Decadal and multidecadal climate variability in the Atlantic sector observational analyses, modeling

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Decadal and multidecadal climate variability in
theAtlantic sector - observational analyses,
modeling studies, and impacts over continental
regions Thomas L. DelworthGFDL/NOAA

• Why study decadal variability?
• Sources of decadal variability
• Atlantic multidecadal variability observations
  and models
• Climatic impacts of Atlantic multidecadal
  variability
• Predictability

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Why study decadal climate variability?

• Societal relevance eg, Dust Bowl, hurricanes
• Decadal variability may modulate interannual
  variability
• Possibility of predictability of decadal scale
  climate variability
• Timescale on which we begin to see human impacts
  on climate

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Sources of decadal variability

• Internal variability of the coupled
  ocean-atmosphere-land-ice system
• Preferred patterns of variability (space and
  time) arising from dynamics of climate system.
• Examples El Nino-Southern Oscillation, Annular
  Modes
• Changing radiative forcing
• Natural forcing from solar and volcanic activity
• Anthropogenic emissions of greenhouse gases and
  aerosols
• ? Observed records of climate are a combination
  of internal variability and forced climate
  change.

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Sources of decadal variability (continued)

• On decadal time scales, ocean is thought to play
  crucial role.
• Longer inherent timescales provide memory to
  the climate system
• Much decadal variability research has looked to
  the ocean as a source of decadal variability,
  with possible atmospheric interactions and
  impacts
• Research has tended to examine patterns (and
  possible mechanisms) of decadal variability that
  focus on particular ocean basin its inherent
  dynamics.
• We focus in this talk mainly on the Atlantic
• Observed variability
• Simulated variability
• Impacts on climate, particularly over continental
  regions

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North Atlantic SST ASO (Aug-Oct) 75W-7.5W,
10N-60N 15 year lowpass filter Anomalies from
1870-1969 time-mean
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Extratropical North Atlantic 75W-7.5W, 30N-60N
Aug-Oct HADISST
Tropical North Atlantic 75W-7.5W, 10N-25N
Warm
Warm
Cold
Warm
Cold
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1905-1925 minus 1881-1900
1935-1960 minus 1905-1925
1971-1990 minus 1935-1960
1996-2005 minus 1971-1990
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North Atlantic Temperature
Atlantic Meridional Overturning Circulation
(AMOC)
What will the next decade or two bring?
Warm North Atlantic linked to
Drought
More rain over Sahel and western India
More intense hurricanes

• Two important aspects
• Decadal-multidecadal fluctuations
• Long-term trend

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Atlantic Multidecadal Variability streams of
evidence

• INSTRUMENTAL ANALYSES
• Bjerknes (1964), Folland (1984,1986), Kushnir
  (1994),
• - Multidecadal Atlantic SST fluctuations have
  large-scale spatial structure
• PALEO/PROXY ANALYSES
• - Enhanced multidecadal variability in a wide
  array of proxy indicators in/around the North
  Atlantic
• MODEL SIMULATIONS
• - Most coupled climate models contain enhanced
  multidecadal variability in the Atlantic,
  associated with MOC fluctuations

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Delworth and Mann, 2000
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What do models tell us?Many models simulate
enhanced multidecadal variability involving
Atlantic MOC Similar spatial structure as
observations Differing timescales in the
multidecadal range Large-scale atmospheric
impact

• GFDL R15, R30 40-80 years (Delworth et al.,
  1993, 1997)
• GFDL CM2.1 20 years
• HADCM3 25 years (Dong and Sutton, 2005)
• HADCM3 centennial (Vellinga and Wu, 2004
  Knight et al., 2005)
• NCAR CCSM3 20 years (Danabasoglou, personal
  communication)
• ECHAM3 35 years (Timmermann et al., 1998)
• ECHAM5 70-80 years (Jungclaus et al., 2005)
• Theoretical work in hierarchy of models te Raa
  et al. (2004)
• Multiple physical processes influencing the
  Atlantic THC may contribute
• to the variety of timescales found.

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SST anomalies associated with interdecadal MOC
fluctuations
MODEL
On decadal timescales, most current models
underestimate SST variability in the ACR and
overestimate variability in the PCR. Santer et
al., 2006
Modest Tropical Amplitude
EOF 1 HADISST OBSERVED SST
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Knight et al., GRL, 2005
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JJA Precipitation Anomalies Associated with
Maximum MOC
Units cm/day
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Hybrid coupled model (based on GFDL CM2.1)
Global Atmosphere/Land System
Heat Water Momentum
Heat
Heat Water Mom.
Atlantic Slab Ocean
Pacific Dynamic Ocean
Indian Dynamic Ocean
Constant Flux Adjustment
Time varying heating to induce AMO-like SST
variations
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Simulated multidecadal JJAS surface air
temperature difference (K) (1931-1960)
(1961-1990)
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Regression of modeled LF JJAS Rainfall Anomaly on
modeled AMO Index
Modeled AMO Index
Regression of observed LF JJAS Rainfall Anomaly
(CRU data) on observed AMO Index
Observed AMO Index
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Reduction in Wind Shear when North Atlantic Warms
relative to South Atlantic
Observations
Model (GFDL CM2.1)
Zhang and Delworth, GRL,2006
Red means less shear, and therefore more
favorable conditions for hurricanes. CONCLUSION
Models demonstrate that a warming North Atlantic
(relative to the South Atlantic) CAUSES
atmospheric circulation changes that are
favorable for tropical activity.
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Modeled AMO Index
Observed AMO Index
Modeled LF anomalous ASO vertical shear of zonal
wind (m/s)
Observed LF major Hurricane number anomaly and LF
anomalous ASO vertical shear of zonal wind (m/s)
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Ensemble starting at year 1101
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Attempts to decompose observed Atlantic signal
into forced and internal variability components
very difficult to do.
Atlantic MDR SST
Observed SST
Indo-Pacific SST
Indian Ocean SST
MDR minus Indian
MDR minus model forced with ANTHRO
Estimated internal variability contribution to
Atlantic MDR SST
MDR minus Indo-Pacific
Mann and Emmanuel
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Observed SST Changes 1995-2003 minus 1966-1985
Modeled response to radiative forcing changes
(GFDL CM2.1 model)
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Summary/Conclusions

• Substantial observational evidence for enhanced
  multidecadal variability in the North Atlantic.
• Observed changes are a mixture of internal
  variability and forced climate change.
• Models simulate enhanced North Atlantic
  multidecadal variability associated with MOC.
• Atlantic variability has large-scale climate
  impacts
• - Wind shear in tropical Atlantic (hurricanes)
• - Rainfall over Africa to India
• - Heat and drought over North America
• - Northern Hemisphere temperature
• Indications of possible predictability
  vigorously pursuing this topic
• - models, observing systems, and theoretical
  understanding

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North Atlantic Temperature
Atlantic Meridional Overturning Circulation
(AMOC)
What will the next decade or two bring?
Warm North Atlantic linked to
Drought
More rain over Sahel and western India
More intense hurricanes

• Two important aspects
• Decadal-multidecadal fluctuations
• Long-term trend

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(No Transcript)
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Zhang et al., in press
Atlantic constrained experiment (ACE)
Radiatively forced experiment (RFE)
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Directions and needed activities

• Sustained observation systems ARGO looks quite
  promising
• Predictability experiments - what might be
  predictable (given current capabilities)
• Improved models physics and resolution!
• Theoretical work on dynamical underpinning of
  phenomena that may give rise to decadal
  predictability (AMO and others)

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Observed SST Changes 1966-1985 minus 1936-1965
Modeled in response to radiative forcing changes
(GFDL CM2.1 model)
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Observed SST Changes 1936-1955 minus 1906-1925
Modeled in response to radiative forcing changes
(GFDL CM2.1 model)