1. 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 20 th Century workshop, 2007. European winter climate and the NAO Stratospheric perturbation experiments: Mean and Extremes Trop-strat vs trop models: Blocking NAO and ENSO NAO trend Conclusions

2. Page 2© Crown copyright 2004 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

3. Page 3© Crown copyright 2004 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 NAO U 50hPa

4. Page 4© Crown copyright 2004 Winter climate and stratospheric change Model TemperatureObserved Temperature European T trends 1960s-1990s HadAM3 ctl 0.15K/decade HadAM3 expt 0.59K/decade Observations 0.53K/decade Model PrecipitationObserved Precipitation Scaife et al. GRL, 2005

5. Page 5© Crown copyright 2004 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 90 th percentile 5d rain10 th percentile Tmin Scaife et al. J.Clim., 2007

6. Page 6© Crown copyright 2004 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) 90 th percentile rainfall10 th percentile Tmin Scaife et al. J.Clim, 2007

7. Page 7© Crown copyright 2004 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 2005February 2006

8. Page 8© Crown copyright 2004 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 1 st Jan Zonal wind at 50hPa

9. Page 9© Crown copyright 2004 Winter 2005/6 Tropospheric model Tropospheric model with stratospheric perturbation Observed Anomalies A cold European signal can be reproduced from observed Atlantic SSTs in the Hadley Centre model The stratospheric mid-winter warming enhanced this cold signal The observed signal is larger than either simulation Scaife and Knight, QJ, in prep.

10. Page 10© Crown copyright 2004 Possible mechanism WEAK Stratospheric Polar Vortex Atlantic SST NAO - NORMAL Stratospheric Polar Vortex NAO --- Rossby waves Atlantic SST Heat fluxes Wind stress + heat fluxes WEAK Stratospheric Polar Vortex and vice versa in +ve NAO winters, simple analytical model?

11. Page 11© Crown copyright 2004 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 1 st Jan  Troposphere-stratosphere-mesosphere model 25 members HadISST as a boundary condition Fully resolved stratosphere Zonal wind at 50hPa

12. Page 12© Crown copyright 2004 Winter 2005/6 Control Perturbed expt Extended model Observations Internal variability may have dominated 2005/6 winter

13. Page 13© Crown copyright 2004 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)

14. Page 14© Crown copyright 2004 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 reproduces maximum blocking frequency in both Pacific and Atlantic sectors. Climate model blocking frequencies (D’Andrea et al. 1997)

15. Page 15© Crown copyright 2004 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

16. Page 16© Crown copyright 2004 Internal Variability NAO (hPa)U50 (m/s)Ratio HadAM310.30.521.0 HadGAM7.83.22.4 OBS11.75.72.1 NAO (hPa) Zonal Wind (m/s) HadAM3ObservationsHadGAM r(U 50,NAO) = 0.0r(U 50,NAO) = 0.5r(U 50,NAO) = 0.8

17. Page 17© Crown copyright 2004 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 reproduced NAO 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