An Orbitally Driven Tropical Source for Abrupt Climate Change

An Orbitally Driven Tropical Source for Abrupt Climate Change

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

Example Younger Dryas Common explanation : Meltwater pulses from the retreating Laurentide ice sheet 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 New explanation : Changes in tropical climate (like ENSO)

Reason : Have global impacts on interannual timescales in present days What is ENSO El Nio/Southern Oscillation Related to the SST of the equatorial Pacific 2 phases Cause by anomalous equatorial winds over

the Pacific ocean El nio : warmer SST La nia : cooler SST Cause of those anomalies is unknown Long-range effect because of the change in the evaporation/precipitation over the equator General picture (for the winter) Sea surface temperature El nio La nia

Surface air temperature 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 More experiments Changing the Earth's orbital parameters (Milankovitch forcing) Changes in seasonal cycle Anomalous heat flux into the ocean 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 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 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 Shutdowns Some orbital configurations lead to an abrupt locking of the ENSO variability to the seasonal cycle (shutdown)

Mean SST similar to a La Nia event Recurs every ~11 kyr ( precession cycle) Variable duration One of them occurred ~12 kyr ago Coincides with the Younger Dryas 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 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 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

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

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