El Niño-Southern Oscillation:Past, Present, and Future

Oceanography lecture, Kim Cobb

Why study the El Niño-Southern Oscillation?1. It is the largest source of year-to-year global climate variability.

2. It carries negative societal consequences, both economic and humanitarian.

3. Improved forecasts minimize negative societal consequences.

4. It’s a fascinating, mysterious, and complex natural phenomenon.

What is the El Niño-Southern Oscillation?

a birds-eye view of 2 of the largest El Niño events of the century:

and the 2002/2003 El Niño event:

A natural climate cycle in the tropical Pacific that alternates between warm (El Niño) and cold (La Niña) phases every 2-7 years.

What is the El Niño-Southern Oscillation?

a birds-eye view of 2 of the largest El Niño events of the century:

and the 2002/2003 El Niño event:

A natural climate cycle in the tropical Pacific that alternates between warm (El Niño) and cold (La Niña) phases every 2-7 years.

Average tropical Pacific conditions

- trade winds blow from East to West

- warm water piles up in West Pacific, driving deep atmospheric convection

- cold, nutrient-rich water is brought to the surface in the East Pacific (upwelling)

strong trade winds

cool in Eastwarm in West

This system is tightly coupled

Peruvian fisherman (1800’s):named mysterious warm, nutrient-poor waters “El Niño”

Sir Gilbert Walker (1924):named East-West seesaw in sea-level pressure the “Southern-Oscillation”

Jacob Bjerknes (1969):explains feedbacks that link the “El Niño” and “Southern Oscillation” phenomena

Tropical Pacific conditions during El Niño

trade windsweaken

upwellingslows

Eastern Pacificwarms

The Bjerknes Feedback:

http://www.pmel.noaa.gov/tao/elnino

trade windsstrengthen

upwellingintensifies

Eastern Pacificcools

…and the reverse for La Niña

El Niño Impacts

- these departures from “normal” climate carry serious economic and social costs

- improved ENSO forecasts minimize the costs

- La Nina’s effects are roughly the opposite of El Nino’s effects

- the impacts are not confined to the tropical Pacific

In Oregon:ENSO influences- coho salmon spawning- agriculture- water resource management- ski conditions

ENSO affects temperatures and rainfall in Georgia

Year

1860 1880 1900 1920 1940 1960 1980 2000

Tem

per

atu

reD

evia

tio

n (

°C)

-3

-2

-1

0

1

2

3Eastern tropical Pacific Temperature

El Niño

La Niña

1997 El Niño1982 El Niño

Predicting ENSO extremes

Devastation caused by the 1982 El Niño made ENSOprediction a top priority for U.S. and other countries.

6-month prediction: - requires careful monitoring of tropical Pacific ocean + atmosphere - first ENSO model unveiled in 1987 - models predicted a weak El Nino event in 1997

I. Short-term Predictions

The tropical Pacific observing system

http://www.pmel.noaa.gov/tao/

Year

1860 1880 1900 1920 1940 1960 1980 2000

Atm

osp

her

ic C

O2 (

pp

m)

280

300

320

340

360

Ice CoreKeeling curve

1860 1880 1900 1920 1940 1960 1980 2000

Tem

per

atu

reD

evia

tio

n (

°C)

-3

-2

-1

0

1

2

3

Tem

per

atu

re (

°C)

0.0

0.5

1.0

Average Global Temperature

Atmospheric CO2

Eastern tropical Pacific Temperature

El Niño

La Niña

1997 El Niño1982 El Niño

II. Long-term Predictions

Are severe El Niño events becoming more frequent?How will ENSO change as the Earth warms?

“This winter's El Niño gives us a taste of the extreme erratic weather that our children and grandchildren can expect more of unless we take action to reverse the trend of global warming.” then Vice President Al Gore, 1998

“Warm episodes of the El Niño-Southern Oscillation (ENSO) Phenomenon . . . have been more frequent, persistent and intense since the mid-1970s,compared with the previous 100 years.” Intergovernmental Panel on Climate Change, 2001

“Confidence in projections of changes in future frequency, amplitude, and spatial pattern of El Niño events in the tropical Pacific is tempered by some shortcomings in how well El Niño is simulated in complex models.” IPCC, 2001

ENSO and Global Warming in the 21st century

The instrumental record of ENSO is too short to answersome key questions:

1. Are late 20th century El Niño events more frequent and more severethan those of the recent past?

2. Is there a correlation between average global temperature and El Niño activity?

3. How much and how fast has ENSO changed in the past?

ENSO and coral 18O in the Central Tropical Pacific (CTP)

During El Niño events, positive SST and precipitation anomalies both contribute to negative coral 18O anomalies in the CTP

Interpretation of coral 18O on lower frequencies relies on assumption that warm SST drives higher precipitation in the CTP, and vice versa

SST and rainfall anomalies during the 1982 El Nino

Corals: The geologic record of ENSO

CORALS from the tropical Pacificrecord ENSO in the geochemistry of their skeletons

Year

1900 1920 1940 1960 1980 2000

SS

T A

nom

oly

(°C

)3

2

1

0

-1

-2

18 O

(‰

)

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Central Pacific TemperatureCoral Geochemistry

This live coral is ~50yrs old

This fossil coral grewfrom ~1320-1390A.D.

CORALS from 125 thousand years ago contain the signature of ENSO

Fossil corals collected during fieldtrips in 1998, 2000, and 2005

Beached fossil corals range from gravel-sized to 2m-diameter(~100-150yrs).

rare longer cores: ENSO & decadal variability

common short cores: mean climate

Palmyra40 cores U/Th dated28 cores undated

Christmas18 cores U/Th dated63 cores undated

Fanning 33 cores undated

The Line Islands Fossil Coral Collection

Strategy- rare long cores (40-100y) for variability- common short cores (10-20y) for mean climate

17th Century Spliced Coral 18O Records

splice

- absolute 18O values agree (give consistent picture of mean climate)- large, well-reproduced El Niño events in mid-17th century- low-amplitude, less well-reproduced decadal variability

Year A.D.

1200 1400 1600 1800 2000

-5.75

-5.50

-5.25

-5.00

-4.75

-4.50

-4.25

Coral reconstructions of tropical Pacific climate1. Raw records

930 960

18 O

(‰)

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

1170 1200Years

1320 1350 1380 1410 1440 1650 1680 1890 1920 1950 1980

2. ENSO only

El Niño

La Niña

1997El Niño

Canals freeze in Europe“Little Ice Age”

Greenland green“Medieval Warm Period”

1°C

warmer

colder

Probing the coral record of ENSO:

1. Are late 20th century El Niño events more frequent and more severe than those of the recent past? Not necessarily.

2. Is there a clear relationship between average global temperature and El Niño activity? No.

3. How much and how fast can ENSO change? ENSO can double in strength in less than 5 years.

Tudhope et al., 2001Woodruffe et al., 2003McGregor et al., 2004Correge et al., 2000Cobb et al., 2003, in progress

Chasing Late Holocene trends in ENSO variance

Equally importantare reconstructionsof tropical Pacificmean climate throughthe late Holocene….

Are corals up to the challenge?

Reduce error bar by usingmany small corals?

Are we up to the challenge?

Moy et al., 2002

Clement et al., 1999

1. The future of ENSO remains uncertain.

2. We have much to learn about ENSO from the geologic record.

Conclusions

Food for Thought

Coral reefs are disappearing at alarming rates worldwide, due to the combined influence of rising ocean temperatures and human disturbances (pollution, dynamite fishing, etc).

Even if ENSO does not change in a “greenhouse world”, Man has perturbed the environment to the point that an El Niño event could be the “straw that breaks the camel’s back”.

Dai and Wigley, 2000

Borneo caves

Goal: generate well-dated records of interannual to millennial climate in an ENSO center of action

Emphasis:-calibration-reproducibility

Potential Resolution: sub-annual

Time coverage: 1 million years ago to present

photos of three Borneo speleothems thatformed during the last70,000 years, includingtwo that contain records of 20th century climate

Annually-resolvedprecipitation records

from Warm Poolstalagmites