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An Orbitally Driven Tropical Source for Abrupt Climate
Change
Amy C. Clement, Mark A. Cane and Richard Seager
by Jasmine RémillardNovember 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
● 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
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
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 Niña event Recurs every ~11 kyr (½ 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