So what is an el nino? Essay

This essay has a total of 3123 words and 17 pages.

So what is an el nino?

So What is an El Nino, Anyway?
A non-technical description

An El Nino is a temporary change in the climate of the Pacific ocean, in the region around
the equator. You can see its effects in both the ocean and atmosphere, generally in
Northern Hemisphere winter. Typically, the ocean surface warms up by a few degrees
celsius. At the same time, the place where hefty thunderstorms occur on the equator moves
eastward. Although those might seem like small differences, it nevertheless can have big
effects on the world's climate.

o What causes it?
o What makes it stop growing?
o What effects does it have?
o How long does it last?
o How often do we get them?
o How well can we predict El Nino?
o A more technical explanation

What causes it?
Usually, the wind blows strongly from east to west along the equator in the Pacific. This
actually piles up water (about half a meter's worth) in the western part of the Pacific.
In the eastern part, deeper water (which is colder than the sun-warmed surface water) gets
pulled up from below to replace the water pushed west. So, the normal situation is warm
water (about 30 C) in the west, cold (about 22 C) in the east.

In an El Nino, the winds pushing that water around get weaker. As a result, some of the
warm water piled up in the west slumps back down to the east, and not as much cold water
gets pulled up from below. Both these tend to make the water in the eastern Pacific
warmer, which is one of the hallmarks of an El Nino.

But it doesn't stop there. The warmer ocean then affects the winds--it makes the winds
weaker! So if the winds get weaker, then the ocean gets warmer, which makes the winds get
weaker, which makes the ocean get warmer ... this is called a positive feedback, and is
what makes an El Nino grow.

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So what makes it stop growing?
The ocean is full of waves, but you might not know how many kinds of waves there are.
There's one called a Rossby wave that is quite unlike the waves you see when you visit the
beach. It's more like a distant cousin to a tidal wave. The difference is that a tidal
wave goes very quickly, with all the water moving pretty much in the same direction. In a
Rossby wave, the upper part of the ocean, say the top 100 meters or so, will be lesirely
sliding one way, while the lower part, starting at 100 meters and going on down, will be
slowly moving the other way. After a while they switch directions. Everything happens very
slowly and inside the ocean, and you can't even see them on the surface. These things are
so slow, they can take months or years to cross the oceans. If you had the patience to sit
there while one was going by, you'd hardly notice it; the water would be moving 100 times
slower than walking speed. But they are large, hundreds or thousands of kilometers in
length (not height! Remember, you can hardly see them on the surface), so they can have an
effect on things. Another wave you rarely hear about is called a Kelvin wave, and it has
some characteristics in common with Rossby waves, but is somewhat faster and can only
exist close to the equator (say, within about 5 degrees of latitude around the equator).

El Ninos often start with a Kelvin wave propagating from the western Pacific over towards
South America. Perhaps you saw, on the TV news, the movie (produced by JPL) for the El
Nino of 1997/98? It showed a whitish blob (indicating a sea level some centimeters higher
than usual) moving along the equator from Australia to South America. That's one of the
hallmarks of a Kelvin wave, the early part of the El Nino process.

When an El Nino gets going in the middle or eastern part of the Pacific, it creates Rossby
waves that drift slowly towards southeast Asia. After several months of travelling, they
finally get near the coast and reflect back. The changes in interior ocean temperature
that these waves carry with it "cancel out" the original temperature changes that made the
El Nino in the first place. I'm being deliberately vague here becuase it's complicated;
look at the "For Further Reading" link or the "More Technical Explanation" link for more
information. The main point is that it shuts off when the these funny interior-ocean waves
travel all the way over to the coast of Asia, get reflected, and travel back, a process
that can take many months.

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What effects does it have?
A strong El Nino is often associated with wet winters over the southeastern US, as well as
drought in Indonesia and Australia. Keep in mind that you aren't guaranteed these effects
even though there is an El Nino going on; but the El Nino does make these effects more
likely to happen.

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How long does it last?
A strong El Nino can last a year or more before conditions return to normal. If you read
the bit above about Rossby and Kelvin waves (you did, didn't you?) then you know that it
lasts more or less as long as it takes the interior-ocean waves to travel all the way over
to the coast of Asia, get reflected, and travel back. You can also look at the Historical
El Nino section, which has a plot showing the last 30 years of El Ninos, and judge for

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How often do we get them?
El Ninos happen irregularly, but if you want to impress people at cocktail parties, you
might mention that we usually get one every three to seven years. Note the word "usually":
sometimes they turn up more frequently, sometimes less. You can also look at the
Historical El Nino section, which has a plot showing the last 30 years of El Ninos, and
judge for yourself (deja vu).

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How well can we predict El Nino?
On average, complex computer models designed to predict El Nino can successfully do so 12
to 18 months in advance. However, it seems to vary by episode; sometimes El Ninos are
predicted quite well, with plenty of advance notice from the models, while other times
they are predicted poorly, with the models not picking them up until the El Nino has
already started. Trying to fix up the models is one of our research topics here at

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This isn't much of an explanation.
A more technical explanation, complete with nifty graphics, has been created by the people
at NOAA's Pacific Marine Environmental Laboratory. Check that out, or look at the "For
Further Reading" link for written material.

Last modified: 25 June 1997
Copyright © 2000 David W. Pierce. All rights reserved.

El Nino
a warm current of water
El Nino (Spanish name for the male child), initially referred to a weak, warm current
appearing annually around Christmas time along the coast of Ecuador and Peru and lasting
only a few weeks to a month or more. Every three to seven years, an El Nino event may last
for many months, having significant economic and atmospheric consequences worldwide.
During the past forty years, ten of these major El Nino events have been recorded, the
worst of which occurred in 1997-1998. Previous to this, the El Nino event in 1982-1983 was
the strongest. Some of the El Nino events have persisted more than one year.

El Nino Years
1902-1903 1905-1906 1911-1912 1914-1915
1918-1919 1923-1924 1925-1926 1930-1931
1932-1933 1939-1940 1941-1942 1951-1952
1953-1954 1957-1958 1965-1966 1969-1970
1972-1973 1976-1977 1982-1983 1986-1987
1991-1992 1994-1995 1997-1998

Selected text from: CPC ENSO Main Page
In the tropical Pacific, trade winds generally drive the surface waters westward. The
surface water becomes progressively warmer going westward because of its longer exposure
to solar heating. El Nino is observed when the easterly trade winds weaken, allowing
warmer waters of the western Pacific to migrate eastward and eventually reach the South
American Coast (shown in orange). The cool nutrient-rich sea water normally found along
the coast of Peru is replaced by warmer water depleted of nutrients, resulting in a
dramatic reduction in marine fish and plant life.

Animation by: Shao
In contrast to El Nino, La Nina (female child) refers to an anomaly of unusually cold sea
surface temperatures found in the eastern tropical Pacific. La Nina occurs roughly half as
often as El Nino.

La Nina Years
1904-1905 1909-1910 1910-1911 1915-1916
1917-1918 1924-1925 1928-1929 1938-1939
1950-1951 1955-1956 1956-1957 1964-1965
1970-1971 1971-1972 1973-1974 1975-1976
1988-1989 1995-1996

1997-1998 El Nino
the most recent event
The most recent El Nino event began in the spring months of 1997. Instrumentation placed
on Buoys in the Pacific Ocean after the 1982-1983 El Nino began recording abnormally high
temperatures off the coast of Peru. Over the next couple of months, these strength of
these anomalies grew. The anomalies grew so large by October 1997 that this El Nino had
already become the strongest in the 50 years of accurate data gathering.

The image below displays the Sea Surface Temperature (SST) Anomalies in degrees Celsius
for the middle of September, 1997. By this time, the classic El Nino pattern has almost
fully ripened, with maxima above 4 degrees Celsius.

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