1. © Crown copyright 2007 Forecasting weeks to months ahead Dr. Alberto Arribas Monthly-to-Decadal area, Met Office Hadley Centre Exeter, April 2014

2. We have a different strategy to the rest of the world Hindcast length Frequency of system upgrades Centre's priorities Japan~ 30 yr5 yr Experimental USA~ 40 yr8 yr Link to re-analysis Europ. Centre~ 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

3. GloSea history -Summer 2009: GloSea4 becomes operational -Nov. 2010: -Vertical high-res (Full stratosphere) -Sea-ice assimilation (concentrations) -Updated hcst (1996-2009) -May 2011: -Monthly system -Nov. 2012: GloSea5 -Horizontal high-res (N216 atm. / 0.25 ocn.) -NEMOVAR (3d-Var ocean initialisation) -2013-2014: -Updated physics -Increased hindcast length and ensemble size

4. Atmos & land surf: ERA-i Ocean: Seasonal ODA reanalysis Atmos & land surf: NWP anal Ocean/sea-ice : NEMOVAR GloSea5 runs completed daily 60 days6 months 30 th Jan 2014 17 th Feb 96-09 Forecasts Hindcasts 15 days

5. Tropical Cyclones: World-wide Precipitation, wind all year

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

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

8. El Nino / La Nina: World-wide precipitation and temperature all year

9. El Niño composites Forecast system (May -> JJA) Observations

10. © Crown copyright Met Office ForecastObserved ENSO Teleconnection: Prec. Nino - Nina DJF JJA Arribas et al., MWR, 2011

11. North Atlantic Oscillation: N.America & Europe Precipitation, temperature, wind DJF

12. Winters depend on which way the wind blows Weak P Gradient Cold advection into Europe Cold, calm and dry Winter 2009/10 Winter 2011/12 Strong P Gradient Warm advection into Europe Mild, stormy and wet

13. © Crown copyright Met Office North Atlantic Oscillation (winter)

14. Why does horizontal resolution matter? Improved Gulf Stream -> Improved Atl.Blocking Scaife et al., Geophys. Res. Lett., 2012. Low-res:1 deg ocean High-res:0.25 deg ocean

15. Predictability of the NAO NAO skill: correlation=0.6 (c.f. ECMWF 0.16, NCEP 0.25: not significant) Significant at the 99.5% level Retrospective winter forecasts from early November (20 yr hcst) r = 0.61 Ensemble Member Ensemble Mean Observations Scaife et al., Nat. Geo., 2014 (under review) MacLachlan et al., QJRMS, 2014 (under review)

16. Individual winters 92/93 – 11/12 Good agreement between pressure patterns in many individual years Scaife et al., Nat. Geo., 2014 (under review)