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El Niño
What Is El Niño?
Normal Conditions In The Tropical Pacific?
Why Is It Called El Niño?
El Niño And Major Weather Disruptions
What Is The Southern Oscillation?
El Niño Related To The Southern Oscillation
Effects Of El Niño and La Niña
What Causes El Niño?
Are All El Ninos The Same?
Getting Ready For El Niño Events
Weathering The Storms
Cleaning Up After El Niño
What Is El Niño?
Interactions between the atmosphere and the ocean in the tropical
Pacific, as well as the physical shape and thermal structure of the
tropical Pacific Ocean, result in alternations between two climate
extremes. Each extreme lasts from one to two years, with two to seven
years between events. The two extremes are frequently referred to as
El Niño and La Nina, while Southern Oscillation is the name
given to the associated atmospheric fluctuations.
El Niño (The Male Child) originally referred to a warm current
that develops almost every year along the coast of Ecuador and Peru
at Christmas time. El Niño is now commonly used to refer to
persistent, large-scale warm events in the central Pacific. Corresponding
cold events have been called anti-El Niño, counter El Niño,
El Viejo (The Grandfather) and La Nina (The Female Child).
The term Southern Oscillation was originally defined as an oscillation
in the atmospheric pressure field between the Pacific and Indian Oceans,
with related changes in temperature and precipitation in those regions.
The term is sometimes used to refer to the self-sustained oscillation
of the combined ocean-atmosphere system in the central Pacific. The
acronym ENSO (El Niño/Southern Oscillation) was first used to
differentiate the large-scale warm events from those confined to Coastal
South America only.
ENSO has been called the single most important influence on extreme
climate events in many regions of the global tropics. The four most
important parameters that contribute to the El Niño/Southern
Oscillation phenomenon are the sea surface temperature, the thermal
structure of the ocean, the atmospheric winds, and the tropical rainfall.
Normal Conditions In The Tropical Pacific
The tropical Pacific Ocean can be considered as comprised of three
layers. A shallow, warm, well-mixed layer overlies a deep cold, stratified
layer. The transition zone in between these layers is called the “thermocline.”
Within the thermocline the water temperature decreases very rapidly
with increasing depth. The warm layer is about 200 meters (656 feet)
deep in the western Pacific, but it is typically only 50 meters (164
feet) deep, or less, in the eastern Pacific.
Surface air pressure over the eastern South Pacific is normally greater
than that over the western South Pacific. This airflow is the “trade
winds.” The frictional drag of the westward flowing air on the
ocean’s surface results in the water also being driven westward
across the Pacific, where it accumulates. The slope of the ocean surface
increases towards the west until it balances the frictional drag of
the winds. To help understand this principle, visualize blowing on
a cup of hot coffee. As you blow on the cup of coffee the liquid flows
towards, and accumulates at, the far side of the cup until a balance
is achieved between the breath and the liquid. The same is true on
a much larger scale in the tropical Pacific Ocean.
Under the influence of the trade winds, sea level in the western
Pacific is normally about 50 cm (20 inches) higher than in the eastern
Pacific. As warm water accumulates in the western Pacific it depresses
the thermocline. Since warm water is less dense than cold water, a
deeper layer of warm water is required to produce the same hydrostatic
pressure as a layer of cold water. In the eastern Pacific the westward
transport of the warm surface waters by the trade winds causes the
thermocline to rise towards the surface.
The surface waters of the western Pacific are very warm, with temperatures
greater than 28°C (82.5°F) common. The surface water of the
eastern Pacific, in contrast, is among the coldest found near the equator,
and can be as cool as 20°C (68°F). This is due to the upwelling
of cold sub-surface water near the South American coast. This water
is brought to the surface by the combination of the persistent southerly
winds along the coast and the Coriolis Effect that results from the
earth’s rotation.
The Coriolis Effect is the apparent deflection of a moving object
to the right in the Northern Hemisphere, or to the left in the Southern
Hemisphere, resulting from the rotation of the earth. The Coriolis
Effect is due to the fact that the observer’s reference system
is rotating. The speed of the object is not affected; only it’s
apparent direction of motion. Thus, water flows down a drain in a clockwise
direction in the Northern Hemisphere and counter-clockwise in the Southern
Hemisphere.
As the wind pushes the water towards the Equator, the Coriolis Effect
results in the water having a component of its flow to the west. As
the surface water moves away from the coast, sub-surface water rises
to replace it. This cold upwelled water is very rich in nutrients and
can have a high oxygen content. Marine life is very abundant in such
cold water. As the surface water flows westward from the South American
coast, additional sub-surface waters along the equator maintain a tongue
of cold water across the equatorial Pacific Ocean.
Over the eastern Pacific, the northeast and southeast trade winds
flow around and out of the subtropical anticyclones at low levels and
into a convergence zone just north of the Equator. A second convergence
zone extends across the South Pacific from northwest to southeast.
Within each zone the converging surface air rises, producing clouds
and abundant rainfall, and then flows back toward the poles at high
altitudes. As the trade winds are the major circulation feature over
the eastern Pacific, a great monsoon circulation dominates the western
Pacific. In the monsoon, the low-level air flows across the Equator
from the winter to the summer hemisphere. There it rises, produces
clouds and abundant rainfalls, and then flows back into the winter
hemisphere at high levels.
Why Is It Called El Niño?
Every year around Christmas time as the winds subside, the temperature
of the water along the coast of Ecuador and Peru increases. This warming
is part of the natural annual variation in the oceanic circulation,
although the degree and extent of the warming varies from year to year.
The Peruvian fisherman as a reference to the Christ Child gave the
name El Niño, Spanish for The Male Child, to this warming event.
As the water temperature increased the fish departed for colder waters
and the fisherman used this time to repair their boats and nets.
El Niño And Major Weather Disruptions
About once every three to seven years, the water along the western
coast of South America remains warmer than normal for a year or longer.
The warmer water extends along the Equator from the coast of South
America to the International Date Line. Significant changes in the
circulation of the atmosphere are associated with these changes in
sea surface temperature. Unusual weather is often experienced throughout
the Tropics, such as droughts in Indonesia and Hawaii, floods in Ecuador,
and hurricanes in Tahiti. Large weather anomalies may occur beyond
the Tropics as well, especially in the winter hemisphere.
What Is The Southern Oscillation?
Southern Oscillation refers to a periodic seesaw in the air pressures
over the eastern and western South Pacific Ocean. Air pressure is a
measure of the mass of air over a given area. The greater the mass
of air over the area, the higher the air pressure. It has been found
that the air pressures over much of the Southern Hemisphere and the
tropical Northern Hemisphere were highly correlated over long periods
of time.
Whenever pressures are higher than normal over the western South
Pacific, they are lower than normal over the eastern South Pacific,
and vice versa. When pressure is high in the Pacific Ocean it tends
to below in the Indian Ocean from Africa to Australia. These conditions
are associated with low temperatures in both these areas, and rainfall
varies in the opposite direction to pressure.
Scientists often monitor the Southern Oscillation by comparing the
air pressures at Darwin, Australia with those at Papeete, Tahiti. The
Southern Oscillation Index is based on the difference between the pressures
at these two locations. The Southern Oscillation Index is positive
when the pressure is above normal in the eastern Pacific and below
normal in the western Pacific. Persistent high values of the index
are associated with La Nina events. The index becomes negative and
remains negative during El Niño events.
El Niño Related To The Southern Oscillation
Strictly speaking, El Niño is an oceanic phenomenon, while
the Southern Oscillation is an atmospheric phenomenon. During the warm
phase of the ENSO cycle, air pressure falls over the eastern Pacific
and rises over the western Pacific. As the pressure difference from
east to west decreases, the easterly trade winds that blow across the
equatorial Pacific become weaker. In some warm years the flow is reversed
and the wind blows from the west. As the winds weaken, the warm water
that has accumulated in the western Pacific flows back to the east.
At the same time subsurface perturbations, called Kelvin waves, move
eastward in a narrow belt centered on the Equator to the coast of South
America, where they depress the thermocline. These waves take about
three months to cross the Pacific from west to east. The result is
an increase in the water temperatures in the central and eastern Pacific,
and a warm event (El Niño) occurs.
During the cold phase of the ENSO cycle, air pressure rises over
the eastern Pacific and falls over the western Pacific. The easterly
trade winds become stronger than normal. During some cold years the
“easterlies” may extend all the way across the Pacific to
Asia. The warm water confined to the western Pacific and cold, subsurface
water is upwelled along the Equator and the South American coast. The
result is a decrease in the water temperatures in the central and eastern
Pacific, and a cold event (La Nina).
Effects Of El Niño And La Niña
El Niño, and its opposite, La Nina, produce major disruptions
in the world’s weather. The extreme El Niño of 1982-1983
resulted in over 1,000 deaths, and nearly nine billion dollars in damage.
Most of the effects are felt within the Tropics, but some higher latitude
regions are often regularly affected as well.
In the South Pacific, tropical cyclones form far to the east of their
normal spawning grounds during El Niño events. Six tropical
storms struck French Polynesia during the summer of 1982-1983, an island
chain where such storms are rarely seen in normal years. In some cases,
twin storms form over warm water in the central Pacific, one on either
side of the Equator.
El Niño also seems to inhibit the formation of hurricanes
over the Atlantic Ocean. The condensational heating of the atmosphere
that occurs in the rainstorms over the eastern equatorial Pacific during
El Niño events increases the strength of the upper tropospheric
westerly winds over the tropical Atlantic. The increased wind shear
between the upper tropospheric “westerlies” and the lower
tropospheric easterly trade winds inhibits the formation of tropical
cyclones. With regard to the number and intensity of Atlantic hurricanes,
years with strong El Niño events average four storms fewer than
normal, while years with moderate El Niño events tend to have
two fewer storms. There have been found to be fewer tropical cyclones
near Australia during El Niño years.
El Niño events also have significant biological consequences.
During El Niño the water along the coast of South America is
warmer, contains fewer nutrients and less dissolved oxygen. Reproduction
rates of some species of marine life drop to almost zero, and many
adult fish and birds die as the food supply is diminished.
What Causes El Niño?
El Niño is a natural occurrence resulting from the size, physical
shape and thermal structure of the tropical Pacific, and the interactions
between the ocean and the atmosphere there. The internal coupling between
the ocean and the atmosphere has been shown to produce recurring El
Niño events, without any external forcing involved. The implication
is that El Niño events are a natural mode of variability of
the ocean-atmosphere system of the tropical Pacific Ocean. Interactions
among the surface winds, the tropical rainfalls, the sea surface temperature
and the ocean currents are sufficient to produce recurring warm and
cold events.
During El Niño events, the east–west temperature gradient
of the sea surface is decreased. As a result, air is removed from over
the eastern Pacific and accumulated over the western Pacific. The Southern
Oscillation Index drops as surface pressures rise in the west and fall
in the east. A positive feedback exists between the ocean and the atmosphere.
The sea surface temperatures are modified by the resulting changes
in the atmospheric circulation. The water becomes warmer than normal
in the central and eastern Pacific, and cooler than normal in the western
Pacific. The atmospheric circulation, in turn, responds to the changes
in the sea surface temperature distribution. Air rises over the central
and eastern Pacific, producing clouds and rains, and sinks over the
western Pacific, resulting in clear skies and drought.
Are All El Niños The Same?
Although there are many features that are common from one El Niño
to the next, there are also significant differences. These include
such factors as the intensity, timing, evolution and duration of each
event. The El Ninos of 1972 and 1982 were very strong; those of 1976
and 1986 were moderate in intensity; while those in 1963 and 1969 were
weak. Intensity here refers to the magnitude of the sea surface temperatures,
the amounts of the anomalous tropical rainfalls and the strength of
the wind anomalies.
During the El Niño of 1972 warm water first appeared off the
coast of Peru late in the year and spread westward, while in 1982 the
warm water first appeared in the western Pacific in May and spread
eastward. While most El Ninos persist for no more than 18 months, the
El Ninos of 1941 and 1991 both lasted more than two years.
The evolution of the thermocline movements is rather consistent from
one El Niño to the next. Conditions in the central equatorial
Pacific are more similar from one event to the next, compared to those
in the eastern and western Pacific.
Getting Ready For El Niño Events
There is much you can do, right now, to be ready when the rain starts
to fall again:
- Review your home and business insurance to see if you have flood
coverage. If you don’t, you may want to purchase a policy, but
act quickly. There is a 30-day delay in the effective date, so you
can’t wait until the rain starts to obtain coverage.
- Develop a family evacuation plan including where you will meet
if a storm strikes, who will pick up the children at school, how you
will take care of your pets, etc.
- Plan evacuation routes, avoiding low-lying areas, and do a couple
of test runs so the routes become familiar.
- Stockpile emergency supplies. Think “self-reliance,”
and you will be on the right track.
- Clean up around your home or business. Trim trees and clear out
rain gutters, as well as any pool, lawn or patio drainage systems.
Run a hose through the drains for 15 minutes to be sure they are clear.
- Check the roof, and make needed repairs now.
- Take photographs of your home, inside and out, to document its
contents and condition. Keep these with you important documents. Video
cameras are particularly useful to enable simultaneous narration of
important property descriptions.
- Re-fill any prescriptions, so that you have a one-month supply
on-hand.
- Be sure your pets are vaccinated.
- Take American Red Cross first aid and CPR classes.
- Teach all family members how and when to turn off gas, electricity
and water.
- Move valuables and appliances to upper floors to prevent damage
from floodwaters, if you are in a flood-prone area.
- Faulty furnaces or damaged chimneys can emit odor-free, invisible
carbon monoxide. Purchase a carbon monoxide detector to check your
home from this deadly gas.
Weathering The Storms
- When a weather advisory is issued, turn on your television or your
radio for regular updates. If you have a NOAA Weather Radio, listen
for continuous updates.
- Head home, or to your prearranged evacuation location, as soon
as possible. If you must be in your car, be sure you have emergency
supplies with you, keep your gas tank full, and tell someone your
route, destination and anticipated arrival time.
- Don’t walk through high water. Move to higher ground, and
away from rivers, creeks and storm drains. Don’t drive around
barricades. As little as two feet of water can set a vehicle afloat,
even a bus.
- If your car stalls out in high water, leave it and head to higher
ground fast. It only takes six inches of fast-moving water to knock
a person down.
- Place sandbags around your property and against vulnerable doorways,
to help protect your home or business. Your local fire department
can supply sandbags and tell you how to use them.
- If winds are high, stay indoors and away from windows. Stay in
the building’s lower levels unless it is susceptible to flooding,
or in a small, protected space.
- In a lightning storm, do not seek shelter under trees. Lightning
seeks out tall objects. Instead, squat as low to the ground as you
can, away from trees and power lines, but don’t lie down. Inside,
stay away from windows, don’t use the telephone or electric appliances
and don’t use metal objects. Turn off all electrical items. Stay
out of the swimming pool.If you are near to the coast, don’t
head to the water’s edge to watch the waves. A three-foot swell
can produce a six-foot wave, and wind-driven waves can accelerate
rapidly. The view isn’t worth the risk.
- If you or your family are evacuated, take your pre-packaged emergency
supplies, a change of clothes, extra blankets and sleeping bags, your
insurance documents and other important papers.
- Lock your windows and doors, and turn off the utilities. If heavy
winds are predicted, nail plywood over the windows.
- Contact your kennel or Humane Society shelter for information on
how to take care of your pets. Health reasons prohibit pets from staying
in evacuation centers.
Cleaning Up After El Niño
- If your property floods, disconnect appliances that are still plugged
in until the ground is completely dry.
- Ventilate the building to speed up the drying process.
- Don’t stand in water when operating switches or using electrical
cords.
- Wear dry, rubber-soled shoes and stand on something non-conductive
(woods work best) when resetting circuit breakers.
- Don’t let your children go barefoot or play in standing water.
- Don’t drink tap water if authorities issue a “boil water
order.” Use bottles or stored water for cooking, drinking, dish
washing, and for your pets until the authorities say the water supply
is clean.
- Stay away from downed power lines and report them immediately to
your local power company.
- Don’t use any gas appliance that has been flooded until the
gas utility or qualified appliance contractor checks it. Immediately
report any gas line breaks to your local gas utility, and don’t
turn your gas back on until the utility company issues an all clear.
- Never cook indoors with a charcoal or gas grill, as the fumes can
be deadly.
- Take pictures of any damage before you start to clean up, then
make temporary repairs until you consult with your insurance company
and begin permanent repairs. Document what you do to repair your property,
and keep receipts for every purchase.
- If you must move out while your home is repaired, secure all entrances
to prevent vandalism or injuries.
- Discard any food, including canned goods that have come into contact
with floodwaters.
- Examine walls, floors, doors and windows to make sure that your
home or office is not in danger of collapsing. Watch for loose plaster
or ceilings that could fall.
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