An%20Orbitally%20Driven%20Tropical%20Source%20for%20Abrupt%20Climate%20Change

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An Orbitally Driven Tropical Source for Abrupt
Climate Change

• Amy C. Clement, Mark A. Cane
• and Richard Seager
• by Jasmine Rémillard
• November 8, 2006

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Introduction

• Climate has undergone abrupt changes
• Those changes occurred within decades
• No external forcing that fast

• from internal processes
• or
• a rapid response to gradual external forcing

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Example Younger Dryas

• Common explanation
• Meltwater pulses from the retreating Laurentide
  ice sheet

• New explanation
• Changes in tropical climate (like ENSO)

• Reason
• Have global impacts on interannual timescales in
  present days

• Problems
• Meltwater pulse prior to the onset and after its
  end
• Deep water formation weaken way before
• Ocean circulation recovered only after
• Deep water formation take a long time to respond
• Impacts on wide regions of the globe

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What is ENSO

• El Niño/Southern Oscillation
• Related to the SST of the equatorial Pacific
• 2 phases
• El niño warmer SST
• La niña cooler SST
• Cause by anomalous equatorial winds over the
  Pacific ocean
• Cause of those anomalies is unknown
• Long-range effect because of the change in the
  evaporation/precipitation over the equator

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General picture (for the winter)
Sea surface temperature
Surface air temperature
El niño
La niña
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Modeling experiments

• Coupled ocean-atmosphere interactions in the
  tropical Pacific
• Linear dynamics
• Nonlinear thermodynamics

• Reproduces well the behavior of the present day
  ENSO
• Quasiperiodic
• Irregular
• Partially locked to the seasonal cycle

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More experiments

• Changing the Earth's orbital parameters
  (Milankovitch forcing)
• Changes in seasonal cycle
• Anomalous heat flux into the ocean

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Decomposing the solar forcing

• First two EOFs describe the precession through
  the year of the perihelion, with most of the
  total variance
• We are near a negative maximum of the 1st EOF
  (perihelion occurs near boreal winter)
• Positive 2nd EOF results in a strengthening of
  the seasonal cycle in the equatorial Pacific

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2 regimes of ENSO behavior

• Increased seasonal cycle strength
• Strong oscillation
• Highly regular
• Period of 3 years

• Damped seasonal cycle
• Strong oscillation
• Fairly irregular
• Period of 4 years

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Transition

• Minimum in total variance
• Oscillations moderately regular
• Happens when perihelion is in winter or summer

• Return period of 11 kyr
• No clearly defined mode of behavior
• Episodically lock to the period of the forcing (1
  yr)
• Shutdown of ENSO
• Maximal length when weak eccentricity
• Not guaranteed to happen
• No preferred timescale

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Shutdowns

• Some orbital configurations lead to an abrupt
  locking of the ENSO variability to the seasonal
  cycle (shutdown)
• Mean SST similar to a La Niña event
• Recurs every 11 kyr (½ precession cycle)
• Variable duration
• One of them occurred 12 kyr ago
• Coincides with the Younger Dryas

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Robustness

• Alteration of the drag coefficient (Cd)
• Measure of the surface wind stress anomalies
• Controls the effective dynamical coupling

• Under modern orbital configuration
• Cd90-100 ? chaotic regime
• Cd80 ? mode locked
• Cdlt80 ? no coupled instability and oscillation
• Cd110 ? stronger and less regular

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More robustness

• Under the orbital forcing
• Cd90
• Regimes qualitatively similar
• More dramatic shutdowns
• Cdlt90
• Always in shutdown
• Cd110
• Regimes qualitatively similar
• Doesn't lock (no shutdown)

• Thus, it is a nonlinear dynamical regime

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Conclusions

• Smoothly variable orbital forcing can provoke
  abrupt climate response
• Character of the response depends on the value of
  Cd and the presence of noise
• Heinrich events could also be paced by the solar
  forcing
• Younger Dryas would be a return of these
  orbitally paced events

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Future

• More complete models
• Influence of additional processes
• Further investigation of the link between abrupt
  climate change and orbital forcing
• Modeling and observational perspectives
• Nature of abrupt climate change
• Possible future behavior of ENSO