1. Strat-trop interaction and Met Office seasonal forecasting
Adam Scaife,
Jeff Knight, Andrew Marshall, Sarah Ineson and Alberto Arribas
Hadley Centre, Met Office, Exeter, UK.
European winter climate and the NAO
Stratospheric influence: Mean and Extremes
Trop-strat vs trop models: NAO and ENSO
GloSea4: tests and plans
Conclusions
Adam Scaife

2. North Atlantic Oscillation
Dominant pattern of Atlantic-European weather variability
Related to changes in the position and strength of the Atlantic Storm track
NAO
r = 0.62
NET*
r = 0.67
r = 0.84
© Crown copyright 2004
* NET: 15W-45E,30-65N
CET
Adam Scaife

3. Stratospheric influences
Change in NAO index
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
NAO
Change in surface pressure
U50hPa
© Crown copyright 2004
Adam Scaife

4. Winter climate and stratospheric change
Model Temperature
Observed Temperature
European T trends 1960s-1990s
HadAM3 ctl K/decade
HadAM3 expt K/decade
Observations K/decade
Model Precipitation
Observed Precipitation
© Crown copyright 2004
Scaife et al. GRL, 2005
Adam Scaife

5. Observed vs forced changes in extremes
Observations
Model (forcings)
Model (forcings + NAO)
Observed changes larger than modelled changes with all anthropogenic forcings
Signs of dipole across Europe in observed data
Scaife et al., J.Clim., 2007
© Crown copyright 2004
Adam Scaife

6. Winter 2005/6: cold Europe case study
December 2005
January 2006
February 2006
Colder than 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
Zonal wind through the winter
(c.f. Baldwin and Dunkerton 2001, Charlton et al. 2004)
© Crown copyright 2004
Adam Scaife

7. Winter 2005/6 Tropospheric models (HadAM/HadGAM) 50/25 members
HadISST as a boundary condition
Tropospheric Model + stratospheric perturbation
25 members
HadISST as lower boundary condition
Perturbed stratosphere from 1st Jan
Troposphere-stratosphere model
Zonal wind at 50hPa
© Crown copyright 2004
Adam Scaife

8. Winter 2005/6 Old model New model New trop-strat model
Old model + strat warming
Observed Anomalies
New model
New trop-strat model
Cold European signal from
Atlantic SSTs
Cold European signal from stratospheric warming
Stratospheric warming unpredictable from SST alone
Scaife and Knight, QJ, in prep.
© Crown copyright 2004
Adam Scaife

9. ENSO effects on Europe L60 HadGAM L38 HadGAM 50hPa gph PMSL
ENSO events produce a –ve NAO response (e.g. Bronniman et al. 2004)
Only the weakest 2/3 of observed events and didn’t occur in trop model (Toniazzo and Scaife 2006)
Upper level component appears in trop-strat models (Hamilton, 1993, Manzini et al., 2006)
L60 HadGAM
L38 HadGAM
50hPa gph
PMSL
© Crown copyright 2004
Adam Scaife

10. Possible mechanism Atlantic SST Atlantic SST
NORMAL
Stratospheric
Polar Vortex
WEAK
Stratospheric
Polar Vortex
WEAK
Stratospheric
Polar Vortex
Rossby waves
NAO -
NAO -
NAO ---
Wind stress +
heat fluxes
Heat fluxes
Atlantic SST
Atlantic SST
and vice versa in +ve NAO winters
© Crown copyright 2004
Adam Scaife

11. Decadal variability Stratosphere-troposphere Model (HadAM3)
Stratosphere-troposphere Model (HadGAM1)
NAO TRENDS 1960/ /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
Troposphere Model (HadAM3)
© Crown copyright 2004
Adam Scaife

12. A simple metric to judge models:
HadAM3
Observations
HadGAM
Zonal Wind (m/s)
NAO (hPa)
NAO (hPa)
NAO (hPa)
r(U50,NAO) = 0.0
r(U50,NAO) = 0.5
r(U50,NAO) = 0.8
NAO (hPa)
U50 (m/s)
Ratio
HadAM3
10.3
0.5
21.0
HadGAM
7.8
3.2
2.4
OBS
11.9
5.6
2.1
© Crown copyright 2004
Adam Scaife

13. GloSea4 – next Met Office seasonal forecast system
We are currently testing an extended stratospheric model as a likely candidate for use in the next Met Office seasonal forecast system GloSea4, due in 2009.
Skill measures and mechanisms using case studies:
L38 vs L60 – overall improvement?
Volcanic years, cold globe but warm Europe?
Weak/strong stratospheric jet years, predictable? Cold/warm Europe?
El Nino years, weak polar vortex and cold Europe?
Development of an operational volcanic “switch”
© Crown copyright 2004
Adam Scaife

14. Conclusions
The NAO dominated mean and extreme European winter climate shift from the 1960s to 1990s and the stratosphere played an important role.
A picture is emerging which links the stratosphere to the NAO (sometimes because of ENSO) and to cold, blocked European winters.
Extended model experiments produce a –ve NAO response to El Nino and potentially larger decadal responses to SST but the winter 2005/6 stratospheric anomaly is not predictable from SST information alone.
Tests are underway to include initial atmospheric conditions to evaluate a trop-strat model as the next Met Office seasonal forecast model.
© Crown copyright 2004
Adam Scaife