Strat-trop interaction and Met Office seasonal forecasting Adam

Strat-trop interaction and Met Office seasonal forecasting Adam

Strat-trop interaction and Met Office seasonal forecasting Adam Scaife, Jeff Knight, Andrew Marshall, Sarah Ineson and Alberto Arribas Hadley Centre, Met Office, Exeter, UK. European winter climate and the NAO Stratospheric influence: Mean and Extremes Trop-strat vs trop models: NAO and ENSO GloSea4: tests and plans Conclusions Adam Scaife 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

Crown copyright 2004 * NET: 15W-45E,30-65N CET Page 2 Adam Scaife Stratospheric influences 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) NAO U50hPa Change in surface pressure Impose a body force in the models stratosphere (c.f. Norton 2003) => Increase in stratospheric wind from 1960s to 1990s

=> Increase in NAO similar to observed value Crown copyright 2004 Page 3 Adam Scaife Winter climate and stratospheric change Model Temperature Observed Temperature European T trends 1960s1990s HadAM3 ctl 0.15K/decade HadAM3 expt 0.59K/decade Model Precipitation Crown copyright 2004 Observed Precipitation

Observations 0.53K/decade Scaife et al. GRL, 2005 Page 4 Adam Scaife Observed vs forced changes in extremes Observations Model (forcings) Model (forcings + NAO) Observed changes larger than modelled changes with all anthropogenic forcings Signs of dipole across Europe in observed data Crown copyright 2004 Scaife et al., J.Clim., 2007 Page 5

Adam Scaife Winter 2005/6: cold Europe case study December 2005 January 2006 Zonal wind through the winter February 2006 Colder than 1970-2000 over much of Europe (c.f. Baldwin and Dunkerton 2001, Charlton et al. 2004) 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 Crown copyright 2004 Page 6

Adam Scaife Winter 2005/6 Tropospheric models (HadAM/HadGAM) 50/25 members HadISST as a boundary condition Tropospheric Model + stratospheric perturbation Zonal wind at 50hPa 25 members HadISST as lower boundary condition Perturbed stratosphere from 1st Jan Troposphere-stratosphere model 25 members HadISST as a boundary condition Crown copyright 2004 Page 7 Adam Scaife Winter 2005/6 Old model New model

Old model + strat warming Observed Anomalies New trop-strat model Cold European signal from Atlantic SSTs Cold European signal from stratospheric warming Stratospheric warming unpredictable from SST alone Scaife and Knight, QJ, in prep. Crown copyright 2004 Page 8 Adam Scaife

ENSO effects on Europe ENSO events produce a ve NAO response (e.g. Bronniman et al. 2004) Only the weakest 2/3 of observed events and didnt occur in trop model (Toniazzo and Scaife 2006) Upper level component appears in trop-strat models (Hamilton, 1993, Manzini et al., 2006) L60 HadGAM L38 HadGAM 50hPa gph PMSL Crown copyright 2004 Page 9 Adam Scaife 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

Crown copyright 2004 Page 10 Adam Scaife Decadal variability Stratosphere-troposphere Model (HadAM3) Stratosphere-troposphere Model (HadGAM1) NAO TRENDS 1960/61-1990/91: Observations: 11.9 hPa Strat-trop HadAM3: 10.3 1.2 hPa Troposphere Model (HadAM3) Strat-trop HadGAM: 7.8 1.5 hPa Trop HadAM3: 5.6 1.4 hPa Crown copyright 2004 Possibility of improved signal to noise ratio in ensemble decadal predictions

Page 11 Adam Scaife A simple metric to judge models: Observations HadGAM NAO (hPa) NAO (hPa) Zonal Wind (m/s) HadAM3 NAO (hPa) r(U50,NAO) = 0.0 Crown copyright 2004 r(U50,NAO) = 0.5

r(U50,NAO) = 0.8 NAO (hPa) U50 (m/s) Ratio HadAM3 10.3 0.5 21.0 HadGAM 7.8 3.2 2.4 OBS 11.9

5.6 2.1 Page 12 Adam Scaife GloSea4 next Met Office seasonal forecast system We are currently testing an extended stratospheric model as a likely candidate for use in the next Met Office seasonal forecast system GloSea4, due in 2009. Skill measures and mechanisms using case studies: L38 vs L60 overall improvement? Volcanic years, cold globe but warm Europe? Weak/strong stratospheric jet years, predictable? Cold/warm Europe? El Nino years, weak polar vortex and cold Europe? Development of an operational volcanic switch Crown copyright 2004 Page 13 Adam Scaife

Conclusions The NAO dominated mean and extreme European winter climate shift from the 1960s to 1990s and the stratosphere played an important role. A picture is emerging which links the stratosphere to the NAO (sometimes because of ENSO) and to cold, blocked European winters. Extended model experiments produce a ve NAO response to El Nino and potentially larger decadal responses to SST but the winter 2005/6 stratospheric anomaly is not predictable from SST information alone. Tests are underway to include initial atmospheric conditions to evaluate a trop-strat model as the next Met Office seasonal forecast model. Crown copyright 2004 Page 14 Adam Scaife

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