Transcript rapid climate change - Atmospheric and Oceanic Sciences

An Orbitally Driven Tropical
Source for Abrupt Climate
Change
Amy C. Clement, Mark A. Cane
and Richard Seager
by Jasmine Rémillard
November 8, 2006
Introduction
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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
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Common explanation :
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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 :
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Changes in tropical
climate (like ENSO)
Reason :
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Have global impacts on
interannual timescales in
present days
What is ENSO
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El Niño/Southern Oscillation
Related to the SST of the equatorial Pacific
2 phases
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Cause by anomalous equatorial winds over
the Pacific ocean
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El niño : warmer SST
La niña : 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
niño
La
niña
Surface air
temperature
Modeling experiments
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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
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Changing the Earth's orbital parameters
(Milankovitch forcing)
➔ Changes
in seasonal cycle
➔Anomalous
heat flux into the ocean
Decomposing the solar forcing
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First two EOFs describe the precession
through the year of the perihelion, with most
of the total variance
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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
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Increased seasonal
cycle strength
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Strong oscillation
Highly regular
Period of 3 years
Damped seasonal
cycle
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Strong oscillation
Fairly irregular
Period of 4 years
Transition
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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)
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Shutdown of ENSO
Maximal length when weak eccentricity
Not guaranteed to happen
No preferred timescale
Shutdowns
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Some orbital configurations lead to an abrupt
locking of the ENSO variability to the
seasonal cycle (shutdown)
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Mean SST similar to a La Niña event
Recurs every ~11 kyr (½ precession cycle)
Variable duration
One of them occurred ~12 kyr ago
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Coincides with the Younger Dryas
Robustness
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Alteration of the drag coefficient (Cd)
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Measure of the surface wind stress anomalies
Controls the effective dynamical coupling
Under modern orbital configuration
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Cd=90%-100%  chaotic regime
Cd=80%  mode locked
Cd<80%  no coupled instability and oscillation
Cd=110%  stronger and less regular
More robustness
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Under the orbital forcing
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Cd=90%
➔Regimes qualitatively similar
➔More dramatic shutdowns
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Cd<90%
➔Always
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in shutdown
Cd=110%
➔Regimes qualitatively similar
➔Doesn't lock (no shutdown)
➔ Thus,
it is a nonlinear dynamical regime
Conclusions
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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
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More complete models
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Influence of additional processes
Further investigation of the link between
abrupt climate change and orbital forcing
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Modeling and observational perspectives
Nature of abrupt climate change
Possible future behavior of ENSO