an orbitally driven tropical source for abrupt climate change
DESCRIPTION
Amy C. Clement, Mark A. Cane and Richard Seager by Jasmine Rémillard November 8, 2006. An Orbitally Driven Tropical Source for Abrupt Climate Change. Introduction. Climate has undergone abrupt changes Those changes occurred within decades No external forcing that fast. - PowerPoint PPT PresentationTRANSCRIPT
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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
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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
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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
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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
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General picture (for the winter)
El niño
La niña
Sea surfacetemperature
Surface airtemperature
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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
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More experiments
● Changing the Earth's orbital parameters (Milankovitch forcing)
➔Changes in seasonal cycle
➔ Anomalous heat flux into the ocean
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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
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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
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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
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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
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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
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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
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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
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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