Decadal Forecast Exchange Doug Smith, Adam Scaife, G. J. Boer, M. Caian, C. Cassou, F. J. Doblas-Reyes, V. Guemas, E. Guilyardi, E. Hawkins, W. Hazeleger, L. Hermanson, C. K. Ho, M. Ishii, V. Kharin, M. Kimoto, B. Kirtman, J. Lean, D. Matei, W. J. Merryfield, J. Mignot, W. A. Müller, H. Pohlmann, A. Rosati, C. Severijns, D. Swingedouw, L. Terray, B. Wouters and K. Wyser © Crown copyright Met Office

Many groups have now developed decadal predictions Key experiments have been done for CMIP5 What about real time predictions? 15th session of the WMO Commission for Climatology recommended action to start the coordination and exchange of decadal predictions Informal exchange of decadal predictions has been coordinated by Met Office since 2011 9 dynamical systems + 2 empirical Annual mean fields of surface temperature, pressure and precipitation (to be expanded…) Exchanged each year in March © Crown copyright Met Office

Max Planck Institute (Germany) 2011 2012 2013 2014 Uninitialised Hadley UK Met Office  EC-Earth anomaly SMHI (Sweden) EC-Earth full field IC3 (Spain) KNMI (Holland) MRI JMA (Japan) MIROC University of Tokyo MPI Max Planck Institute (Germany) CCCMA Canada CERFACS France IPSL GFDL USA RSMAS University of Miami, USA NCEP NCAR NorESM Norway NRL USA (empirical) Reading University of Reading (empirical) © Crown copyright Met Office

http://www-hc/~hadmt/web/decadal_multimodel.html © Crown copyright Met Office

© Crown copyright Met Office

Surface temperature predictions (five year means) Skill of initialised predictions Initialised - Uninitialised Skilful almost everywhere (positive correlations) Mostly due to external forcing Initialisation gives improved skill mainly in North Atlantic and tropical Pacific (Smith et al. 2010, also Doblas-Reyes et al 2013 for CMIP5 multi-model) © Crown copyright Met Office © Crown copyright Met Office

CMIP5 decadal predictions Predictions (yr 2-5) from 6 CMIP5 systems Initialized solid, uninitialised dashed Observations grey bars Global-mean T and Atlantic multi- decadal variability Correlations and RMSE below Initialisation improves skill, especially for AMV Forecast lead time Doblas-Reyes et al. (2013)

Physical basis for improved skill No historical observations – must rely on models Consistent signal: increase from 1960 to 1995, decrease thereafter Agrees with related observations Some skill in initialised predictions, but not in uninitialised predictions Initialised hindcasts Uninitialised hindcasts © Crown copyright Met Office (Pohlmann et al. 2013) © Crown copyright Met Office

Atlantic multi-decadal variability (AMV) Smith et al, 2011 © Crown copyright Met Office

Atlantic tropical storms Initialised Uninitialised Persistence Skill from both initialisation and external forcings Improved through initialisation Consistent with remote influences from improved SST predictions 1995 transition could have been predicted (Smith et al. 2010, 2014) © Crown copyright Met Office © Crown copyright Met Office

Remote influences on tropical Atlantic atmosphere Idealised predictions of rainfall 2 – 5 years ahead Tropical Atlantic atmosphere is relatively predictable Skill originates from sub-polar North Atlantic © Crown copyright Met Office © Crown copyright Met Office Dunstone et al, 2011

North Atlantic sub-polar gyre (SPG) Meridional heat transport Overturning circulation SPG 500m temp Observations Initialised (DePreSys) Uninitialised (NoAssim) Improved skill for 1995 rapid warming results from initialisation of increased Atlantic overturning circulation and meridional heat transport (Robson et al. 2012, also Yeager et al. 2012, and Robson et al 2014 for 1960s cooling event) © Crown copyright Met Office © Crown copyright Met Office

Impacts of 1995 SPG warming Model Observations Temperature Pressure Precipitation © Crown copyright Met Office (Robson et al. 2013)

Predicted cooling of SPG SPG predicted to cool… …in response to weakening of Atlantic overturning Likely to cause climate impacts around the Atlantic basin Not a reversal, but impacts associated with warm SPG less likely: cold winters and wet summers in Europe less likely fewer hurricanes than recent peaks reduced Sahel rainfall reduced risk of drought in SW USA (Hermanson et al, submitted) © Crown copyright Met Office

Observed weakening of AMOC (Smeed et al, 2013) © Crown copyright Met Office

Ratio of predictable components in reality and models (RPC), years 2-5 Temperature Pressure RPC<1 (blue)  models overconfident (agree with each other but not with reality) RPC>1 (red)  models under confident (unexpected!) Implies reality is more predictable than models  models respond too weakly to SSTs? Members are not potential realisations of reality  affects skill and predictability assessment Can make skilful predictions now, but need mean of large ensemble and to adjust variance Higher skill possible with improved models (Eade et al, in prep) © Crown copyright Met Office

Summary CMIP5 enables assessment of historical performance of decadal predictions Need to focus on actual predictions as well Informal exchange of decadal predictions started Needs formalising under WMO to support GFCS Encouraging skill in North Atlantic (including hurricanes), related to AMOC Expect impacts over land, but too weak in models Models underestimate predictability  respond too weakly to SSTs? AMOC is weakening now (observations) predicted to continue climate impacts likely © Crown copyright Met Office

Potential role of aerosols… North Atlantic SST Observations Model Booth et al., 2012 Dunstone et al., 2013 © Crown copyright Met Office

Predicted cooling of SPG: JJA impacts (Hermanson et al, submitted) © Crown copyright Met Office

Impact of initialisation 2012-16 Initialised – uninitialised, stippled where not significant (Smith et al 2013) © Crown copyright Met Office