An Orbitally Driven Tropical Source for Abrupt Climate Change

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