1. © Crown copyright 2007 A fully resolved stratosphere: Impact on seasonal forecasting Alberto Arribas Monthly-to-Decadal area, Met Office Hadley Centre Reading, April 2013

2. -Ensemble forecast -Dealing with model drift obs. climatology Forecast simulations How do we make seasonal forecasts?

3. obs All Hindcast runs: ~ 12 members 14 years (96-09) Hindcast mean How do we make seasonal forecasts?

4. obs Hindcast mean Forecast members How do we make seasonal forecasts?

5. Bias corrected forecast obs How do we make seasonal forecasts?

6. Skill of seasonal forecasts Between 1990s and late 2000s, skill had barely improved, still minimal outside tropics

8. Change of strategy Hindcast length Frequency of system upgrades Centre's priorities JMA ~ 30 yr? ? NCEP ~ 40 yr8 yr Link to re-analysis ECWMF ~ 25 yr5 yr Med-range UK Met Office * 14 yr1 yr Link to model development * Arribas et al., 2011: GloSea4 ensemble prediction system for seasonal forecasting. MWR. 139, 1891-1910

9. GloSea history -Summer 2009: GloSea4 becomes operational -Nov. 2010: -Vertical high-res (L85 strat. / L75 ocean) -Sea-ice assimilation (concentrations) -Updated hcst (1996-2009) -May 2011: -Monthly system -Nov. 2012: GloSea5! -Horizontal high-res (N216 atm. / 0.25 ocn) -NEMOVAR ODA

10. Improved GloSea5 Range: 7months Model: HadGEM3 Resolution: N216L85O(0.25)L75 Initialisation: daily, NWP state + NEMOVAR 0.25deg Method: Full field + hindcast bias correction Ensembles: SKEB2 stoch. physics, lagged initialisation Forecasts: Seasonal 2 day -1 : 42 members updated weekly Monthly 2 day -1 : 28 members updated daily Reference: MacLachlan et al., 2013, in prep.

11. Atmos & land surf: ERA-i Ocean: Seasonal ODA reanalysis Atmos & land surf: NWP anal Ocean/sea-ice : NEMOVAR A coupled system for med-range and ocean forecasting 60 days6 months 6 th Dec 2012 9 th Jan 96-09 Forecasts Hindcasts 15 days - 24 members - IC perts. from MOGREPS-G - Coupled model as in GloSea5 - Run once a day - Comparison PS32 – PS34 - Operational end13 or 14? New suite:

12. GloSea5: Skillful NAO predictions! Scaife et al., 2013 Source: MetOffice.gov.uk R=0.60 NAO: November forecast for Dec-Jan-Feb observations forecast

13. PMSL anomalies from Nov for DJF 05/06 to 09/10 winters

14. Reasons for skilful NAO forecasts Gulf Stream Bias Wly wind bias => Blocking Deficit No Gulf Stream Bias No Wly wind bias => Good Blocking in N. Atl New Model Scaife et al., Geophys. Res. Lett., 2012. Low-res: 1 deg ocean High-res: 0.25 deg ocean

15. - Courtesy of Warwick Norton -

16. Not December! As found by Warwick Norton, there is very little skill in December Not a simple persistence of anomalies over the timescale of a few weeks Hints at a possible mechanism in late winter: ENSO/stratosphere => strongest NAO (Ineson and Scaife 2008) From Nick Dunstone

17. Looking at stratosphere: Data available L38 L85 GloSea4: N96 GloSea5: N216 89-0289-09 96-09

18. SSWs in L38 and L85 From David Fereday

19. Obs No Strat Moderate NinoStrong Nino From Anna Maidens

20. A quick diversion: How good are ENSO forecasts in GloSea5?

21. Improving ENSO forecasts Obs The westward extension of Nino is a common error in many climate models. It affects remote regions. High-res model has better ENSO pattern and teleconnections Low resolution High resolution

22. Nino 3.4 SST:ACC / RMSE&Spread ACC higher (good) RMSE reduced (also good) May JJANov DJF GloSea5 (red) GloSea4 (blue)

23. © Crown copyright Met Office ForecastObserved ENSO Teleconnection: Prec. Nino - Nina DJF JJA

24. Back to the stratosphere

25. Importance of stratosphere. Winter 2012-2013 Strat. Winds Observations (winter 2012/2013) From Jeff Knight

26. Forecasts Easterly winds appear at high altitude first First signs in forecasts made on 21 December… Strat. Winds Observations Monthly Forecasts: January 2013 From Jeff Knight

27. © Crown copyright Met Office First significant Snow Fall ~18 th Jan Forecasters notified from late Dec => increased risk ACTIONABLE DfT notified on 4 th January: …there is now an increasing risk of cold conditions returning later in January and an increased chance of wintry conditions starting later this month… Wintry weather brings disruption to airports, road and rail networks across the UK, with more snow and ice on the way The Guardian 20 th Jan Metro 18 th Jan

28. Conclusions - GloSea5: High skill predicting winter NAO - Why? High horizontal res.; sea-ice; stratosphere - Skill highest in late winter, role of stratosphere? - Consistent with ENSO/Europe teleconnection through stratosphere and better SSW

29. Thanks Any questions?

30. Tropical storm tracks: impact of resolution 1990s ~240 km horizontal resolution 2000s ~120 km horizontal resolution Observations

31. Today: GloSea5 ~53 km horizontal resolution Observations Tropical storm tracks: impact of resolution