The El Niño Southern Oscillation - ENSO
Background
ENSO is an atmosphere and ocean phenomenon. It is associated with quasi-periodic, seasaw like changes in the distribution of sea surface temperature, elevation of sea surface level, sea
surface level atmospheric pressure that results in large scale perturbations to the mean, averaged
atmospheric and oceanic circulation. The phenomenon is primarily confined to the Equatorial
Pacific Ocean, although its influence upon the state of the atmosphere and ocean can be felt
around the planet.
The existence of ENSO was first recognized as a reoccurring atmospheric oscillation in
atmospheric pressure differences between Darwin, Australia and Tahiti, French Polynesia at the
end of the 19th century. The pressure difference is now expressed through the Southern
Oscillation Index (SOI). It sea-saws between values of about +30 and –30 (Figure 1). At about
the same time, reports of reoccurring, quasi-periodic changes in sea surface temperature, i.e. an
unusual warming off the South American coast became known to western scientists as El Niño.
More recently, scientists found that the atmosphere and ocean phenomena independently
discovered and observed are really a result of one and the same coupled ocean-atmosphere
process, hence, the term ENSO. It is now known as the largest global anomaly of the climate
system. The discovery of this link between the ocean and atmosphere sparked major international
research efforts. Soon it was recognized that El Niño is quite often followed by an event that
results in an unusual cooling of the eastern Pacific Ocean and an associated warming in the west.
In some way, the Pacific Ocean behaves like a huge bath tab, in which water sloshes from one
end to the other quasi-periodically, raising sea surface level and temperature at one end and
lowering both at the other end. The unusual cooling in the east is now referred to as La Niña. One
cycle of warm, normal, cold, normal and warm conditions in sea surface temperature takes about
3-4 years to complete. El Niño and La Niña are both extreme phases of ENSO and are often also
referred to as the ENSO warm and ENSO cold phase. During the warm event, the SOI value is
positive and during the cold event it is negative.
Figure 1: A time series of the Southern Oscillation Index based upon the sea level pressure
difference (anomaly) between Tahiti and Darwin for the period 1970 to 2000. The index
oscillates between values of about –30 and +30 with a period of about 3-4 years. Strongly
negative values are associated with an unusual cooling (La Niña or ENSO cool phase) and
strongly positive values are associated with warming (El Niño or ENSO warm phase) of the
eastern equatorial Pacific Ocean. A very distinct, clear-cut example is the warm event in 1987-88
which was followed by a cold event in 1988-1989.
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Activity 1: Build your ow n El Niño
What You Need
Rectangular clear container (glass or plastic--approx.18"x4"x4")
Cold water
Cooking oil
Blue food coloring
Hair dryer
Blank newsprint paper to set the container on
Markers for making map
Handout: world map showing the Pacific Ocean
What You Do
1. Make your Pacific Ocean—
a. Place the container in the center of the blank piece of newsprint and in the margins mark
the coast of South America on the east side and Indonesia/Australia on the west side.
b. Fill container half full of water.
c. Mix in a few drops of blue food coloring.
2. Create your “warm water” layer
a. Pour cooking oil over the surface of the water until you have about 1 inches of oil. (It’s OK
if it seems to mix. Just let it rest until everything is separate again. )
3. Let the tradewinds blow!
a. Plug in your hair dryer (Keep it away from any water, including spills. If you DO happen
to drop it into your ocean, DON’T fish it out; unplug it.)
b. Turn on your blow-dryer and blow the air stream across the water from East (South
America to West (Australia/Indonesia)
Questions:
What do you see in terms of the "warm" and "cold" water.
What happens when you stop the tradewinds?
What’s Going On?
The container represents a slice of Pacific Ocean between Indonesia and South America. The blue
is the cooler nutrient-rich water and the oil layer is the layer of warm water created naturally
when the sun’s rays hit the surface. Where the two layers meet is the thermocline. The hair dryer
represents the trade winds.
As the air blows over the surface of the water, the hot water will move to the other end of the
container. The cold water will rise to the surface replacing the hot water. This represents the
usual winds that drive the warm water to Australia. Turn off the hairdryer and look at the slope
between the hot and cold water. Does the water return to El Nino conditions now that the wind
has stopped? Of course, the ocean's water is not exactly half hot, half cold. The warm layer is
really a thin surface layer. While this model helps you understand the processes at work, it doesn't
give an accurate representation of El Niño.
When the tradewinds blow you should notice that the “warm water” (oil) piles up in the West as it
is blown by the trade winds. This is the normal condition for the equatorial Pacific Ocean.
Use your red markers to show the normal location and direction of the warm water and air in the
Pacific (along the eastern Australian coast and around Indonesia). Label these “normal
conditions.”
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Figure 2a shows the actual winter pressure zones and winds for normal conditions. Note the high pressure
cell in the eastern Pacific off South America driving the south easterly trade winds.
Figure 2b shows normal Walker circulation in the Equatorial Pacific
Discuss what happens to the air above the warm water in terms of how much moisture the air can
hold. Notice how the blue water moves upwards and mixes up more towards the surface at the
east end. This mix-up is called upwelling and it’s essential for bringing up nutrient-rich water
from the bottom to the surface. Discuss how plankton feed on the nutrients, and in turn fish feed
on the plankton, so these areas tend to be rich in fish and other sea life.
Isn’t it interesting that food chains begin with weather conditions.
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When the tradewinds stop the layer of warm water flows across the ocean from West to East.
This is the warm water part of the El Niño condition. Notice that the upwelling that you saw with
the tradewinds has stopped. Now a thick layer of warm water (oil) covers the surface in the East,
choking off the cool, nutrient-rich waters from rising up to the surface. Take out the maps and use
blue markers to show the El Niño location and direction of the warm water and air in the Pacific
(across the Ocean from Australia to South America.). Label them “El Niño conditions.” Show
the patterns of El Niño in the world's oceans using different colors to represent warmer and cooler
water, and arrows to represent the direction in which the water is moving.
Figure 2c shows El Niño conditions in the Equatorial Pacific
Activity 2: Make graphic representations of El Nino and La Nina Conditions, sea surface
pressure
In this first example, the sea-saw mechanism in sea surface level atmospheric pressure associated
with ENSO and referred to as the Southern Oscillation is revisited. As previously indicated, the
sequence of ENSO warm and cold events during the late 1980s represented a clear signal of the
phenomenon and is used here as an example. In the following section, you will be lead through
stepped through the procedure of graphically and interactively displaying the sea surface level
pressure. It is recommended that the you are connected to the internet and follow the instructions
outlined in Table 1. Changes in pressure, temperature etc. (see below) are always represented as
anomalies, that is the difference between the actual, present-day temperature or pressure and a
climatological value. The latter is calculated as a mean value from many years of observations;
the period is also referred to as the reference period and is representative of the climatological
mean state of the Earth’s climate system.
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Step
1
Action
Connect to the NCEP Atlas: http://www.esrl.noaa.gov/psd/cgibin/gcos_wgsp/printpage.pl
Select: Surface Pressure Timeseries Analysis
Check Data Set: NCEP/NCAR reanalysis sea level pressure
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3
4
Select Jan-May 1987, select standardized anomaly, plot type anomaly
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Under Plot Options check the following boxes: Color Plot, and Shaded Check plot
contour labels, for map projection check Pacific Basin
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Check on Create Plot. If all option were selected properly the graphic shown in Figure
3 or 4 will appear.
Table 1: Instructions to graphically display sea surface level pressure during the ENSO
warm and cold event in 1987 – 1988. For the warm event choose data averaged for January to
May 1987, for the cold event choose data averaged for August to December 1988.
The distribution in sea surface level pressure anomaly, i.e. the difference in the pressure observed
during the period January to May 1987 and the climatological mean value is characterized by
higher than usual values (positive anomaly) in the western and lower than usual values (negative
anomaly) in the eastern Pacific Ocean. During this period, unusually warm surface water
appeared in the eastern Pacific Ocean (see exercise below). This is the so called El Niño, or
ENSO warm event. Print your map to turn in.
Figure 3: The graphic shows the anomaly in sea surface level pressure (in millibars) during the period
January to May 1987. Australia on the left, South America on the right. Positive contour labels east of
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about 180 indicate higher than normal sea surface level pressure. During this period a very strong El
Niño event, i.e. an unusual warming of the eastern Equatorial Pacific Ocean was observed.
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About 12-14 months after the warm event, ENSO approaches another extreme state
referred to as an ENSO cold event or La Niña. While the eastern Pacific surface ocean is
unusually cold during this period, the distribution of sea level surface pressure is the opposite to
that shown in Figure 3. The regions in the western Pacific Ocean characterized by higher than
usual pressure are now characterized by lower than usual pressure (Figure 4), while the eastern
Pacific Ocean exhibits higher than usual pressure. The figure is obtained by replacing the
averaging period of step 4 (Table 1) with the period August to December 1988.
The process, which is identified here from plots of the atmospheric observations is the so-called
sea-saw mechanisms in sea surface level pressure. The Southern Oscillation fluctuates between
these two distinct phases with a period of about 3-4 years, and the SOI is a measure for the
current state of the oscillation.
Figure 4: The graphic shows the anomaly in sea surface level pressure during the period August
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to December 1988. Positive contour labels east of about 180 indicate higher than normal sea
surface level pressure. During this period a very strong La Niña event, i.e. an unusual cooling of
the eastern Equatorial Pacific Ocean was observed.
Activity 3: Make graphic representations of El Nino and La Nina Conditions, sea surface
Temperatures
Changes in sea surface temperature are dramatic during ENSO warm and cold events and
oscillate in a manner similar to that of atmospheric pressure. This exercise involves displaying
graphically the sea-saw mechanism in sea surface temperature associated with ENSO, data are
used for the period 1987 – 1988, and the instructions are identical to those listed in Table 1 with
the exemption that the variable to be selected is that of sea surface temperature. So, when
following the instructions of step 2 in Table 1, choose Hadlsst, select time begin and end with
either April 1987 (Figure 4) or August 1988 (Figure 5). The resulting graphic displays the
anomaly in sea surface temperature during the ENSO warm event (Figure 5) and cold event
(Figure 6).
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The sea-saw mechanism already established in pressure data (Figure 3 and 4), is clearly evident
from the record of sea surface temperature. Within a period of about 18 months the temperature
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distribution reverses from cold to warm. This represents a temperature change of more than 4 C
over the width of the Pacific Ocean.
Figure 5: Anomaly in sea surface temperature during the 1987 El Niño. The eastern equatorial
ocean is unusually warm with temperature more than 2 degree higher than normally, i.e. contour
labels are positive numbers in the east and negative in the west.
Figure 6: Anomaly in sea surface temperature during the August 1988 La Niña event. The eastern
equatorial ocean is unusually cold with temperature more than 2 degrees lower than normally,
i.e. contour labels are negative numbers in the east and positive in the west.
(Modified from http://eprints.usq.edu.au/1074/1/Ribbe_2002_Climate.pdf)
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The data and images were provided by the NOAA-CIRES Climate Diagnostics Centre, Boulder,
Colorado, USA, from their Web site at http://www.cdc.noaa.gov/ and by the Pacific Marine
Environmental Laboratory, Seattle, USA, from their Web site at http://www.pmel.noaa.gov/.
Exercise 4: More on SST’s
Go to the following website:
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensocycle/meansst.shtml
It describes the average or "normal" sea surface temperatures in the Southern Pacific over a year's
time.
1. Describe the sea surface temperatures along the equator going from west to east.
2. What process contributes to the low temperatures you observe in the eastern equatorial Pacific?
3. Look at the four diagrams carefully. Do you observe any differences between the various
seasons? If yes, are they significant?
The animations on the following web-page will show you the sea surface temperature anomalies
in the Southern Pacific before, during and after four recent El Niño events. The data originally
come from the Along Track Scanning Radiometers (ATSRs):
http://www.cdc.noaa.gov/map/clim/sst_olr/old_sst/sst_anim_4panel.shtml
4. Look carefully at every event individually. What is the feature on these graphs that tells you
that there is an El Niño event taking place in every case? How is that feature different from
normal conditions?
5. Do the four El Niños look the same? Which seems to be the strongest and which lasts the
longest?
And a word about La Niña… Look at the images on the following web-site:
http://www.pmel.noaa.gov/tao/elnino/la-nina-anomaly.html
6. Describe in a few sentences how La Niña conditions are different from normal conditions.
Exercise 5: Look at Sea Surface Height
Go to the following web page: http://sealevel.jpl.nasa.gov/science/elninopdo/elnino/
This site shows data from the TOPEX/Poseidon satellite altimeter indicating sea surface height
changes in the period of time between March 1997 and December 2002.
Look at the first few months of this time period (at the bottom of the page) and answer the
following questions.
1. What happened in March of 1997 to trigger the onset of the El Niño event?
2. What is the name of the wave that propagates from the Western to the Eastern Pacific?
3. How long did it take for it to reach South America?
4. What does the wave help transport and how does it affect sea level in the South-eastern
Pacific?
5. How did this El Niño event affect sea level and temps?
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6. Estimate the duration of the 1997-98 El Niño event in months (note beginning and end dates).
Did the conditions return to normal after this event?
Exercise 6: El Niño, La Niña and Seasonal Precipitation
Year
Average
El Niño Years
La Niña Years
1972-1973
1982-1983
1991-1992
1997-1998
1973-1974
1988-1989
1995-1996
1998-1999
Winter Precipitation in Total Snowfall in
San Francisco, CA
Urbana, Illinois
19.70”
27.06”
25.09”
15.16”
37.28”
14.48”
12.26”
21.40”
15.52”
26”
5”
15”
10”
12”
32”
24”
38”
29”
Number of
Hurricanes in
Atlantic Ocean
6
3
2
4
3
5
12
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1. What correlations do you see about winter precipitation in San Francisco and El
Niño /La Niña years?
2. What correlations can you make about snowfall in Urbana and El Niño /La Niña
years?
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3. What correlations can you make about hurricanes in the Atlantic Ocean and El
Niño /La Niña years?
Exercise 7: Current Conditions (June 2011)
Look at the following recent image of sea level height anomalies:
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_update/sealev.gif
The lines connect points of equal height. The brighter the colors, the more positive the sea level
anomaly.
1. Record the date the image was released: ______________________
2. How does the sea level compare to the long-term average in:
· the western equatorial Pacific?
· the eastern equatorial Pacific?.
Can you guess from what you observe whether there is an El Niño event in progress? Explain
your answer in a few sentences (i.e. compare your observations above to normal or unusual
conditions in each area).
Now, go to the following web-page: http://www.pmel.noaa.gov/tao/jsdisplay (this web-site
contains data collected by the TAO/Triton array Select the image on the left titled "Sea Surface
Temperature and Winds". The top figure shows temperatures (in colors) and winds (arrows - the
length of the arrows shows how strong the winds are). The bottom figure shows anomalies, i.e.
deviations from the normal. This bottom figure is actually quite useful too, because it may hint at
processes not going on as usual! Concentrate on this diagram!
3. In general, are the temperatures along the equator warmer, colder or close to normal?
4. What is the direction of the wind anomalies?
Return to the web-page: http://www.pmel.noaa.gov/tao/jsdisplay. You will now plot the
thermocline across the equator in the Pacific. Carry out the following steps:
· Click the blue-green button called “Data Display”
- Click the blue-green button called “Section plots”
· Click the second orange button from the left called “Depth” right under the word “Section”
· Click the red button called “Make plot!”
5. Describe the similarities or differences in thermocline thickness and depth between the eastern
and western Pacific.
6. Is an El Niño in progress? Why or why not?
Look at the last two months' sea surface temperature data by going to the following page:
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml
Select the "SST Animation - Tropical Pacific." You will see sea surface temperatures and
anomalies for the last twelve weeks.
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7. Do you see any Kelvin waves propagate from west to east along the equator in the form of
higher temperatures?
8. How would you describe the temperatures along the South American coast, especially near the
equator? Normal, warmer or colder?
9. Summarize your observations on sea surface heights and temperatures over the past few weeks
Taking everything into consideration, what’s your description of the
conditions: normal or unusual?
Look at the El Niño Advisory by the Climate Prediction Center of NOAA:
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/
10. According to this agency, is there an El Niño event under way?
11. How extensive in area is the sea surface warming?
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