trop models: BlockingNAO

and ENSONAO

trend

Conclusions

© Crown copyright 2004 Page 2

North Atlantic Oscillation

Dominant pattern of Atlantic-European weather variability

Related to changes in the position and strength of the Atlantic Storm track

CET

NET*

NAO

r = 0.67

r = 0.62

r = 0.84

* NET: 15W-45E,30-65N


Stratospheric perturbation experiment

Change in NAO index

Change in surface pressure

Observations have a large increase in NAO

Control run has very little increase in NAO(includes GHG, aerosols, observed SST etc, c.f. Rodwell et al. 1999)

Impose a body force in the model’s stratosphere(c.f. Norton 2003)

=> Increase in stratospheric wind from 1960s to 1990s

=> Increase in NAO similar to observed value

NAOU50hPa


Winter climate and stratospheric change

Model Temperature Observed Temperature

European T trends 1960s-1990s

HadAM3 ctl 0.15K/decade

HadAM3 expt 0.59K/decade

Observations 0.53K/decadeModel Precipitation Observed Precipitation

Scaife et al. GRL, 2005


Observed vs forced changes in extremes

Frost days

Observed changes larger than modelled changes with all anthropogenic forcings

Signs of dipole across Europe in observed data

90th percentile 5d rain10th percentile Tmin

Scaife et al. J.Clim., 2007


Including modelled change in the NAO (?hPa)

Frost days

Observed changes similar to modelled changes due to NAO

Signs of dipole across Europe in observed data, large Baltic signal (c.f. Koslowski and Loewe, 1994)

90th percentile rainfall10th percentile Tmin

Scaife et al. J.Clim, 2007


Winter 2005/6: a good case study

Zonal wind through the winter (c.f. Baldwin and Dunkerton 2001, Charlton et al. 2004)

• Colder than 1970-2000 over much of Europe

• 2nd coldest in 10 years using area mean T

• Record snowfall in parts of central Europe • Late winter colder than early winter

• Extreme stratospheric sudden warming in January

January 2006December 2005 February 2006


Winter 2005/6

Tropospheric model (HadAM3)• 50 members• HadISST as a boundary condition

Tropospheric model + stratospheric perturbation• 25 members• HadISST as a boundary condition• Perturbed stratosphere from 1st Jan

Zonal wind at 50hPa


Winter 2005/6

Tropospheric modelTropospheric model with stratospheric perturbation

Observed Anomalies

A cold European signalcan be reproduced fromobserved Atlantic SSTsin the Hadley Centre model

The stratospheric mid-winter warming enhanced this cold signal

The observed signal is largerthan either simulation

Scaife and Knight, QJ, in prep.


Possible mechanism

WEAKStratospheric Polar Vortex

Atlantic SST

NAO - NAO -

NORMALStratospheric Polar Vortex

NAO ---

Rossby waves

Atlantic SST

Heat fluxesWind stress + heat fluxes

WEAKStratospheric Polar Vortex

and vice versa in +ve NAO winters, simple analytical model?


Winter 2005/6

Tropospheric model (HadAM3)• 50 members• HadISST as a boundary condition

Tropospheric model + stratospheric perturbation• 25 members• HadISST as a boundary condition• Perturbed stratosphere from 1st Jan

Troposphere-stratosphere-mesosphere model• 25 members• HadISST as a boundary condition• Fully resolved stratosphere

Zonal wind at 50hPa


Winter 2005/6

Control Perturbed expt

Extended model Observations

Internal variability may have dominated 2005/6 winter


NAO trends

Stratosphere-troposphere Model (HadAM3)

Troposphere Model (HadAM3)

NAO TRENDS 1960/61-1990/91:

Observations: 11.9 hPa

Strat-trop HadAM3: 10.3 ± 1.2 hPa

Strat-trop HadGAM: 7.8 ± 1.5 hPa

Trop HadAM3: 5.6 ± 1.4 hPa

Possibility of improved signal to noise ratio in ensemble decadal predictions

Stratosphere-troposphere Model (HadGAM1)


Atmospheric blocking

Tibaldi and Molteni (1990) index for 40 years (ERA40 and HadAM3 models)

19L HadAM3 model underestimates blocking frequency

Weather forecasts underestimate blocking frequency after >5 days

Climate models in general underestimate blocking frequency

Strat-trop HadAM3 model reproducesmaximum blocking frequency in bothPacific and Atlantic sectors.

Climate model blocking frequencies (D’Andrea et al. 1997)


ENSO effects on Europe

On average, ENSO events reproduce a –ve NAO response (e.g. Bronniman et al. 2004)

Only true for the weakest 2/3 of observed events and didn’t occur in HadAM3 (Toniazzo and Scaife 2006)

50hPa gpheight

L38 HadGAM

PMSL

L60 HadGAM


Internal Variability

NAO (hPa) U50 (m/s) Ratio

HadAM3 10.3 0.5 21.0

HadGAM 7.8 3.2 2.4

OBS 11.7 5.7 2.1

NAO (hPa) NAO (hPa) NAO (hPa)

Zo

na

l Win

d

(m/s

)

HadAM3 Observations HadGAM

r(U50,NAO) = 0.0 r(U50,NAO) = 0.5 r(U50,NAO) = 0.8


Conclusions

Changes in observed European winter climate and frequency of extremes from 1960s to 1990s can be largely explained by the NAO

The stratosphere played a crucial role in these changes

Extended model experiments have tested the hypothesis that the response to SSTs is amplified by the stratosphere:

A large improvement in blocking and more –ve NAO events occur-ve NAO response to ENSO is reproducedNAO changes are larger in extended models but less than observed change.

A clear picture is emerging which links the stratosphere to the NAO (and sometimes because of ENSO) to cold, blocked European winters

Documents

Influence of the stratosphere on surface winter climate Adam Scaife, Jeff Knight, Anders Moberg, Lisa Alexander, Chris Folland and Sarah Ineson. CLIVAR