Download 09B R-El Nino Reading

Oceanography 10, T. James Noyes, El Camino College
9B-1
El Niño
“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 1 whose birth is celebrated during December. The warm
ocean water 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. 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 NiñoSouthern Oscillation).
Normal Conditions
During normal conditions, the 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 on the western side of the ocean by about 3 feet.
Gravity wants to push the water “downhill” towards the east, but the winds hold it in the west 2,
so instead it flows “downhill” along the east coasts of Asia and Australia as western boundary
currents 3. 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 4, 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
This has always struck me as odd, because El Niño conditions typically have disastrous consequences for local
people, but the birth of the baby Jesus is “good,” right?
2
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.”
3
the Kuroshio and East Australian Current
4
The entire circulation is a “convection cell.”
Oceanography 10, T. James Noyes, El Camino College
9B-2
Normal Conditions
W
ar
m
Northern
Australia
& Indonesia
Land
South
America
Land
Warm
Cold
El Niño Conditions
W
ar
m
Northern
Australia
& Indonesia
Land
South
America
Warm
Cold
Land
During El Niño conditions, the east side
of the Pacific Ocean is unusually warm
(white/red color), while the east side of
the ocean is cooler than usual (purple/
blue colors). During La Niña conditions,
the opposite is true. NASA/JPL.
Oceanography 10, T. James Noyes, El Camino College
El Niño Conditions
9B-3
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.
During El Niño conditions, the trade winds weaken.
This allows some warm water to flow “downhill” to
the east, back across the Pacific. Since the water is
no longer as warm in the west, the air above it does
not get as warm and less air rises. Since less air rises,
less air comes in from the west to replace the rising
air. In other words, the winds become weaker. This allows more warm water to 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.
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.
Important: 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.)
Once the warm water of El Niño hits the coast of South
El Niño conditions typically occur once
America, it flows north and south along the coast of the every 4 to 7 years, though there are
Americas, shutting down upwelling and increasing the
examples of El Niño’s only 2 years apart
amount of rainfall. During some El Niño years, warm
and a decade passing without an El Niño.
water gets farther north than during others, so it not
El Niño conditions typically last for
about 12 to 18 months.
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 there 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 highaltitude air current (“wind”), with important implications for weather throughout the Northern
Hemisphere.
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.
Oceanography 10, T. James Noyes, El Camino College
9B-4
El Niño is an example of a change in climate, not weather. The word “weather” is used to describe
temperature, rainfall, and so forth on a particular day. The word “climate” refers to what conditions are
usually like, though they may vary from day to day. For example, Canada has a cooler climate that we
do. That does not mean that it never gets warm in Canada, just that it is not as warm overall or as often.
Another example: southern California has a dryer climate than the southeastern United States. This does
not mean that it never rains in southern California, just that it does not rain as often here. During an
El Niño, our climate becomes warmer and wetter (more warm days, more rainy days).
Other Climate Phenomena like El Niño
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. Expect to hear more about the PDO’s effects
on California within your lifetime.
El Niño illustrates how scientists’ understanding of climate has changed over the last few decades.
We use to think that climate change was slow and steady over long-periods of time, but El Niño and
similar climate phenomena show how climate can shift rapidly once a “positive feedback loop” is
triggered. (This is one reason that some climate scientists are particularly worried about global
warming: it could trigger a “positive feedback loop” which leads to a much larger change in the
Earth’s climate.) In other words, climate is often like a “light switch” that flips from the “on”
setting (e.g., warm) to the “off” setting (e.g., cold) without spending much time in between. (In the
past, we use to think climate was more like a light with a “dimmer” knob that was slowly being
turned up or down.)
El Niño and its Implications for Global Warming
During El Niño conditions, the water along the coast of the America’s changes by as much 3-4oC in
the places that are hardest hit like the west coast of South America, and about half as much or less in
California’s coastal waters. Thus, what may seem like a small change in temperature can have
significant effects that it would be imprudent (and risky) to ignore. Remember this as we turn our
attention to global warming, which has raised the temperature of the Arctic by 3-4oC (and the world
overall by less than 1oC).