Influence of the stratosphere on surface winter climate

1. Adam Scaife, Jeff Knight, Anders Moberg, Lisa Alexander, Chris Folland and Sarah Ineson. CLIVAR Climate of the 20th Century workshop, 2007. Influence of the stratosphere on surface winter climate • 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. 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 * NET: 15W-45E,30-65N CET

3. Stratospheric perturbation experiment 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

4. 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/decade Model Precipitation Observed Precipitation Scaife et al. GRL, 2005

5. Observed vs forced changes in extremes Frost days 10th percentile Tmin 90th percentile 5d rain Observed changes larger than modelled changes with all anthropogenic forcings Signs of dipole across Europe in observed data Scaife et al. J.Clim., 2007

6. Including modelled change in the NAO (?hPa) Frost days 10th percentile Tmin 90th percentile rainfall 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) Scaife et al. J.Clim, 2007

7. Winter 2005/6: a good case study December 2005 January 2006 February 2006 • 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 Zonal wind through the winter (c.f. Baldwin and Dunkerton 2001, Charlton et al. 2004)

8. 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

9. Winter 2005/6 Tropospheric model with stratospheric perturbation Observed Anomalies Tropospheric model 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. Possible mechanism 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, simple analytical model?

11. 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

12. Winter 2005/6 Control Perturbed expt Extended model Observations Internal variability may have dominated 2005/6 winter

13. NAO trends Stratosphere-troposphere Model (HadAM3) NAO TRENDS 1960/61-1990/91: Observations: 11.9 hPa Strat-trop HadAM3: 10.3 ± 1.2hPa Strat-trop HadGAM: 7.8 ±1.5 hPa Trop HadAM3: 5.6 ± 1.4hPa Possibility of improved signal to noise ratio in ensemble decadal predictions Stratosphere-troposphere Model (HadGAM1) Troposphere Model (HadAM3)

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

15. 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) L60 HadGAM L38 HadGAM 50hPa gpheight PMSL

16. 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 Internal Variability

17. 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