An Orbitally Driven Tropical Source for Abrupt Climate Change

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An Orbitally Driven Tropical Source for Abrupt Climate Change. Amy C. ... are near a negative maximum of the 1st EOF (perihelion occurs near boreal winter) ...
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Title: An Orbitally Driven Tropical Source for Abrupt Climate Change 1 An Orbitally Driven Tropical Source for Abrupt Climate Change
  • Amy C. Clement, Mark A. Cane
  • and Richard Seager
  • by Jasmine Rémillard
  • November 8, 2006
  • 2 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
  • 3 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
  • 4 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
  • 5 General picture (for the winter) Sea surface temperature Surface air temperature El niño La niña 6 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
  • 7 More experiments
  • Changing the Earth's orbital parameters (Milankovitch forcing)
  • Changes in seasonal cycle
  • Anomalous heat flux into the ocean
  • 8 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
  • 9 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
  • 10 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
  • 11 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
  • 12 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
  • 13 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
  • 14 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
  • 15 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
  • Recommended
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