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Transcript of Page 1 Hadley Centre © Crown copyright 2004 Evidence for the Atlantic Multidecadal Oscillation as...
© Crown copyright 2004 Page 1
Hadley Centre
Evidence for the Atlantic Multidecadal Oscillation as an internal climate mode from
coupled GCM simulationsJeff Knight
Hadley Centre, Exeter, UK
4th International CLIVAR Climate of the 20th Century Workshop,
Hadley Centre, Exeter, UK
Wednesday, 14th March 2007
© Crown copyright 2004 Page 3
Hadley Centre
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 1870-1999
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?
© Crown copyright 2004 Page 5
Hadley Centre
AR4 Ensembles
Single model North Atlantic mean SST
Grey = annual means for 3 ensemble membersRed = ensemble meanBlack = 90% limits of estimated ens meanBlue = Observed SST from HadSST2All data relative to 1900-99 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
© Crown copyright 2004 Page 6
Hadley Centre
AR4 Ensembles
Multi-model ensemble
Red = super-ensemble mean (34 members)Black = 90% limits of estimated ens meanBlue = Observed SST from HadSST2All data relative to 1900-99 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
© Crown copyright 2004 Page 7
Hadley Centre
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
AR4AVG
OBS minusAR4
Linear change = Trend (K/year) x Period (year)
© Crown copyright 2004 Page 8
Hadley Centre
AR4 Ensembles
A better AMO index?
Black = 90% limits of estimated ens meanBlue = Observed SST from HadSST2All data relative to 1900-99 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
© Crown copyright 2004 Page 9
Hadley Centre
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*
© Crown copyright 2004 Page 11
Hadley Centre
Control Simulation
1400 Year Coupled Model Representation of the AMO70-180 Year band Observed 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.
© Crown copyright 2004 Page 12
Hadley Centre
Control Simulation
1400 Year HadCM3 control simulation
Maximum overturning streamfunction at 30°N Persistent band of variability between 70-120 years Compares with observed period of ~65 years (instrumental) and 40-130 years (palaeo – Gray et al. 2004).
© Crown copyright 2004 Page 14
Hadley Centre
Control Simulation
THC-Mean temperature cross-correlations
NorthernHemisphere
SouthernHemisphere
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
© Crown copyright 2004 Page 16
Hadley Centre
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)
© Crown copyright 2004 Page 17
Hadley Centre
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)
© Crown copyright 2004 Page 19
Hadley Centre
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
© Crown copyright 2004 Page 20
Hadley Centre
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).
© Crown copyright 2004 Page 21
Hadley Centre
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)
© Crown copyright 2004 Page 22
Hadley Centre
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
1944 1998
Ma
jor
Hu
rric
an
es
Emanuel (2005) suggests a more direct link between SST and the integrated intensity of storms.
© Crown copyright 2004 Page 23
Hadley Centre
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 200-850
hPa shear
August-October (ASO) (1951-60)-(1971-80)
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
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Hadley Centre
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.
© Crown copyright 2004 Page 29
Hadley Centre
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
1870-2002
Decadal Northern North Atlantic SST as a
statistical predictor
© Crown copyright 2004 Page 30
Hadley Centre
Reconstruction and forecast of the THC
Look for points in the control simulation where the THC index rises through present day (decade 1997-2003) 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 -0.70 Sv)
THC Predictability
© Crown copyright 2004 Page 31
Hadley Centre
Motivation
Large scale SST patterns (after Folland et al., 1999)
HadISST Low-pass (> 13.3y) EOFs
1911-2002
40ºS - 70ºN
Projections 1870-2002
© Crown copyright 2004 Page 32
Hadley Centre
Control Simulation
Coupled Model Representation of the AMO70-180 Year band 25-125 Year band
0°
60°
120°
180°
© Crown copyright 2004 Page 33
Hadley Centre
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
Te
mp
era
ture
(°C
)T
ren
d (
°C d
ec
ad
e-1)
Anomalies difficult without bias 30 year trends Uncertainty in ensemble mean trend 90% limits (shaded) Inconsistent (1900-1930) to (1925-1955) Also (1945-1975) to (1965-1995) Uncertainty still large with 4 members
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Hadley Centre
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
© Crown copyright 2004 Page 35
Hadley Centre
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