Chap. 7 ENSO

Chap. 7 ENSO

‣ To learn more about ENSO, see this web site http://www.educnet.education.fr/obter/appliped/elnino/theme/nino.htm

7.1 Oceanic climatology

7.2 Historical and theoretical development

7.3 Main Features of ENSO (ex. with El Niño 97)

7.4 Description of the Southern Oscillation

7.5 ENSO : regional climatic changes (Pacific and Africa)

7.6 Teleconnections

7.7 Seasonal forecasts

sommaire général

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Sea Surface Temperature

7.1 Oceanic climatology : SST

Source : NOAA/PMEL/TAO. Site : http://www.pmel.noaa.gov/toga-tao

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Pourquoi des eaux plusFraiches sur les cotes est des continents ?

7.1 Oceanic climatology : SST


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7.1 Oceanic climatology :Ekman transport and coastal upwelling

Northern Hemisphere :Ekman transport d’Ekman at rightangle to the right of the wind stress

In the southern hemisphere, It’s the opposite : Ekman Transport at right angle to the leftof the wind stress


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In the southern hemisphere, the Ekman transport is locatedat right angle to the left of thewind stress

Wind stress

Ekman transport

Peru

Chile

7.1 Climatologie océanique :Transport d’Ekman et upwelling côtier


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Wind stress

Coastal Upwelling Equatorial undercurrent

7.1 Oceanic climatology :3D circulations in the Pacific


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Sea level more elevated over West Pacific

7.1 Oceanic climatology : Sea-level

Source : figure extraite du site du Jet Propulsion Laboratory (JPL). site : http://sealevel.jpl.nasa.gov

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Opposition of phase of the pressure between Sydney – Buenos Aires (Hildebrandsson – 1897)

Southern Oscillation (pressure, temperature, rain) – G. Walker (~1920)

Link between Pacific SST and Southern Oscillation (Bjerkness – 1966, 1969) – Apparition of the notion of Teleconnection

« Build-up » of Wyrtki (1975-1979) Composite based on EL Niño events from the 1950s to

1970s– Rasmusson and Carpenter (1982) Modelisation of the coupled ocean/atmosphere (1980 ….) Theoretical developments (1980 …)

7.2 Historical and theoretical developments

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« Build-up of Wyrtki– Ocean governed by the atmosphere (weakening of the trades –

displacement of the warmpool in the direction of the East Pacific)

The delayed oscillator – Reflection of the Rossby wave into Kelvin wave (West coast) causing a

deepening of thermocline. Reflection of the Kelvin waves (East coast). Alternation of warm anomalies (kelvin «downwelling») and cold (kelvin «upwelling»).

Couples instabilities– Link SST - thermocline (positive feed back with a convergent wind at

surface)– Low-frequency unstable coupled mode (periode of 3 to 4 years and 6

months)– «SST mode» - advection of zonal mean gradient of SST by the anomalies of

the current – advection of the mean vertical gradient of temperature by the anomalies of the vertical velocities – modulation of the vertical advection by the mean equatorial upwelling

– Salt Barrier role : move of the east area of the Warmpool – role of the zonal advection of the SST gradient

7.2 Historical and theoretical developements

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Ocean/Atmosphere Coupled phenomenon ENSO : main source of variability of the climate at

interannual timescales Oceanic modifications « El Niño/La Niña » in the

Pacific, at surface but also at subsurface. Alternations periods « El Niño/ La Niña » from 3 to

7 years Planetary consequences of the ENSO via the changes of the general circulation :

Teleconnections. Oceanic phenomenon partially predictable (causes

of the onset of El Niño ?) Interactions ENSO/Pacific Decadal

Oscillation(PDO), ENSO/MJO, MJO/Synoptic-scale

7.3 Principales caractéristiques d’El Niño

et illustration avec El Niño 97

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El Nino 97El Nino 97

SST

Anomalies of + 5°c over East Pacific

Source : NOAA/ NWS/NCEP/ Climate Prediction Center http://www.cpc.noaa.gov/products/precip/CWlink/MJO/enso.shtml/

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El Nino 97 :El Nino 97 :Dynamical anomalies height of the sea-level

Sea-level anomalies height ‣ - 25 cm over West Pacific ‣ + 30 cm over East Pacific

Source : figure extraite du site du Jet Propulsion Laboratory (JPL). site : http://sealevel.jpl.nasa.gov

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El Nino 97 :El Nino 97 :

Source : NOAA/ NWS/NCEP/ Climate Prediction Center http://www.cpc.noaa.gov/products/precip/CWlink/MJO/enso.shtml/

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Subsurface Pacific structure and its evolutionEl Nino 97 :El Nino 97 :

The area of strong vertical gradient of SST determines the thermocline whose depth varies also with the ENSO phase


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El Nino 97 :El Nino 97 :Subsurface Pacific structure and its evolution


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Back-upstart animation

Subsurface Pacific structure and its evolution


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3D image coming from the french-american satelliteTopex-Poséidon, (CNES-NASA)

Surface waters measured by the HVRR sensor of the satelliteSubsurface waters measured by stationary buoys , TAO

red = 30°cblue= 8°cthermocline = between dark blue and pale blue, between 10 et 20°C,

located at 600 ft in West Pacific and at 150 ft in East Pacific



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The thermocline is flattened by theKelvin wave (westerly wave) in the directionof the South American coasts.

Raising of the sea-level of 34 cm along the equatorial East Pacificbecause of the weakening of the trades winds



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Tahiti

Darwin

7.4 Description of the southern oscillation

‣ Southern Oscillation = periodic phenomeneon of reversal of the mean sea-level pressure (MSLP) pattern between the Central Pacific (Tahiti)

and the Western Pacific (Darwin) associated with a displacement of the Walker cell.

‣ G. Walker (1932) has defined an index based on the difference of MSLP between Tahiti and Darwin which is :

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SOI

= monthly difference of MLSP between Tahiti and Darwin

= monthly mean difference (1941-1980) of MLSP between Tahiti and Darwin

= standard deviation of the monthly difference of MLSP between Tahiti and Darwin. The period of reference is 1941-1980

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0SOI during an El Niño event

0SOI during a La Niña event

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Tahiti

Darwin

7.4 Description of the southern oscillation

‣ Determination of an El Niño event: the SOI must be lower than the lower quartile value of SOI

‣ Determination of a La Niña event : the SOI must be higher than the upper quartile value of SOI

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Source : NOAA/ NWS/NCEP/ Climate Prediction Center.Site : http://www.cpc.noaa.gov/products/analysis_monitoring/bulletin/figt1.html

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7.5 El Niño : climatic changes over Pacific


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Normal year (top), El Niño (bottom)

ITCZ

ITCZ

SPCZ

SPCZ

Indonesianmonsoon trades winds

trades winds


Source : d’après Trenberth, 91a

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El NinoWesterlyanomalies

la Nina

Easterlyanomalies = trades strenghtened

El Nino

la Nina



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El Nino

la Nina

El Nino

la Nina



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7.5 ENSO : climatic changes over Pacific


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7.5 La Niña : climatic changes over Pacific

La Niña :The westerly subtropical jet (STJ)is less zonal than usual:upper westerlies overthe Equatorial East Pacifique arestrenghtened (+10 m/s).As upper westerlies acts as a Rossby canal duct , these westerly anomalies are conducive to favourthe equatorward propagation of the Rossby waves.

It’s the opposite forEl Niño events since theSTJ is more zonal than usual (figure not shown)

Source :NOAA/PMEL/TAO

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⇨ drier over Maghreb

7.5 El Niño : climatic changes over North Africa

(northen winter)

Source : figure from website IRI(InternationalResearch InstituteFor Climate and Society) http://iri.columbia.edu/

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⇨ rain monsoon weakened

7.5 El Niño : climatic changes for the African monsoon


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⇨ rain monsoon enhanced

7.5 La Niña : climatic changes for the African monsoon


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7.6 Teleconnections : planetary consequences (rain and T)

El Niño (december to february)

Sources : d’après Ropelewski et Halpert, 1987 et 1989

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El Niño (june to august)



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La Niña (december to february)



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La Niña (june to august)



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7.7 Seasonal forecast of SST (CEP)

Source : figures extraites du site du CEP

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‣ These seasonal forecasts (1 to 6 months) of the SST (products called ‘Niño Plumes’) are available on the CEP web site : http://www.ecmwf.int/products/forecasts/d/charts/seasonal/forecast/

7.7 Seasonal forecast of the CEP

References (1)

- El Niño references: TAO refereed journal articles and other TAO papers.

-Hayes, S. P., L. J. Mangum, J. Picaut, A. Sumi, and K. Takeuchi, 1991 : TOGA-TAO: A moored

array for real-time measurements in the tropical Pacific Ocean ’. Bull. Am. Meteorol. Soc., Vol.72, p.339-347

- Lee, Martin E., and helton, Dudley, Oceanic Kevin/rossby Wave Infleunce on North American West COast Precipitation, NOAA Tecnhical Memorandum (NWS, WR-253)

- McPhaden, M.J., 1993 : TOGA-TAO and the 1991-93 El Niño-Southern Oscillation Event . Oceanography, Vol.6, p.36-44

- NOAA Reports to the Nation – El Niño and Climate Prediction - Philander, S. G. H., 1990: El Nino, La Nina and the Southern Oscillation. Academic Press, San Diego, CA, 289 p.

-Ropelewski C. F. et Halpert M. S., 1987 : ‘Global and Regional scale précipitations and temperature patterns associated with El Nino/Southern Oscillation’. Mon. Wea. Rev., Vol. 115, p. 1606-1626

- Ropelewski C. F. et Halpert M. S., 1989 : ‘Précipitations patterns associated with the high index of the Southern Oscillation’. J. Clim, Vol.2, p.268-284

References (2)

- Trenberth , K. E., 1991a : General characteristics of El Nino-Southern Oscillation. Teleconnections Linking Worlwide Climate Anomalies. M. Glantz, R.W. Katz, and N. Nicholls, Eds., Cambridge University Press, p.13-41

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Chap. 7 ENSO