AOS 101 Weather and Climate
Lisha M. Roubert
University 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 oceanatmosphere 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
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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/geoa
nimations/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 Niña
La 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 Events
Blue=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