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Decadal and Multidecadal Climate Variability in the Atlantic - DecVar_


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									       Decadal and multidecadal climate variability in the
Atlantic sector - observational analyses, modeling studies, and
                impacts over continental regions

                               Thomas L. Delworth

 1.   Why study decadal variability?

 2.   Sources of decadal variability

 3.   Atlantic multidecadal variability – observations and models

 4.   Climatic impacts of Atlantic multidecadal variability

 5.   Predictability
     Why study decadal climate variability?

•   Societal relevance – eg, Dust Bowl, hurricanes

•   Decadal variability may modulate interannual variability

•   Possibility of predictability of decadal scale climate

•   Timescale on which we begin to see human impacts on
               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.
    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
North Atlantic SST ASO (Aug-Oct)
75W-7.5W, 10N-60N
15 year lowpass filter
Anomalies from 1870-1969 time-mean
Extratropical North Atlantic
75W-7.5W, 30N-60N

Tropical North Atlantic
75W-7.5W, 10N-25N

 Warm          Cold            Warm   Cold   Warm
1905-1925   1935-1960
minus       minus
1881-1900   1905-1925

1971-1990   1996-2005
minus       minus
1935-1960   1971-1990
                                                        North Atlantic Temperature
Atlantic Meridional
Overturning Circulation
    (AMOC)                                    What will the next decade
                                                   or two bring?

          Warm North Atlantic
             linked to …

      More intense hurricanes   More rain
                                over Sahel
                                and western

                                                 Two important aspects:
                                                 a. Decadal-multidecadal fluctuations
                                                 b. Long-term trend
            Atlantic Multidecadal Variability
                      … streams of evidence

     Bjerknes (1964), Folland (1984,1986), Kushnir (1994), …
     - Multidecadal Atlantic SST fluctuations have large-scale
         spatial structure

      - Enhanced multidecadal variability in a wide array of
      proxy indicators in/around the North Atlantic

      - Most coupled climate models contain enhanced
      multidecadal variability in the Atlantic, associated with
      MOC fluctuations
Delworth and Mann, 2000
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.
            SST anomalies associated with interdecadal MOC fluctuations

                                               “On decadal timescales, most current
                                               models underestimate SST variability
                                               in the ACR and overestimate
                                               variability in the PCR.”
                                               Santer et al., 2006


                  EOF 1 HADISST
                  OBSERVED SST
Knight et al., GRL, 2005
JJA Precipitation Anomalies Associated with Maximum MOC

                                  Units: cm/day
             Hybrid coupled model
                      (based on GFDL CM2.1)

            Global Atmosphere/Land System
Heat Water Momentum             Heat           Heat Water Mom.

     Pacific                    Atlantic            Indian
  Dynamic Ocean               Slab Ocean         Dynamic Ocean

                         Constant Flux   Time varying heating
                         Adjustment      to induce AMO-like SST
Simulated multidecadal JJAS surface air temperature
      difference (K) (1931-1960) –(1961-1990)
  Regression of modeled LF JJAS Rainfall   Modeled AMO Index
  Anomaly on modeled AMO Index

Regression of observed LF JJAS Rainfall
Anomaly (CRU data) on observed AMO Index       Observed AMO Index
Reduction in Wind Shear when North Atlantic Warms relative to South Atlantic

     Observations                                    Model (GFDL CM2.1)

                                                     Zhang and Delworth,

    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.
           Observed AMO Index                            Modeled AMO Index

Observed LF major Hurricane number anomaly and        Modeled LF anomalous ASO vertical
LF anomalous ASO vertical shear of zonal wind (m/s)   shear of zonal wind (m/s)
Ensemble starting at year 1101
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 model forced with ANTHRO                   MDR minus Indian

                                                                                    Estimated internal
                                                           MDR minus Indo-Pacific
                                                                                    variability contribution
                                                                                    to Atlantic MDR SST

                                           Mann and Emmanuel
Observed SST Changes: 1995-2003 minus 1966-1985

        Modeled response to radiative forcing changes
              (GFDL CM2.1 model)

1.   Substantial observational evidence for enhanced multidecadal
     variability in the North Atlantic.

2.   Observed changes are a mixture of internal variability and forced
     climate change.

3.   Models simulate enhanced North Atlantic multidecadal variability
     associated with MOC.

4.   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

5.   Indications of possible predictability – vigorously pursuing this topic
        - models, observing systems, and theoretical understanding
                                                        North Atlantic Temperature
Atlantic Meridional
Overturning Circulation
    (AMOC)                                    What will the next decade
                                                   or two bring?

          Warm North Atlantic
             linked to …

      More intense hurricanes   More rain
                                over Sahel
                                and western

                                                 Two important aspects:
                                                 a. Decadal-multidecadal fluctuations
                                                 b. Long-term trend
                                         Zhang et al.,
 Atlantic constrained experiment (ACE)
                                         in press

Radiatively forced experiment (RFE)
        Directions and needed activities

• Sustained observation systems – ARGO looks quite

• 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)
Observed SST Changes: 1966-1985 minus 1936-1965

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

        Modeled in response to radiative forcing changes
              (GFDL CM2.1 model)

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