2
Student Packet
El Niño / Southern Oscillation
Use contents of this packet as you feel appropriate. You are free to copy and use any of the material in
this lesson plan.
Packet Contents
Introduction on Climate Variability Concentrating on El Niño / Southern Oscillation (ENSO)
– can be copied to hand out to students
ENSO Worksheet -What is ENSO?
ENSO Worksheet - ENSO Phases Defined
ENSO Worksheet - Historical Perspective
ENSO Worksheet - Other Phenomena Affecting Climate Variability
Approximate Locations of the Niño Regions in the Pacific Ocean – to be used to make a
transparency
Introduction on Climate Variability Concentrating on
El Niño / Southern Oscillation (ENSO)
What is ENSO?
El Niño / Southern Oscillation (ENSO)
refers to the ocean-atmosphere system in the
tropical Pacific Ocean.
Changes in this
system have important consequences for
seasonal climate variability worldwide.
ENSO as a physical occurrence is a proven
fact; as real as snowstorms and
thunderstorms.
Scientists, however,
currently do not completely understand the
way ENSO works.
An important fact of weather and climate
is its variability. Although definitions vary,
one useful definition is climate refers to
weather conditions occurring over a period
of two weeks or longer. Weather is what is
experienced over a time period of less than
two weeks. Different time periods are used
to define variability. Time periods range
from daily weather differences, to seasonal
differences, to multi-year or longer time
periods. An example of a longer timeperiod variability would be two winters in a
row with warmer than normal temperatures.
Another example would be a colder than
normal winter followed by a winter of
normal temperatures followed by a winter of
warmer than normal temperatures. Such
year-to-year variations in the climate
patterns are associated with changes in the
wind, air pressure, storm tracks, and jet
streams that encompass areas larger than
that of the region you are interested in.
Year-to-year changes in climate variability
are linked to the ENSO phenomena.
Knowledge of ENSO conditions provides
the basis for most current forecasts of
climate conditions more than one month into
the future.
Unfortunately, terms used to describe the
changes that occur in the ocean-atmosphere
system in the Pacific Ocean basin are not
standardized. However, ENSO is becoming
the term that scientists are using to describe
both changes in Pacific sea surface
temperatures and air pressures differences
relative to a long-term average.
The
acronym, ENSO, is composed of El NiñoSouthern Oscillation, where El Niño is the
oceanic component and the Southern
Oscillation is the atmospheric component of
the phenomenon. El Niño is sometimes
used interchangeably with ENSO because
the term El Niño is often used in the popular
media. In the Learning Modules, ENSO
refers to general changes in the oceanatmosphere system, whereas El Niño will be
used to refer to a specific change.
ENSO Phases Defined
In April 2005, twenty-six nations of the
World
Meteorological
Organization’s
Regional Association IV agreed on a
definition for ENSO phases. The definitions
for the three phases, El Niño, La Niña, and
Neutral are as follows.
El Niño is characterized by positive
(warmer) sea surface temperatures
change from normal in the Niño 3.4
region (see definition below) greater
than or equal to 0.5 degrees Celsius
averaged over three consecutive months.
La Niña is characterized by negative
(cooler) sea surface temperatures change
from normal in the Niño 3.4 region (see
definition below) greater than or equal to
0.5 degrees Celsius averaged over three
consecutive months.
Here, negative
means temperatures are 1.5 degrees less
than normal.
regions is provided at the end of this
Learning Module.
Neutral is characterized by what are
considered to be “normal” conditions.
Normal conditions are defined as when
the sea surface change in temperature in
the Niño 3.4 region is between -0.5 and
0.5 degrees Celsius averaged over three
consecutive months.
Historical Perspective - Optional
In these definitions, sea surface
temperatures in region Niño 3.4 are used to
classify the season as an El Niño, La Niña,
or neutral phase. As shown in the definition,
it is not the absolute temperature in the
region that is important, but rather changes
in the temperature from normal or a base
temperature. The current base period to
define normal temperatures is the years
1971-2000.
Changes from normal are
known as anomalies. In an El Niño or the
warm phase, the anomalies are warmer than
the base. Colder anomalies indicate a cold
or La Niña phase.
The location of the Niño 3.4 region is
shown in the following figure. This region
is in the tropical Pacific Ocean between
Australia and South America. A world map
with the approximate location of these
Sir Gilbert Walker, a mathematician,
preformed the first rigorous scientific study
of ENSO.
In 1924, he observed and
reported that the sea surface air pressure
“oscillates” across the Pacific Ocean. When
the sea surface air pressure rises in the
eastern Pacific, it tends to fall in the western
Pacific. Likewise, when the sea surface air
pressure falls in the eastern Pacific it tends
to rise in the western Pacific. Walker named
this oscillation the Southern Oscillation.
He identified relationships between the
Southern Oscillation and precipitation
patterns in Australia, Africa, the Indian
subcontinent, and South America. Walker
thought some oceanic mechanism was
responsible for the oscillation. However, he
could not establish any link with the ocean.
Walker’s work resulted in the Southern
Oscillation being the first identifiable
meteorological process that can be
associated with global climate variability.
Interestingly, initial reaction to Walker’s
work was highly skeptical, because his work
had no theoretical justification.
Importantly, scientists never completely
dismissed Walker’s study because of the
rigor of his analysis. Consequently, a few
scientists continued to explore the
possibility of a relationship between the sea
surface air pressure in the equatorial Pacific
and climatic variability. Jacob Bjerknes
provided a key breakthrough by associating
changes in sea surface temperatures to
differences in the trade-wind circulation
patterns in both the Northern and Southern
Hemispheres of the Pacific.
Bjerknes’
theoretical developments lead to the
conclusion that the ocean’s influence cannot
be ignored in describing the Southern
Oscillation.
Acknowledging Walker’s
earlier work, Bjerknes named this airflow
the Walker Circulation. This early work
provides the basis for climate science today.
In fact, recently one scientist, Michael H.
Glantz of the National Center for
Atmospheric Research, has gone as far as
stating knowledge of ENSO is science’s gift
to the twenty-first century.
Other Phenomena Affecting
Variability - Optional
Climate
The ENSO phenomenon is the most
highly recognizable physical phenomena
affecting global seasonal climate. Other
physical phenomenon and interactions
between the phenomenons are, however,
important. Of all the many phenomena
affecting climate, four are briefly
mentioned. One is similar to ENSO, but is
found in the Atlantic Ocean, one has a major
impact on a large population base, and the
last two are longer cycles.
Similarities have been established
between the North Atlantic Oscillation and
ENSO; however, the North Atlantic
Oscillation is not as well understood. The
North Atlantic Oscillation is a large
seesaw pattern in atmospheric mass between
a subtropical high and a polar low.
Associations between the North Atlantic
Oscillation and climate variability have been
established for parts of Europe, northern
Africa, southeastern United States, and
northeastern Brazil.
The Indian monsoon cycle refers to
changes in seasonal winds in the Indian
Ocean. During the summer winds blow
from the Indian Ocean towards the Indian
sub-continent; bringing with them moisture.
This causes a wet season during the summer.
During the winter, the winds reverse,
blowing from land toward sea. Thus, the
winds are dry and a dry season occurs.
Gaining a fuller understanding of this
process is important because such a large
population (over one billion people) is
dependent on the Indian monsoon cycle for
food production.
The Pacific Decadal Oscillation is
associated with sea surface temperatures in
the northeastern and tropical Pacific Ocean
basin. This oscillation has been described as
a long-lived ENSO-like pattern of Pacific
climate variability. The Pacific Decadal
Oscillation is distinct from ENSO in that
Pacific Decadal Oscillation phases are
persistent with periods lasting between 20
and 30 years instead of approximately six to
18 months that ENSO lasts. Causes of the
Pacific Decadal Oscillation are not known.
The North American climate anomalies
associated with Pacific Decadal Oscillation
are similar to those associated with ENSO,
but are usually not as extreme. The affect of
ENSO events on climate in North America
may be dependent on the Pacific Decadal
Oscillation phase.
A cycle of approximately 80 years in
length, the Gleissberg Cycle, appears to
influence climate by causing the atmosphere
to cool for 40 years and then warm for 40
years. Such cycles will affect predictions of
global warming. Currently, it appears we
maybe entering a cold cycle whose
minimum is expected around 2030.
Further studies are necessary to fully
understand the relationships the different
phenomena and climate variability. This
creates interesting career paths for students.
For additional information
North
Atlantic
Oscillation
http://www.ldeo.columbia.edu/NAO/
Indian
Monsoon
http://yang.gmu.edu/~yang/nasacd/www/ind
ian_monsoon.html
Pacific
Decadal
Oscillation
http://tao.atmos.washington.edu/pdo/
Gleissberg
Cycle
http://mitosyfraudes.8k.com/Calen/Landsche
idt-1.html
ENSO Worksheet -What is ENSO?
1. What does the acronym ENSO stand for?
2. What is the definition of ENSO?
3. Do scientists completely understand the way ENSO works?
Yes
No
4. What is the difference between climate and weather?
5. Year-to-year variations in climate patterns are associated with climate changes between
years. List 2 examples of year-to-year climate variability.
6. What phenomena is the basis for most current forecasts of climate conditions for more than
one month into the future?
7. The acronym ENSO is not always used by everyone. What other term / acronym is often
used in the popular media to describe changes in Pacific sea temperature and air pressure
differences?
8. Why might knowledge of ENSO be important to Joe Soccer?
ENSO Worksheet - ENSO Phases Defined
1. Indicate on the following map the location of the three Niño regions, Niño 3, Niño 3.4 and
Niño 4 in the Pacific Ocean.
2. Label the equator on the above map.
3. What Niño region is used to classify the different ENSO phases?
4. List the three common phases of ENSO. What type of sea temperature anomalies in the Niño
Region 3.4 are observed with each phase?
5. What are anomalies?
ENSO Worksheet - Historical Perspective
1. Why are not all scientific discoveries accepted by scientists and the general public when they
are discovered?
2. What mechanism was the first identified as affecting global climate variability? Who
discovered this mechanism?
3. Why were Sir Gilbert Walker’s studies not completely dismissed at the time they were
published?
ENSO Worksheet - Other Phenomena Affecting Climate Variability
1. Why is improved understanding of the Indian Monsoon so important to society?
2. Which cycle may be important in help explaining global warming?
3. Name a phenomenon that is similar to ENSO that is found in the Atlantic Ocean.
4. Name two ways the Pacific Decadal Oscillation varies from ENSO.
5. Why are there important and exciting career opportunities in climate science?
Approximate Locations of the Niño Regions in the Pacific Ocean
Niño 3.4
Equator
Niño 4
Niño 3