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 ● 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 surface temperature Surface air temperature

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 1 st EOF (perihelion occurs near boreal winter)  Positive 2 nd 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 (C d )  Measure of the surface wind stress anomalies  Controls the effective dynamical coupling ● Under modern orbital configuration  C d =90%-100%  chaotic regime  C d =80%  mode locked  C d <80%  no coupled instability and oscillation  C d =110%  stronger and less regular

More robustness ● Under the orbital forcing  C d =90% ➔ Regimes qualitatively similar ➔ More dramatic shutdowns  C d <90% ➔ Always in shutdown  C d =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 C d 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