1

El Niño - Southern Oscillation(ENSO)

Ocean-atmosphere interactions

ENSO“EN” = El Niño; “SO” = Southern OscillationEl Niño is a warming of the tropical eastern PacificSouthern Oscillation is a see-saw in atmospheric

pressureThe ocean-atmosphere work together (are “coupled”) to

produce ENSO climate variabilityENSO is an Ocean-Atmosphere “dance” involving:

- Surface and upper level winds- East-west SST gradients- Upwelling and ocean productivity- Thermocline depth- Sea surface height

Global Atmospheric Circulation

Tradewinds

Hadley Cells

Walker circulation

The key is to appreciate how odd the “normal” state is!

Q1: A strong east-west SST gradient in thePacific Ocean is partly responsible for:

A. Rain over IndonesiaB. The Hadley circulationC. The Walker circulationD. Answers A, B, CE. Answers A, C

Trade winds on the equator bringcold waters to surface

Equatorial upwelling

Eastern tropical Pacific iscooler than west for tworeasons:

1) Trade winds pile up warmwater in the west

2) Wind-driven upwelling inthe east

2

Normal Conditions

Note features:

- East-west SST gradient- eastern upwelling- E-W Thermocline depth- E-W Sea surface height- Surface pressure- trade winds

Hadley and Walkerwinds vary in phase:

when Hadley cell isstrong, so is thewalker circulation

Normal Conditions in the tropical Pacific Transition to El Niño

Full El Niño conditions

What are changes in

- East-west SST- upwelling- Thermocline depth- Sea surface height- Surface pressure- trade winds- rainfall?

El Niño Conditions

What are changes in

- East-west SST- upwelling- Thermocline depth- Sea surface height- Surface pressure- trade winds- rainfall?

3

El Niño SST anomaly movie

AnomaliesBlue = coldRed = warm

Largest El Niños of 20th century

SST Anomaly (relativeto the average state)

1982

1997

La Niña Conditions

What are changes in

- East-west SST- upwelling- Thermocline depth- Sea surface height- Surface pressure- trade winds- rainfall?

La Niña SST anomaly movie

AnomaliesBlue = coldRed = warm

No two events alike…

La Niña

Warm Phase

Cold phaseCool SST

warm SST

4

Walker CirculationBritish mathematician, director general of

observations for India (formed aftermonsoon failure of 1877- worstfamine in Indian history)

Arrived in 1904, shortly after hugefamine caused by drought

Goal to predict Indian MonsoonFound that many global climate

variations, including Monsoon rains inIndia, were correlated with theSouthern OscillationSir Gilbert Walker

Q2: Place in order of low to high surfaceatmospheric pressure

A. A, B, CB. C, B, AC. B, C, AD. A, C, BE. Can’t tell

ABCOcean temperatures

Pink = warmgreen = cool

SOI = Tahiti SLP - Darwin SLP

Average Climatology: 1933-1992

Tahiti

Darwin

5

Southern Oscillation Index

• SOI is a “proxy” for El Nino - La Nina state• High SOI = Tahiti SLP > Darwin SLP = La

Niña• SOI is the ATMOSPHERIC expression of

ENSO.• The OCEANIC expression of ENSO: SSTs,

upwelling, thermocline, SST height, oceanproductivity

+

-

**

∆SLP

Difference in Jan. SST between the two extreme ENSOepisodes of this century

nino3

Blue = Atm pressure at DarwinRed = SST anomaly for the NINO3 region

ENSO: Physical coupling between the ocean and atmosphere

Important Slide!

6

Connecting El Niño and the SO (ENSO)

The link between SO and El Niño wasmade convincingly by Bjerknes

Made extensive use of data gatheredduring 1957 (strong El Niño year)

Realized that unusual events separated byhalf the circumference of Earth could belinked together as parts of a hugecoupled phenomenon ENSO–involving both the ocean and theatmosphere.

Prof. Jacob Bjerknes

General Description of ENSO Processes

Bjerknes established the empirical connection between EN and SO.He also provided a hypothesis about the mechanism that underliesour present understanding.

The key is to appreciate how odd the “normal” state is– The easterly trades drive westward currents, bringing the cold

waters of the Peru Current from the South American coast– The coriolis force turns westward surface currents poleward,

causing divergence and upwelling– The trade winds push the warm upper layer waters poleward as

well as westward, pulling the thermocline to the surface in theeast

– All are due to the easterly winds, but the easterly winds are alsodue to temperature contrasts along the equator. Thus there arepositive feedbacks to reinforce both El Niño and La Niñaphases

A very simplified view ofocean-atm coupling

Why does ENSO State tend to Oscillate?Equatorial ocean dynamicsKey observations by Klaus Wyrtki in 1970’sEl Nino is preceded by a transfer of warm water

from west to eastThis transfer is thought to trigger a warm eventWhat triggers the movement of water?

– In cold phase, waters cold (and low) in east, warm(and high) in west

– Warm water from west sloshes back and overshootsequilibrium- positive feedbacks mentioned before setof El Niño conditions

Why does ENSO State tend to Oscillate?

Shortest Answer:Equatorial ocean dynamics

Short answer: Differing timescales of Kelvinand Rossby Wave propagation

Longer answer: The oscillation is madepossible due to the asymmetry betweeneastward and westward oceanic motions (see2-D Animation in notes).

Along the equator there is a relatively fasteastward motion called an equatorial Kelvinwave. Peaking somewhat off the equator arewestward motions called Rossby waves.Time scale: <1 yr for the Kelvin waves to shiftthe warm pool eastward [once trades relax]; 2yrs for the Rossby waves to return the warmpool to the west, to await another wind relax.

KelvinRossby

Here is a 3-D animation the tropical Pacific as it cycles through anEl Niño then La Niña event. The surface shown is sea-level (incm) and the surface is colored according to the SST anomaliesassociated with each event.

7

Prediction for Winter ‘08

“normal”

Global SST anomalies - last week

http://iridl.ldeo.columbia.edu

ENSO and Global Warming

Persistent El Niño?

Surface ocean warmsuniformly.

Warming penetrates tothermocline.

Ocean-atm couplingsustains El Niño mode.

Persistent La Niña?

Surface ocean warmsdifferentially; Westwarms more.

WEP SST warming drivesconvection there

Strengthens Walker Cell.Ocean-atm coupling

sustains La Niña mode.

Still no consensusEl Niño Forecasting

Stephen E. ZebiakDirector General

International Research Institutefor Climate Prediction

Mark CaneChair, DEES

Vetlesen Professor