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.

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?

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

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.

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 temperaturesand 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”.

Web Resources

This talk: http://www.gps.caltech.edu/~kcobb/osu.pdf

General El Niño info: http://www.pmel.noaa.gov/tao/elnino

NOVA El Niño page: http://www.pbs.org/wgbh/nova/elnino/