El Niño / Southern Oscillation (ENSO)

and inter-annual climate variability

seasonal cyclewhat is “normal”?

monthly average conditions through a calendar year

sea level pressure and surface winds

surface ocean currents

coastal upwelling

equatorial upwelling

every year is differentinter-annual variability of the seasonal cycle

El Niño/Southern Oscillation (ENSO) is the most important coupled ocean-atmosphere phenomenon to cause climate variability on inter-annual time scales.

what is El Niño?

The term El Niño originally used by Peruvian fishermen to describe the warm current

appearing off the western coast of Peru around Christmas time.

Today El Niño describes the warm phase of a naturally occurring sea surface

temperature oscillation in the tropical Pacific Ocean.

This oscillation is associated with the atmospheric phenomenon known as the

southern oscillation.

what is the Southern Oscillation?

A seesaw shift in surface pressure at Darwin, Australia and the South Pacific Island of Tahiti. When the pressure is high at Darwin it is low at

Tahiti and vice versa.

El Niño, and its sister event – La Niña – are the extreme phases of the southern oscillation, with El Niño referring to a warming of the eastern tropical

Pacific, and La Niña a cooling.

Southern Oscillation Index (SOI)

The (SOI) measures the monthly/seasonal fluctuations in surface air pressure differences at Tahiti and Darwin (SOI = Tahiti – Darwin), and thus the SOI has a negative value during an El Niño event.

Southern Oscillation Index (SOI)

The (SOI) measures the monthly/seasonal fluctuations in surface air pressure differences at Tahiti and Darwin (SOI = Tahiti – Darwin), and thus the SOI has a negative value during an El Niño event.

Walker circulation

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)

Every few years, the

trade winds weaken…

In other years the trade

winds strengthen…

schematic of changes during an El Niño event

quantifying & comparing ENSO events

ENSO can be quantified using a Multivariate ENSO Index (MEI) based on the six main observed variables over the tropical Pacific. These six variables are: sea-level pressure, zonal and meridional components of the surface wind, sea surface temperature, surface air temperature, and total cloudiness fraction of the sky.

MEI comparison of recent El Niño events

MEI comparison of recent La Niña events

Western Pacific “warm pool”

Upwelling zones

El Niño happens roughly every 3-7 years, lasts 12-18 months, and peaks at the end of the calendar year

El Niño is often followed by or preceded by La Niña: an unusual cooling of the tropical Pacific

1997-1998 ENSO event

El Niño shifts the probability of droughts, floods, heat waves, and extreme weather events in large regions of the globe.

Teleconnections and Global Weather Patterns

Teleconnections and Global Weather Patterns

Social and Economic Consequences

El Niño can affect life, property, and economic vitality due to weather related hazards.

Ocean Biological Productivity (Chlorophyll Concentrations)

Jun-Aug ‘98 Dec ‘97-Feb ‘98La Niña El Niño

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Strong Upwelling

Weak Upwelling

NOAA Report to the Nation, 1995

Anchovy Catch (millions of tons)

1972-73 1997-98

El Niño and Peruvian Anchovy

Peruvian Anchovy Catch and ENSO