El Niño-Southern Oscillation

Current News and WeatherENSO TerminologyNeutral, Warm-Phase, and Cold-PhaseENSO Impacts

For Next Class: Read Moran Ch. 8 (pp. 265-276)

El Niño-Southern Oscillation

What is ENSO and what are its three major phases?Why is ENSO important?

ENSO

El Niño-Southern Oscillation

ENSO is a coupled phenomenon, depending on feedbacks between the ocean and the atmosphere.Changes in the oceanic sea surface temperatures (SSTs) drive changes in atmospheric circulation which feedback and further alter the ocean.

ENSO

Coupling of the ocean and atmosphere in the tropical Pacific OceanEl Niño (warm-phase of ENSO): above average sea-surface temperaturesLa Niña (cold-phase of ENSO): below average sea-surface temperaturesLa Nada (ENSO-neutral): average sea-surface temperatures

El Niño-Southern Oscillation

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ENSO

Neutral Conditions in the Tropical Pacific Contrast between relatively

high air pressure over the central and eastern tropical Pacific and relatively low air pressure over the western tropical Pacific ultimately drives the trade winds

Walker Circulation: east-west oriented atmospheric circulation across the equatorial Pacific

Andes

Trade winds

Coastal upwelling

Figure: University of Washington

Cold water

Warm water

Thermocline

Temperature

Dep

th

ThermoclineEkman

transport

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ENSO

Warm Phase With the onset of El Niño, air

pressure falls over the eastern tropical Pacific and rises over the western tropical Pacific• Trade winds slacken in the

western and central equatorial Pacific

• Warm surface waters in the eastern tropical Pacific reduces upwelling

El Niño (Warm Phase ENSO)

1997 e-mail from Anton Seimon in Peru

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El Niño, La Niña, and the Southern Oscillation

Cold Phase• During La Niña, surface waters

are colder than usual over the central and eastern tropical Pacific and somewhat warmer than usual over the western tropical Pacific

• Lower than usual SST in the east topical Pacific inhibit rainfall while higher than usual SST in the west enhance rainfall

Thermocline

PeruAustralia

Figura: NOAA PMEL

Positive feedback between ocean and atmosphere (Bjerknes 1969)

1

1. Warming

2

2. Displacement of convection

3

3. Weakening of easterly winds

4

4. Eastward currents and deepening of

thermocline

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El Niño, La Niña, and the Southern Oscillation

Frequency• Multivariate ESNO Index forms the basis for operational

definitions of El Niño and La Niña Based on six variables measured in the tropical Pacific

• sea-level air pressure• zonal component of surface wind• meridional component of surface wind• surface air temperature• sky cloud cover• sea-surface temperature

• ENSO Alert System launched in February 2009

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1982-83 El Niño 1997-98 El Niño

1998-99 La Niña

Piura - Perú

The rainfall in Piura and El Niño

1982-83

1997-98

19921972

1987

2002

Rainfall occurs when sea surface temperature (SST) exceeds a threshold

Woodman, 1999

Nevado Chacaltaya (2003)

Nevado Chacaltaya (2015)

Francou et al. 2004

32© AMS

El Niño, La Niña, and the Southern Oscillation

Predicting and Monitoring ENSO• Atmosphere/ocean observational data from the

tropical Pacific obtained using the ENSO Observing System

Consists of an array of moored and drifting instrumented buoys, island

and coastal tide gauges, ship based measurements, and satellites

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ENSO Links

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_update/sstanim.shtml

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.html