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