EL NINO SOUTHERN OSCILLATION (ENSO)

AOS 101 Weather and Climate

Lisha M. RoubertUniversity of Wisconsin-Madison

Department of Atmospheric & Oceanic Sciences

El Niño• El Niño is an oscillation of the

ocean-atmosphere system in the tropical Pacific having important consequences for weather around the globe.

• El Niño is characterized by unusually warm ocean temperatures in the Equatorial Pacific, as opposed to La Niña, which is the opposite phase of El Niño.

• ENSO events occur irregularly, every 2-7 years.

How did ENSO get its name?•El Niño was originally recognized by fisherman

off the coast of South America as the appearance of unusually warm water in the Pacific ocean, occurring near the beginning of the year. El Niño means The Little Boy or Christ child in Spanish. This name was used for the tendency of the phenomenon to arrive around Christmas.

•La Niña means The Little Girl. La Niña is sometimes called anti-El Niño.

What causes ENSO?•El Niño results from interaction between

the surface layers of the ocean and the overlying atmosphere in tropical Pacific.

•The internal dynamics of the coupled ocean-atmosphere system determine the onset and termination of El Niño events. The physical processes are complicated, but they involve unstable interaction between the air and planetary scale oceanic waves.

Teleconnections: Linking ENSO to other locations• Teleconnections mean that weather

changes at one location might be related to weather changes at other remote locations.

• The relationship between El Niño events and climatic variations in the Equatorial Pacific region is extremely strong . The relationship outside this area is harder to prove.  Weather anomalies occur all over the globe every year, but some tend to recur with most or all El Niño events and are referred to as teleconnections.

• El Niño impacts are strongest and most widespread when water temperatures reach the annual maximum (boreal winter).

• Above normal warmth of the water, el Niño redistributes convection, which causes changes in the jet stream.

• During the austral winter, El Niño impacts are weaker and confined mostly to the Southern Hemisphere.

Boreal Winter (Winter for NH)

Austral Winter (Winter for SH)

Impacts of ENSO on North America and the Atlantic Basin• Drier than normal

conditions over the North American monsoonal region.

• The Pacific Northwest experiences drier than normal conditions during Fall and Winter.

• Atlantic hurricane activity is suppressed due to cooling.

The Atmosphere during non-ENSO conditions • In normal, non-El Niño condition, the

trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at Indonesia than at Ecuador.

• Sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels.

• Rainfall is found in rising air over the warmest water, and the east Pacific is relatively dry. 

During ENSO• During El Niño the trade winds

relax in the central and western Pacific leading to a depression of the thermocline in the eastern Pacific, and an elevation of the thermocline in the west.

• The result was a rise in sea surface temperature.

•  Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in regions far removed from the tropical Pacific.

ENSO Animation

http://esminfo.prenhall.com/science/geoanimations/animations/26_NinoNina.html

How do we recognize ENSO?• ENSO can be seen in measurements of Sea

Surface Temperatures.

•SST and winds are near normal with warm water in western Pacific and the “cold tongue” (cold water) in the eastern Pacific.•Winds in the western Pacific are weak and winds in the eastern Pacific blow towards the west.

•Warm water has spread from the western Pacific towards the east , the "cold tongue" has weakened.•The winds in the western Pacific are blowing strongly towards the east, pushing the warm water eastward. 

ENSO associated with change in Sea Level• The images show sea surface topography from NASA's

TOPEX satellite, sea surface temperature's from NOAA's AVHRR satellite sensor and sea temperature below the surface as measured by NOAA's network of TAO moored buoys.Normal ENSO

Developing La NiñaLa Niña

So many measurements to monitor ENSO, but where does all this data come from?

• There are many resources to obtain data. We call this the ENSO Observing System.

• There is even a system of moored buoys located in the Atlantic that are specifically designed to monitor ENSO.

How do we monitor ENSO?• A variety of indices are used

to characterize ENSO because it affects so many elements of the atmosphere-ocean climate system. The main ones are: ▫ the Southern Oscillation

Index (SOI): it is given by the difference in sea-level pressure between Tahiti and Darwin, Australia

▫ the Niño 3 index: it refers to the anomalous SST within the region bounded by 5N-5S and 150W-90W.

Red=El Niño EventsBlue=La Niña Events

What is happening now?

• La Niña is present across the equatorial Pacific.• Negative sea surface temperature anomalies

persist across much of the Pacific Ocean.• La Niña is expected to last into the Northern

Hemisphere spring 2011.

Homework:•Study for quiz #2. The material covered

in the quiz will be: -Lab 3: Temperature Trends -Lab 4: Extreme Temperature Events -Lab 5: Precipitation