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El Nino - and What is the Southern Oscillation Anyway?!
El Nino, an abnormal warming of surface ocean waters in the eastern tropical Pacific,
is one part of what's called the Southern Oscillation. The Southern Oscillation is the
see-saw pattern of reversing surface air pressure between the eastern and western
tropical Pacific; when the surface pressure is high in the eastern tropical Pacific it is
low in the western tropical Pacific, and vice-versa. Because the ocean warming and
pressure reversals are, for the most part, simultaneous, scientists call this phenomenon
the El Nino/Southern Oscillation or ENSO for short. South American fisherman have
given this phenomenon the name El Nino, which is Spanish for "The Christ Child,"
because it comes about the time of the celebration of the birth of the Christ ChildChristmas.
Scientists do not really understand how El Nino forms. It is believed that El Nino may
have contributed to the 1993 Mississippi and 1995 California floods, drought
conditions in South America, Africa and Australia. It is also believed that El Nino
contributed to the lack of serious storms such as hurricanes in the North Atlantic
which spared states like Florida from serious storm related damage.
Unfortunately not all El Nino's are the same nor does the atmosphere always react in
the same way from one El Nino to another. This is why NASA's Earth scientists
continue to take part in international efforts to understand El Nino events. Hopefully
one day scientists will be able to provide sufficient warning so that we can be better
prepared to deal with the damages and changes that El Nino causes in the weather.
To really understand the effects of an El Nino event, compare the normal conditions
of the Pacific region and then see what happens during El Nino.
Normal Conditions (Non El Nino)
In general, the water on the surface of the ocean is warmer than at the bottom because
it is heated by the sun. In the tropical Pacific, winds generally blow in an easterly
direction (Trade Winds). These winds tend to push the surface water toward the west.
As the water moves west it heats up even more because it's exposed longer to the sun.
Meanwhile in the eastern Pacific along the coast of South America an upwelling
occurs. Upwelling is the term used to describe when deeper colder water from the
bottom of the ocean moves up toward the surface away from the shore. This nutrientrich water is responsible for supporting the large fish population commonly found in
this area. Indeed, the Peruvian fishing grounds are one of the five richest in the world.
Normally, strong trade winds blow from the east along the equator, pushing warm
water into the Pacific Ocean. The thermocline layer of water is the area of transition
between the warmer surface waters and the colder water of the bottom.
Because the trade winds push surface water westward toward Indonesia, the sea level
is roughly half a meter higher in the western Pacific than in the east. Thus you have
warmer, deeper waters in the western Pacific and cooler, shallower waters in the east
near the coast of South America. The different water temperatures of these areas
effects the types of weather these two regions experience.
In the east, the water cools the air above it, and the air becomes too dense to rise to
produce clouds and rain (high pressure). However; in the western Pacific the air is
heated by the water below it, decreasing density of the air (low pressure) in the lower
atmosphere thus increasing the likelihood of rain. This is why heavy rain storms are
typical near Indonesia while Peru is relatively dry.
El-Nino Conditions
El Nino happens when weakening trade winds (which sometimes even reverse
direction) allow the warmer water from the western Pacific to flow toward the east.
This flattens out the sea level, builds up warm surface water off the coast of South
America, and increases the temperature of the water in the eastern Pacific.
An El Nino condition results from weakened trade winds in the western Pacific Ocean
near Indonesia, allowing piled-up warm water to flow toward South America.
The deeper, warmer water in the east limits the amount of nutrient-rich deep water
normally surfaced by the upwelling process. Since fish can no longer access this rich
food source, many of them die off. This is why these conditions are called "El Nino",
or "the Christ Child", which is what Peruvian fisherman call the particularly bad
fishing period around December. More importantly, the different water temperatures
tend to change the weather of the region.
What happens to the ocean also affects the atmosphere. Tropical thunderstorms are
fueled by hot, humid air over the oceans. The hotter the air, the stronger and bigger
the thunderstorms. As the Pacific's warmest water spreads eastward, the biggest
thunderstorms move with it. If you look on a map, you will see that suddenly islands
like Tahiti, normally tropical paradises, experience massive storms.
El Nino can have impacts on weather at various locations around the globe. Off the
east coast of southern Africa, drought conditions often occur. In countries such as
Zimbabwe, the effects of drought can be devastating.
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The clouds and rainstorms associated with warm ocean waters also shift toward the
east. Thus, rains which normally would fall over the tropical rain forests of Indonesia
start falling over the deserts of Peru, causing forest fires and drought in the western
Pacific and flooding in South America. Moreover the Earth's atmosphere responds to
the heating of El-Nino by producing patterns of high and low pressure which can have
a profound impact on weather far away from the equatorial Pacific. For instance,
higher temperatures in western Canada and the upper plains of the United States,
colder temperatures in the southern United States. The east coast of southern Africa
often experiences drought during El Nino.
http://kids.earth.nasa.gov/archive/nino/intro.html
Updated: January 22, 2003
Responsible NASA Official: Ruth Netting
Curator: NASA Science Content Manager
La Nina
La Niña is caused by a build-up of cooler-than-normal waters in the tropical Pacific,
the area of the Pacific Ocean between the Tropic of Cancer and the Tropic of
Capricorn. Unusually strong, eastward-moving trade winds and ocean currents bring
this cold water to the surface, a process known as upwelling.
Upwelling can cause a drastic drop in sea-surface temperature. Coastal sea-surface
temperatures near Ecuador and Peru dropped nearly 4 degrees Celsius (7 degrees
Fahrenheit) during the 1988-89 La Niña event.
Effects of La Niña
Both El Niño and La Niña affect patterns of rainfall, atmospheric pressure, and global
atmospheric circulation. Atmospheric circulation is the large-scale movement of air
that, together with ocean currents, distributes thermal energy on the surface of the
Earth. These changes are the main sources of variability in climate for many areas
worldwide.
La Niña is characterized by lower-than-normal air pressure over the western Pacific.
These low-pressure zones contribute to increased rainfall.
Rainfall associated with the summer monsoon in Southeast Asia tends to be greater
than normal, especially in northwest India and Bangladesh. This generally benefits the
Indian economy, which depends on the monsoon for agriculture and industry.
However, strong La Niña events are associated with catastrophic floods in northern
Australia. The 2010 La Niña event correlates with one of the worst floods in the
history of Queensland, Australia. More than 10,000 people were forced to evacuate,
and damage from the disaster was estimated at more than $2 billion.