An Orbitally Driven Tropical Source for Abrupt Climate Change Amy C. Clement, Mark A. Cane and...

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

General picture (for the winter)

El niño

La niña

Sea surfacetemperature

Surface airtemperature

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

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