Page 1 Hadley Centre © Crown copyright 2004 Evidence for the Atlantic Multidecadal Oscillation as an internal climate mode from coupled GCM simulations Jeff Knight Hadley Centre, Exeter, UK 4th International CLIVAR Climate of the 20th Century Workshop, Hadley Centre, Exeter, UK Wednesday, 14th March 2007

Page 2 Hadley Centre © Crown copyright 2004 AMO in observations

Page 3 Hadley Centre © Crown copyright 2004 AMO in observations Mean North Atlantic SST  ‘AMO index’  Low-pass (> 13.3y) filtered detrended HadISST  Larger than trend or interannual Surface temperature anomaly  Regression  HadCRUTv blended SST/air temp  90% confidence interval accounting for autocorrelation Palaeoclimate – AMO back to C16-17th?  Tree rings (Gray et al., 2004)  Multiproxy (Delworth and Mann, 2000) Models show some THC-SST links  e.g. Delworth and Mann, 2000 Is the AMO long-lived/periodic? Forced or internal?

Page 4 Hadley Centre © Crown copyright 2004 AR4 20 th Century Forcings Coupled Ensembles

Page 5 Hadley Centre © Crown copyright 2004 AR4 Ensembles Single model North Atlantic mean SST Grey = annual means for 3 ensemble members Red = ensemble mean Black = 90% limits of estimated ens mean Blue = Observed SST from HadSST2 All data relative to average Example of an AR4 20c3m ensemble with few members High inter-member variability leads to very broad uncertainty in the ensemble mean Not easy to distinguish the observations from the possible model estimates of the forced response

Page 6 Hadley Centre © Crown copyright 2004 AR4 Ensembles Multi-model ensemble Red = super-ensemble mean (34 members) Black = 90% limits of estimated ens mean Blue = Observed SST from HadSST2 All data relative to average Super-ensemble (34) using data from 11 models with natural + anthro forcings and available SST Narrower uncertainty on ensemble mean Range is now a function of both internal climate variability and model and forcing differences Represents a ‘best estimate’ of the forced response Atlantic SST is inconsistent with the forced response for much of the last 150 years

Page 7 Hadley Centre © Crown copyright 2004 AR4 Ensembles North Atlantic SST trends Obs show clear multidecadal trend oscillations Model trends are present but relatively weak Difference therefore resembles obs Obs trends almost always significantly different from the forced signal OBS AR4 AVG OBS minus AR4 Linear change = Trend (K/year) x Period (year)

Page 8 Hadley Centre © Crown copyright 2004 AR4 Ensembles A better AMO index? Black = 90% limits of estimated ens mean Blue = Observed SST from HadSST2 All data relative to average Removing a model-based estimate of the historical forced response as an improvement on linear detrending subtracting ‘background’ estimates based on global mean temperature

Page 9 Hadley Centre © Crown copyright 2004 AR4 Ensembles Implications The inconsistency between observed North Atlantic SST and the ensemble estimate of the forced response suggests several possibilities: *In the latter 2 cases, the errors would have to be specific to the Atlantic as the models perform well for the global mean The AMO is an internal mode Models are inadequate to represent the effects of known forcings on climate* The forcings used are incorrect or incomplete*

Page 10 Hadley Centre © Crown copyright 2004 HadCM3 Control Simulation

Page 11 Hadley Centre © Crown copyright 2004 Control Simulation 1400 Year Coupled Model Representation of the AMO Year bandObserved AMO Pattern 0° 60° 120° 180°  Similar pattern and time scale to observed AMO fluctuations.  Similar magnitude – North Atlantic low frequency (>45 year) standard deviation is 0.10K, 0.14K in observations.  Observed AMO likely to be long-lived climate mode.

Page 12 Hadley Centre © Crown copyright 2004 Control Simulation 1400 Year HadCM3 control simulation  Maximum overturning streamfunction at 30°N  Persistent band of variability between years  Compares with observed period of ~65 years (instrumental) and years (palaeo – Gray et al. 2004).

Page 13 Hadley Centre © Crown copyright 2004 Large-scale temperatures

Page 14 Hadley Centre © Crown copyright 2004 Control Simulation THC-Mean temperature cross-correlations Northern Hemisphere Southern Hemisphere Global 0.09°C Sv -1 (0.55)0.01°C Sv -1 (0.13) 0.05°C Sv -1 (0.59) Suggests potential predictability of climate for several decades into the future

Page 15 Hadley Centre © Crown copyright 2004 Mechanism

Page 16 Hadley Centre © Crown copyright 2004 Mechanism Density anomalies related to the THC  Regress 0-800m averaged density onto THC  At THC peak, high densities in the mid-latitude and sub-polar ocean  Low densities in sub-tropical ocean  Density anomalies at 60°N mostly result from the contribution of salinity anomalies, rather than thermal anomalies. From Vellinga and Wu (2004)

Page 17 Hadley Centre © Crown copyright 2004 Mechanism Coupled ocean-atmosphere interactions  Precipitation change associated with an ITCZ shift caused by SST anomalies supplies the tropical fresh water flux forcing  Coupled mechanism involving a delayed oceanic salinity feedback. From Vellinga and Wu (2004)

Page 18 Hadley Centre © Crown copyright 2004 Climate Impacts

Page 19 Hadley Centre © Crown copyright 2004 Climate Impacts North East Brazil Rainfall  NE Brazil has large multidecadal wet season (MAM) rainfall variability  Simulated ITCZ shifts north and away when N Atlantic warm (AMO+)  drier NE Brazil  Simulated rainfall changes similar in size to observations

Page 20 Hadley Centre © Crown copyright 2004 Climate Impacts Sahel Rainfall  African Sahel has large multidecadal rainfall variability  JJA simulated ITCZ shifts north when N Atlantic warm (AMO+)  wetter Sahel  Simulated changes about one-third of those observed.  Compare ITCZ shifts with Caribbean palaeo salinity variations (Schmidt et al., 2004).

Page 21 Hadley Centre © Crown copyright 2004 Climate Impacts North Atlantic-European circulation response to the AMO Simulated MSLP regression with AMO index DJF MAM JJA SON Simulated precipitation regression with AMO index No winter NAO signal at any lead/lag Anomalies typically smaller than observed multidecadal NAO change Broadest signal in summer and autumn Summer/Autumn signal in Europe Little sign of US summer signal (Sutton & Hodson,2005)

Page 22 Hadley Centre © Crown copyright 2004 Climate Impacts Atlantic Hurricanes – the observed relationship Goldenberg et al. (2001) claim a link between the frequency of major Atlantic hurricane formation and AMO variations in North Atlantic SST. Suggest AMO affects vertical shear in the hurricane formation region via circulation changes Major Hurricanes Emanuel (2005) suggests a more direct link between SST and the integrated intensity of storms.

Page 23 Hadley Centre © Crown copyright 2004 Climate Impacts Atlantic Hurricanes – obs model comparisons Model supports an AMO relationship with hurricane development shear, but also shows an IPO relationship. AMO and IPO are uncorrelated (0.06). NCEP/NCAR reanalysis hPa shear August-October (ASO) ( )-( ) HadCM3 decadal AMO-shear correlation Goldenberg main development area highlighted HadCM3 AMO index (red), versus mean Goldenberg area shear (black) Correlation of simulated main development area shear with SST

Page 24 Hadley Centre © Crown copyright 2004 Conclusions

Page 25 Hadley Centre © Crown copyright 2004 Conclusions  The AMO is inconsistent with an estimate of the response of Atlantic SST to natural and anthropogenic forcings from the AR4 models  Either the AMO is internal or the models or their forcings are wrong  This analysis shows an increasing AMO in recent decades  A 1400 year HadCM3 control simulation suggests the AMO is a long- lived coupled mode of climate variability associated with modern-day variations in the strength of the THC  Diagnosis of the simulated mechanism reveals a delayed salinity feedback via displacements of ITCZ rainfall caused by THC-related temperature anomalies  The simulation confirms AMO links with a range of important regional climate phenomena such as NE Brazil and Sahel rainfall, Atlantic Hurricane formation and European circulation.

Page 26 Hadley Centre © Crown copyright 2004 Questions & Answers

Page 27 Hadley Centre © Crown copyright 2004 Climate Impacts

Page 28 Hadley Centre © Crown copyright 2004 Reconstruction and Forecast of the THC

Page 29 Hadley Centre © Crown copyright 2004 Reconstruction and forecast of the THC Use HadCM3 simulation to make a statistical model between SST-THC Use SST from HadISST dataset to reconstruct running decadal THC   Decadal Northern North Atlantic SST as a statistical predictor

Page 30 Hadley Centre © Crown copyright 2004 Reconstruction and forecast of the THC  Look for points in the control simulation where the THC index rises through present day (decade ) reconstructed value (0.63 Sv)  Track the subsequent THC evolution for each of these ‘analogues’ for 6 decades.  Use these to represent the next ~35 years (observed period shorter than in model).  Natural downturn in THC in next decade, to levels of 1960s before 2030 (on average Sv) THC Predictability

Page 31 Hadley Centre © Crown copyright 2004 Motivation Large scale SST patterns (after Folland et al., 1999) HadISST Low-pass (> 13.3y) EOFs   40ºS - 70ºN  Projections

Page 32 Hadley Centre © Crown copyright 2004 Control Simulation Coupled Model Representation of the AMO Year band Year band 0° 60° 120° 180°

Page 33 Hadley Centre © Crown copyright 2004 AR4 Ensembles North Atlantic mean temperature  North Atlantic (0°-80°W, 10°-70°N)  Annual mean SST  4 member ensemble with HadCM3 (black)  Solar+Volc+Anthro. Stott et al. (2000)  Observed SST data from HadISST (blue) Centre year of 30-year trend Year Temperature (°C) Trend (°C decade -1 )  Anomalies difficult without bias  30 year trends  Uncertainty in ensemble mean trend  90% limits (shaded)  Inconsistent ( ) to ( )  Also ( ) to ( )  Uncertainty still large with 4 members

Page 34 Hadley Centre © Crown copyright 2004 Mechanism Salinity leading density anomalies  0-800m salinity contribution to density regressed onto zonal mean density at 60°N  First signs of positive salinity anomalies in subtropics 6 decades (half a period) before a THC peak

Page 35 Hadley Centre © Crown copyright 2004 Mechanism Salinity budget analyses  0-800m mean salinity driven density tendencies regressed onto the THC  In tropics (0-35°N) density increases ~ 6 decades before the peak THC, induced by surface flux forcing and removed by transport  In mid-latitudes (35-48°N) density increases ~ 4 decades before, caused by transport and removed by surface flux forcing  Sub-polar (48-65°N) density increases ~ 2 decades before by transport

Page 36 Hadley Centre © Crown copyright 2004 Mechanism Transport time scale  100 year run with a unit tracer at the surface between 0-15°N  Follow tracer concentrations averaged over 0-800m  Slow buildup in sub-polar ocean