1. The role of the stratosphere in extended- range forecasting Thomas Jung Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research Germany

2. Overview Early experiments Relaxation experiments Extended-range predictions Dynamics of two recent cold European winters o The winter of 2005/06 o The winter of 2009/10 Case study: The January 2009 sudden stratospheric warming event

3. Early experiments: Motivation

4. Early experiments F is the mean optimal forcing (adjoint method, 18 cases) F is applied in the stratosphere only Three 40-day forecasts for each of the winters 1980- 2001 Control Positive forcing (strong vortex) Negative forcing (weak vortex) T L 95L60 (cycle 28r1)

5. Stratospheric response (Z50) Jung and Barkmeijer (2006); MWR

6. Stratospheric response (Z1000) Jung and Barkmeijer (2006), MWR

7. Downward propagation: ERA-40 Jung and Leutbecher (2007), QJRMS

8. Downward propagation: T L 95L91 Jung and Leutbecher (2007), QJRMS

9. Conclusions: Part I Classical stratospheric forcing experiments indicate downward control of stratospheric circulation anomalies in the ECMWF model Downward control is also found in long unforced integrations The magnitude of the response in unforced experiments looks smaller than in forced experiments

10. Relaxation experiments Study the impact of Northern Hemisphere stratosphere on tropospheric forecast skill Strategy: Perform set of control experiments (standard NWP setting) Perform additional experiments with forecast error suppressed in the NH stratosphere Perform similar experiments for other areas (e.g. tropics)

11. Experimental setup T L 159L60 (32r1) 88 30-day forecasts (15 th of Nov, Dec, Jan and Feb. of the period 1980/81-2000/01) Initial and boundary conditions from ERA-40 Relaxation towards ERA-40 Persisted SST and sea ice Control, stratospheric and tropical relaxation experiments

12. Relaxation regions Stratospheric relaxation Tropical relaxation

13. NH Z500 scores Jung et al (2010a), MWR

14. NH Z500 forecast error reduction Jung et al (2010a), MWR

15. Conclusions: Part II Relaxation experiments suggest that reducing stratospheric forecast error leads to better tropospheric forecasts Overall the impacts on Northern Hemisphere forecast skill moderate Locally larger impacts (e.g. North America and North West Europe) Results in good agreement with Newman and Sardeshmukh (LIMs)

16. The cold European winter 2005/06 Jung et al. (2010b), MWR

17. Seasonal forecasts Statistical techniques predicted negative NAO Dynamical techniques also predicted negative NAO (ie cold winter) However, observed circulation anomaly not strictly the NAO

18. Suggested mechanisms Folland et al. (2006) Reemergence of North Atlantic SST anomalies Scaife and Knight (2008) Atlantic SST January SSW Croci-Maspoli and Davies (2009) SST and 2mT anomalies in the western North Atlantic and close to the eastern US seaboard Bader and Latif (2003) Cold SST anomalies Indian ocean NAO- Greatbatch and Jung (2007) La Nina NAO+

19. Experimental setup T L 95L60 (32r1) Atmosphere-only with observed SST/sea ice Lagged ensemble (17 members) started in the middle of November 2005 Calibrations runs with and without relaxation (1990- 2006) Relaxation experiments (various regions)

20. Z500 anomalies: DJF 2005/06

21. Z50 anomalies: DJF 2005/06

22. Z500 anomalies: DJF 2005/06

23. Ensemble mean anomalies: Polar Z50

24. Conclusions: Part III Origin of the cold European 2005/06 has been studied Various mechanisms have been proposed including an important role of the Northern Hemisphere stratosphere Our results suggest that the tropics have been important (for the NH troposphere and stratosphere) Tropical stratosphere: QBO Tropical troposphere o South America o Indian Ocean

25. The extreme negative NAO winter 2009/10 Jung et al. (2011), GRL

26. Operational ECMWF forecasts Jung et al. (2011), GRL

27. Experimental setup Jung et al. (2011), GRL T L 159L60 (36R1) ECMWF Monthly Forecasting System Forecasts started on 1 November 2009 40 Ensemble members Control integration Various sensitivity experiments Hindcasts for each of the configurations 1991-2008 4 ensemble members

28. Sensitivity experiments I Jung et al. (2011), GRL

29. Sensitivity experiments II D+18-D+32 Verifying analysis Jung et al. (2011), GRL

30. Conclusions: Part IV The winter 2009/10 was one of the most negative NAO winters on record Extremely good monthly and seasonal forecast skill during the second half of the winter Sensitivity experiments imply that external forcing and the Northern Hemisphere stratosphere played a secondary role Internal tropospheric dynamics (highly predictable)?

31. The January 2009 SSW: A case study Sensitivity experiments have been strongly forced Consider unforced experiments Case selection Good extended-range forecasts during the late part of the winter 2008/09 Strong SSW in January 2009 Did the SSW contribute to the high skill? Work with Frederic Vitart

32. Forecasts data ECMWF VarEPS T255 up to D+9 T399 from D+10 62 levels in the vertical (up to 5 hPa) Atmosphere-ocean coupling from day-10 50 ensemble members (operationally)

33. The January 2009 SSW Start: 8. January 2009 Start: 15. January 2009

34. Separation into good and poor forecasts Focus on forecasts started on 15. January 2009 Increase ensemble size to 100 members Separate into best and worst 10 SSW cases

35. Z500 anomalies Z500 forecast anomalies (initialized on 15. January 2009)

36. The role of vertical resolution Start: 8. January 2009Start: 15. January 2009

37. Conclusions: Part V Major SSW occurred in January 2009 Extended-range forecasts for Europe have been quite skilful following the SSW Role of the SSW has been assessed by splitting ensemble into two sub-groups (good and poor SSW forecasts) Impact of the SSW seem to have played a secondary role More cases need to be considered in future studies

38. Some general thoughts Stratosphere provides a source of extended-range forecast skill in the Northern Hemisphere troposphere Some areas might benefit in particular (e.g. Europe) However, the influence is likely to be moderate at best (disagreement with Douville et al?) It is somewhat surprising that there is such a spread regarding possible mechanisms of recent cold European winters

39. Further reading: