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amy c clement mark a cane and richard seager by jasmine r millard november 8 2006 n.
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An Orbitally Driven Tropical Source for Abrupt Climate Change PowerPoint Presentation
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An Orbitally Driven Tropical Source for Abrupt Climate Change

An Orbitally Driven Tropical Source for Abrupt Climate Change

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An Orbitally Driven Tropical Source for Abrupt Climate Change

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  1. Amy C. Clement, Mark A. Cane and Richard Seager by Jasmine Rémillard November 8, 2006 An Orbitally Driven Tropical Source for Abrupt Climate Change

  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. New explanation : Changes in tropical climate (like ENSO) Example – Younger Dryas • Common explanation : • Meltwater pulses from the retreating Laurentide ice sheet • 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. Damped seasonal cycle Strong oscillation Fairly irregular Period of 4 years 2 regimes of ENSO behavior • Increased seasonal cycle strength • Strong oscillation • Highly regular • Period of 3 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 • Cd=90%-100%  chaotic regime • Cd=80%  mode locked • Cd<80%  no coupled instability and oscillation • Cd=110%  stronger and less regular

  13. More robustness • Under the orbital forcing • Cd=90% • Regimes qualitatively similar • More dramatic shutdowns • Cd<90% • Always in shutdown • Cd=110% • 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