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El Niño Unit (Topic 8B) – page 1
El Niño Unit
Section:
“El Niño” is a warm ocean current that flows along the Equator and towards the west coast of
South America approximately every 4 to 7 years. It often reaches the coast in December, and is
named “El Niño” after the baby Jesus whose birth is celebrated during December. The warm
ocean water remains along the coast for about a year to 18 months on average. It pushes down
cool, upwelling water, cutting off the supply of nutrients to phytoplankton. With less food, many
animals like fish, birds, and marine mammals die, so fishermen’s incomes plummet. In addition,
more rain falls, leading to floods and mudslides.
At the same time that El Niño affects the east side of the Pacific, the west side of the Pacific
Ocean experiences a cooling and thus less rain. Thus, atmospheric pressure increases in the
western Pacific and decreases in the eastern Pacific. This reversal of the normal conditions in the
atmosphere is called the “Southern Oscillation.” During the last century, scientists began to
recognize that that El Niño, the Southern Oscillation, and other changes across the entire Pacific
Ocean (the world’s largest ocean) were all linked together, and came to call the phenomenon
ENSO (El Niño-Southern Oscillation).
1. What is an El Niño?
A ____________________ current that moves towards _______________________
along the Equator.
2. How do El Niño conditions affect the temperature of the ocean on each side of the
Pacific? In other words, do they get warmer or cooler?
The water on the eastern side of Pacific Ocean at the Equator gets ___________, and
the water on the western side of the Pacific Ocean at the Equator gets ____________.
3. How do El Niño conditions affect the temperature of the climate on each side of the
Pacific? In other words, do they get more clouds and rain or less?
The eastern side of Pacific Ocean at the Equator gets ______________ clouds & rain,
and the western side of the Pacific Ocean gets _______________ clouds & rain.
4. During El Niño conditions, is there more upwelling or less along the coast of the
Americas?
5. During what time of year do El Niño conditions typically begin?
__________________
6. About how long do El Niño conditions typically last?
__________________
7. About how often do El Niño conditions happen?
__________________
El Niño Unit (Topic 8B) – page 2
Normal Conditions
W
During normal conditions, the
ar
m
trade winds push ocean water near
the Equator to the west. As the
water travels near the Equator, it
becomes warmer and warmer.
Eventually it hits Asia, and begins
to pile up, raising the sea surface
Northern
on the western side of the ocean by Australia
South
America
about 3 feet. Gravity wants to push & Indonesia
the water “downhill” towards the
east, but the winds hold it in the
Land
Land
Warm
west 1, so instead it flows
“downhill” along the east coasts of
Cold
Asia and Australia as western
boundary currents 2. The trade
winds also pull water away from the west coast of South America, and water rises up from below
to replace it (upwelling).
The warm water on the west side of the Pacific warms the air above it, lowering its density and
causing it to rise. The air is moist owing to lots of evaporation from the warm ocean water, so as
it rises and cools, the water vapor condenses, producing lots of clouds and rain. Cooler air from
the east replaces the rising air, and is itself warmed, and rises. Air sinks on the cooler, east side
of the Pacific 3, so clear skies with little rain are common there.
Notice that the temperature difference between the two sides of the ocean creates winds which
blow to the west. These winds cause warm water to pile up on the western side of the ocean,
creating the temperature difference. Thus, the ocean water and winds reinforce one another; they
are part of a “positive feedback loop.”
1
In fact, some water does leak back along the Equator in the “doldrums” where the air is rising instead of pushing
the water to the west. This current is call the “Equatorial Countercurrent,” because it flows along the Equator
against the wind. Once the winds weaken, this current becomes the much stronger “El Niño.”
2
the Kuroshio and East Australian Current
3
The entire circulation is a “convection cell.”
El Niño Unit (Topic 8B) – page 3
El Niño Conditions
During El Niño conditions, the
W
ar
trade winds weaken. This allows
m
some warm water to flow
“downhill” to the east, back across
the Pacific. Since the water is no
If the air does not rise on the western side of the ocean,
longer as warm in the west, the air
air will not move west to replace it.
Northern
above it does not get as warm and
Australia
South
less air rises. Since less air rises,
& Indonesia
America
less air comes in from the west to
replace the rising air. In other
Land
Land
Warm
words, the winds become weaker.
This allows more warm water to
Cold
flow east, which weakens the
winds, which allows more warm
water to flow east, which weakens the winds, which allows…you get the idea. As during normal
conditions, there is a “positive feedback loop” in which the atmosphere and ocean keeping
pushing one another to change more and more.
8. What causes winds?
Winds occur when one location is warmer than another location. The warmer spot warms
the air above it. This (A)______________________ the density of the air and thus causes
it to (B)______________. Cooler air from the cooler spot comes in to replace the rising
air.
A: increases, decreases
B: rise, sink
9. Why to the winds near the Equator weaken or even reverse during El Niño conditions?
The western side of the ocean is (C)__________________ during El Niño conditions
and the eastern side of the ocean is (D)__________________ during El Niño conditions,
so air on the western side of the ocean has a HIGHER density than normal (not as low)
compared to the other side of the ocean. Therefore, (E)__________________ air rises on
the western side of the ocean, and (E)__________________ air comes west to replace it.
C: warmer, less warm
D: cooler, less cool
E: more, less
El Niño Unit (Topic 8B) – page 4
As the winds weaken, they can no longer push warm water away from the coast of South
American, and the warm water of El Niño arrives to push down the cooler, upwelling water.
With no upwelling to fertilize the phytoplankton with nutrients, there is less food for animals, so
many animals die. In addition, the warm water of El Niño warms the air and produces lots of
evaporation. Thus, more moist air rises along the coast of South America, leading to more clouds
and rain than usual.
Once the warm water of El Niño hits the coast of South America, it flows north and south along
the coast of the Americas, shutting down upwelling and increasing the amount of rainfall.
10. How and why does the amount of rainfall on each side of the ocean change
during El Niño conditions?
Water becomes warmer on the (A)______________________ side of the ocean.
Warm water evaporates (B)__________________ easily than cold water,
so there is (B)______________ water vapor in the air available to become clouds & rain.
Water becomes cooler (less warm) on the (A)_____________________ side of the ocean,
so there is (B)______________ water vapor in the air available to become clouds & rain.
A: eastern, western
B: more, less
11. How and why is there less upwelling along the coast of the Americas
during El Niño conditions?
There is less upwelling along the coast of the Americas (on the eastern side of the ocean),
because at the Equator, water is flowing (A)___________________,
which is (B)_________________________ the coast of the Americas.
Upwelling occurs when winds and currents take water (B)_________________________
a coast, and water rises up to replace the water that is taken away.
A: east, west
B: towards, away from
El Niño Unit (Topic 8B) – page 5
El Niño conditions mainly affect the Pacific Ocean at the Equator, changing water temperatures
by up to 4oC or so (that’s 7oF or more) in places. (However, these changes then ripple outwards
an affect other parts of the world.) For example, the coast of California might typically change
by 1oC (2oF) during El Niño conditions. This may not seem like much, but remember that ocean
water’s heat capacity is about 14 times greater than the heat capacity of the land. That means that
the amount of heat needed to increase the temperature of ocean water by 2oF would raise the
temperature of the land by 28oF! This is impressive, but it is not likely that all this additional heat
will be transferred from the ocean to the land. As we have noted many times, the most important
effect is the change in the amount of evaporation from the ocean and therefore the amount of
water vapor that is available to become clouds and rain.
During some El Niño years, warm water gets farther north than during others, so it not does
always have the same effect on California. For example, during the 1976 El Niño, California had
one of its worst droughts. The strongest and most reliable effects take place in the Pacific Ocean
near the Equator.
The changes that take place in the Pacific Ocean ripple outwards and can affect climate on the
other side of the world. For example, El Niño shifts the position of the Jet Stream, a high-altitude
air current (“wind”), with important implications for weather throughout the Northern
Hemisphere.
Scientists are not sure what causes normal conditions to shift to El Niño conditions. Most
believe that a local change in the atmosphere kick-starts the feedback loop that eventually leads
to full-blown El Niño conditions, because the atmosphere can change faster due to its lower
density and heat capacity.
Note: The gyres do not reverse during El Niño conditions. The trade winds weaken during El
Niño conditions and sometimes do go backwards near the Equator, but the westerlies and
momentum keep the gyres going in their normal direction. (This is a common misunderstanding,
so I thought it was important to mention it.)
12. Does California always get more rain during El Niño conditions?
13. Do El Niño conditions only affect weather and climate near the Pacific Ocean,
or do El Niño conditions change weather and climate in other parts of the world too?
14. Which are more likely to get El Niño conditions started,
natural changes (fluctuations) in the atmosphere or ocean?
15. Does the California Current reverse and flow north during El Niño conditions?
El Niño Unit (Topic 8B) – page 6
Summary of Changes in the Ocean during typical El Niño conditions
West Side
of Pacific
East Side
of Pacific
Answer the following questions about changes in the ocean during El Niño conditions.
16. Do the winds near the Equator get stronger or weaker?
17. Does the ocean current near the Equator begin to flow east or west?
18. Does the ocean current near the Equator begin to flow towards
or away from the Americas?
19. Does the water on the eastern side of the ocean by the Americas get warmer or colder?
20. Does the water on the western side of the ocean by Asia get warmer or colder?
21. Is there more or less upwelling along the coast of the Americas?
22. Is there more or less life in the ocean near the coast of the Americas?
23. How does the climate of the west coast of the Americas change?
In other words, does it get warmer or colder? Are there more clouds and rain, or less?
24. How does the climate of the east coast of the Asia change?
In other words, does it get warmer or colder? Are there more clouds and rain, or less?
El Niño Unit (Topic 8B) – page 7
Effects of El Niño
El-Niño-related changes in climate cost the state of California about $1 or $2 billion every time
an El Niño occurs. By feeding data from the TAO array of ocean sensors (cost: about $10 million
a year) into computer simulations of climate, we can now issue El Niño warnings with some
degree of accuracy. Spending about $100 million on things like sandbags, clearing storm drains,
giving appropriate training to disaster relief and reconstruction teams, and so on can cut the cost
of an El Niño nearly in half and save lives. Clearly, this is a good investment: if there are two El
Niño events each decade, we would spend about $300 million to prevent $1 billion or more in
damage. The United States, Japan, and France maintain the array of ocean sensors.
Climate vs. Weather
In this class we focus on climate, not weather. Climate is the long-term average of weather
conditions (what the weather is usually like). For example, Southern California has a warm, dry
climate. This does not mean that it is always warm (we have our cooler days) or that it does not
rain in Southern California; it means that our weather is warm most of the time and that rain is
less common here than elsewhere. Another way to think about it: weather is what conditions are
like a particular day, while climate is what conditions are typically like over a season or a year.
El Niño is just one example of a number of large-scale climate phenomena that scientists are
studying. La Niña conditions, for example, are the opposite of El Niño conditions. The trade
winds become stronger and push more warm water to the west side of the Pacific. This leads to
strong upwelling and abundant life on our side the ocean, but drought conditions on land. La
Niña conditions tend to follow El Niño conditions, and then the atmosphere and ocean return to
“normal.”
Recently, scientists identified a variety of El Niño that is being called El Niño Modoki. (The
Japanese word “modoki” means “a similar but different thing.”). In these events, the warm water
stops in the middle of the Pacific Ocean and the winds in the east Pacific get stronger, so more
water is pushed away from the west coast of the United States. Thus, water along the west coast
of the United States gets cooler than normal and the west coast gets less rain than normal.
Other examples of El-Niño-like climate phenomena under investigation include the Pacific
Decadal Oscillation (PDO), the North Atlantic Oscillation (NAO), and and the Antarctic
Circumpolar Wave (ACW). None are as well understood as El Niño, because they do not happen
as often, so we have fewer well-studied examples. For example, the PDO “flips” from its
“positive” phase to its “negative” phase once every 20 years or so, unlike El Niño conditions and
“normal” conditions which alternate about once every 5 years or so. Expect to hear more about
the PDO’s effects on California within your lifetime.
25. Are El Niño conditions a change in climate or a change in weather?
El Niño Unit (Topic 8B) – page 8
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