Evolution of the El Ni ño Southern Oscillation (ENSO) from the Last Ice Age to Today

Evolution of the El Niño Southern Oscillation (ENSO) from the Last Ice Age to Today

Andy BushDept. of Earth & Atmospheric Sciences University of Alberta

Winds of Change:



Winds of Change:

A.B.G. Bush, 2006, Journal of Climate, in press.



Winds of Change:

A.B.George Bush, 2006, Journal of Climate, in press.



Winds of Change:

A.B.George Bush, 2006, Journal of Climate, in press. [email protected]

Motivation: To understand the climatological factorsthat determine the period and intensity of interannualvariability (ENSO).

Motivation: To understand the climatological factorsthat determine the period and intensity of interannualvariability (ENSO).

Past climates provide altered mean states within which interannual variability exists.

Animation of TOPEX/Poseidon sea surface height data

QuickTime™ and aCinepak decompressor

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Some human impacts of ENSO: 1) Impact on disease spread (malaria and dengue) 2) Food production

El Niño winter snow anomalies

Observed CompositeTemperature Anomalies

El Niño

La Niña

SST anomalies, Aug.-Sept. 2006: El Niño’s coming…

Existing numerical models for ENSO prediction are anomaly models in which a background climate state is assumed. Predicted variables are perturbations on that background state. The two climate variables that must be assumed are:1) Mean depth of the thermocline2) Strength of the climatological easterly trade winds

These quantities are known for today’s climate, so anomaly models work quite well for ENSO prediction.

Existing numerical models for ENSO prediction are anomaly models in which a background climate state is assumed. Predicted variables are perturbations on that background state. The two climate variables that must be assumed are:1) Mean depth of the thermocline2) Strength of the climatological easterly trade winds

These quantities are known for today’s climate, so anomaly models work quite well for ENSO prediction.

However, one or both of these quantities appear to havebeen different in the past (aeolian deposits, upwelling indices,planktonic foraminifera, etc.). Changes in the strength of thegeneral circulation can cause changes in these quantities.

Our atmosphere exhibits dynamic variability associated with midlatitude baroclinic waves, or eddies.

Our atmosphere exhibits dynamic variability associated with midlatitude baroclinic waves, or eddies.

Eddies may be either TRANSIENT (not fixed to a specificgeographic location) or STATIONARY (fixed geographically; caused by mountain ranges, continent-ocean contrasts, etc.)

Eddies play a very important role in governing the strength of the general circulation.

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Atmospheric eddies are the primary mechanism by which low latitude HEAT is transported poleward (v’T’>0). This occurs in the growth phase of baroclinic waves. (Idealized life cycle)

Atmospheric eddies are the primary mechanism by which low latitude HEAT is transported poleward (v’T’>0). This occurs in the growth phase of baroclinic waves.

They are also the primary mechanism by which the zonal mean(and, by angular momentum conservation, the meridional mean)flow is forced (u’v’>0). This occurs in the Rossby wave decayphase of the baroclinic wave, in which easterly momentum istransported equatorward.



Global Implications?



Global Implications?

More Eddy ActivityStronger Circulation

Eddy activity depends on the meridional temperature gradients of the climatological background state.

Stronger temperature gradients increase the rate of eddy formation (can be shown from linear theory).



Meridional temperature gradients were quite different in the past for a variety of reasons (ice sheets, orbitalparameters, greenhouse gases, etc.).



Meridional temperature gradients were quite different in the past for a variety of reasons (ice sheets, orbitalparameters, greenhouse gases, etc.).

Also, during an Ice Age, topographic forcing of stationary waves was very different because of the massive ice sheets.

The numerical experiments

A global coupled atmosphere-ocean general circulation model isused to simulate 80 years of climate for:



1) Last Glacial Maximum (LGM, 21,000 years ago) -massive continental ice sheets -decreased atmospheric carbon dioxide -sea level lowering of 120 m -surface vegetation different

Schematics of ice sheet extent at the Last Glacial Maximum




2) 9,000 years ago -orbital parameters -remnants of Laurentide ice sheet

Obliquity was high in the early-mid Holocene (9,000-6,000 years ago).This accentuates the seasonal cycle; warmer summers and colder winters.





3) 6,000 years ago -orbital parameters






4) Today (control)






4) Today (control)

5) Doubling of atmospheric carbon dioxide (2xCO2)

El Niño and La Niña events are defined by sea surfacetemperature anomalies in the Nino 3.4 region. Valuesare typically normalized by the standard deviation.

Control simulation produces good statistics for ENSO.

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S.D.=0.83

S.D.=0.87

Control

Observations

Wavelet analysisPower Spectrum:

Averaged in time:

Observations

S.D.=0.83

S.D.=0.87

Control

Observations

Values Normalized byTheir standardDeviation

LGM

9,000 B.P.

6,000 B.P.

Control

Observations

2xCO2

Values NOTNormalized byThe standardDeviation

S.D.=0.58

S.D.=0.55

S.D.=0.81

S.D.=0.83

S.D.=0.87

S.D.=1.05

IncreasingPeriod of ENSO

(decrease inFrequency)

Not much changein mean thermoclinedepth (except forCO2 case)

Change in east-west tiltof thermocline consistentwith change in strength of mean atmospheric trade winds

Changes in the climatological mean states

Not much changein mean thermoclinedepth (except forCO2 case)

Change in east-west tiltof thermocline consistentwith change in strength of mean atmospheric trade winds

Changes in the climatological mean states

~20% reduction in Easterly trade winds from LGM to today

Linear Stability Analysis of the coupled atmosphere-ocean system (Fedorov and Philander, Science, 2001)

(increases withdecreasing windspeed)

Control

Growth Rate

PERIOD

Linear Stability Analysis of the coupled atmosphere-ocean system (Fedorov and Philander, Science, 2001)

(increases withdecreasing windspeed)

Control

Growth Rate

PERIOD

* LGM* 9,000* 6,000

*2xCO2

Is there a reason why the strength of the atmospheric winds should be different in these simulations?


Yes. The eddy fields are very different.



LGM: enhanced meridional temperature gradient (transient eddies) presence of massive continental ice sheets (stationary eddies)




9,000 and 6,000 B.P.: more seasonal climate because of enhanced obliquity of the planet and summertime perihelion. Effect is stronger at 9,000 B.P. than 6,000 B.P. Hotter summers and colder winters should produce more wintertime transient eddy activity.




9,000 and 6,000 B.P.: more seasonal climate because of enhanced obliquity of the planet and summertime perihelion. Effect is stronger at 9,000 B.P. than 6,000 B.P. Hotter summers and colder winters should produce more wintertime transient eddy activity.

CO2: reduced meridional temperature gradient (transient eddies)

The Eliassen-Palm (E-P) flux vector:

Z

Y

Poleward heat flux(v’T’>0)

Equatorward momentum flux(u’v’>0)

E-P flux

Increased midlatitudeeddies drive strongersubtropical subsidence,a stronger Hadleycell and, through angular momentumconservation,stronger equatorialeasterlies.

Peak easterly (i.e. negative)winds over the Pacificoccur near the solstices ofend-December and end-June when baroclinic eddy activity is greatest

Coupled modelTAO winds

Zonally symmetricatmosphere-onlyintegration for aland-covered planet--wind changes are notrelated to the Asian monsoon.

Changing TeleconnectionPatterns:Temperature

LGM

9K BP

6K BP

Today

2xCO2

LGM

9K BP

6K BP

Today

2xCO2

ChangingTeleconnection Patterns:Precipitation

Implications of changing teleconnection patterns:


Caution must be used when interpreting paleoclimate proxy recordsfor ENSO. For example, Rodbell et al (1999) interpreted the absence of a distinct ENSO signal from early Holocene sediments ofcoastal Peru to mean that ENSO was absent between 15,000 and~6,000 years ago. This assumed “stationarity” of the teleconnectionpattern is incorrect.



Also, Koutavas et al (2002) interpreted a reduced zonal SST gradientin the tropical Pacific at the LGM to mean the glacial climate was in an El Niño state. This assumption does not take into account the changed mean state of the LGM climate.



Also, Koutavas et al (2002) interpreted a reduced zonal SST gradientin the tropical Pacific at the LGM to mean the glacial climate was in an El Niño state. This assumption does not take into account the changed mean state of the LGM climate.

Teleconnections produce high latitude fingerprints of ENSO changes.

Conclusions

The period of ENSO increases from the LGM to today

Conclusions


The amplitude of ENSO increases from the LGM to today

Conclusions



These changes are consistent with the decrease in strength of the climatological easterly trade winds overthe Pacific

Conclusions




Decrease in trade wind strength is consistent with thedecrease in midlatitude eddy activity

Conclusions




Decrease in trade wind strength is consistent with thedecrease in midlatitude eddy activity

Decrease in eddy activity related to topographic forcingat the LGM, to orbital forcing at 9,000 and 6,000 B.P., andto radiative forcing in the 2xCO2 environment

What’s been observed?

Increased tropical Pacific wind speeds during the LGM(aeolian deposits, upwelling indices from ocean cores)



Increased tilt of the tropical Pacific thermocline duringthe LGM (planktonic foraminifera)




Increase in ENSO amplitude and period from LGM to todayfrom corals (Tudhope et al, 2001)




Increase in ENSO amplitude and period from LGM to todayfrom corals (Tudhope et al, 2001)

Decrease in easterly trade wind strength (Nature, May 2006)

With many thanks to…

Canadian Foundation for Climate andAtmospheric Research (CFCAS; PolarClimate Stability Network)

With many thanks to…

Canadian Foundation for Climate andAtmospheric Research (CFCAS; PolarClimate Stability Network)

…and all of you!

Wavelet analysisPower Spectrum:

Averaged in time:

Seasonal Cycle

Seasonal Cycle

QBO?

Seasonal Cycle

QBO?

Decadal Variability

Seasonal Cycle

QBO?

ENSO

Decadal Variability

Changing teleconnection patterns: composite temp. anomaliesControl 6,000 B.P.

9,000 B.P. LGM

Evolution of the El Ni ño Southern Oscillation (ENSO) from the Last Ice Age to Today

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