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El Nino and La Nina |
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An El Nino or La Nina is a temporary change
in the climate of the Pacific
Ocean. It can be seen in
measurements of the sea surface temperature in the region around the equator.
Although the changes in sea temperatures may seem small, they can have huge
effects on the world's climate.
El Nino was originally recognized by fishermen off the
coast of South America as the appearance of unusually warm
water in the Pacific Ocean, occurring near the beginning
of the year. El Nino means The Little Boy or Christ Child
in Spanish. This name was used for the tendency of the phenomenon
to arrive around Christmas.
La Nina is essentially the opposite of El
Nino. The name La Nina ("the girl child") was coined to represent
the opposite of El Nino ("the boy child"). The terms El Viejo and
anti-El Nino are also used.
What causes El Nino?
What causes La Nina?
What is the El Nino/Southern
Oscillation (ENSO)?
What does ENSO-neutral mean?
Maps of water temperatures
during El Nino and La Nina episodes
Why are El Nino and La Nina
typically strongest during the winter and early spring?
How do El Nino and La Nina
affect weather patterns in the United States?
How do El Nino and La Nina
affect weather in the Lower Mississippi River Valley?
How do El Nino and La Nina
affect the Atlantic hurricane season?
How do El Nino and La Nina
affect tornado activity in the Southeast United States?
What
causes El Nino?
In normal, non-El Nino conditions, the
trade winds blow towards the west across the tropical Pacific. Warmer water
near the surface of the ocean, heated by the sun, is blown to the west by the
trade winds. Warmer water "piles up" in the west Pacific (the sea
surface is about one and one half feet higher at
Indonesia than at Ecuador) as cooler water rises from deep in the ocean in
the east Pacific to replace the water that was moved away by the winds. The
sea surface temperature is about 8 degrees C higher in the west, with cooler
temperatures in the east Pacific due to upwelling of colder water from deep
in the ocean. Rainfall develops much more frequently in rising air over warm
water so normal rainfall amounts are higher over the west Pacific and the east
Pacific area is relatively dry.
During El Nino, the trade winds relax in
the Pacific. This leads to warmer water temperatures in the eastern and,
especially, central Pacific because upwelling is reduced. As the pool of warmer
water moves eastward, the areas of best rainfall development also move to the
east, with associated flooding in Peru and drought in Indonesia and Australia.
The eastward displacement of this heat source (the
warmest water) results in large changes in the global atmospheric
circulation, which in turn forces changes in weather in regions far removed
from the tropical Pacific.
El Nino episodes typically
occur every 3 to 5 years. However, this interval has varied from 2 to 7 years. They typically last 9 to 18 months.
El Nino episodes tend to develop from March to June and reach peak intensity
during the December to April period, weakening from May to July. Prolonged El
Nino episodes have lasted 2 years and even as long as 3 to 4 years. Every El Nino is somewhat different in magnitude and
in duration.
What
causes La Nina?
La Nina, also called El Viejo, is
essentially the opposite of El Nino. La Nina exists when cooler than usual
ocean temperatures occur in the eastern Pacific. La Nina occurs almost as
often as El Nino, but has been lesser known. La Nina and El Nino are two
stages of the same larger phenomenon.
Stronger than usual trade winds, which blow from east to west across the
Pacific Ocean, define La Nina. As warmer water near the
surface of the ocean is blown to the west, cooler water
rises from deep in the ocean in the east Pacific to replace
water that was moved away. The colder water cools the overlying
air which hampers the formation of clouds and tropical thunderstorms
in central and eastern areas of the Pacific Ocean. This
suppression of rain-producing clouds leads to dry conditions
from near the Date Line east to South America.
La Nina is characterized by unusually cold
ocean temperatures in the equatorial Pacific, as compared to El Nino, which
is characterized by unusually warm ocean temperatures in the equatorial
Pacific.
La Nina episodes typically occur every 3 to 5 years. However,
this interval has varied from 2 to 7 years. La Nina episodes
typically last 1 to 3 years. They tend to develop from March
to June and reach peak intensity during the December to
April period, weakening from May to July.
What
is the El Nino/Southern Oscillation (ENSO)?
El Nino and La Nina are abnormal episodes
of warming or cooling of surface ocean waters in the eastern tropical
Pacific. The fluctuations in ocean temperatures during El Nino and La Nina
are accompanied by even larger-scale fluctuations in air pressure between the
western and eastern tropical Pacific. 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.
When the waters of the eastern
Pacific are abnormally warm (an El Nino
event) sea level pressure drops in the eastern Pacific
and rises in the west. The reduction in the pressure gradient is accompanied
by a weakening of the low-latitude easterly trades. These pressure
departures are reversed during La Nina, which features below-average air
pressure over Indonesia and the western tropical Pacific and above-average
air pressure over the eastern Pacific.
The Southern Oscillation Index (SOI) is designed to measure the strength
and phase of the Southern Oscillation. The SOI is calculated
using departures from normal in the surface air pressure
difference between Tahiti, French Polynesia, and Darwin,
Australia. These stations are used because of their long
data records.
During El Nino episodes the SOI has a large negative value due to
lower-than-average air pressure at Tahiti and
higher-than-average pressure at Darwin.
During La Nina episodes the SOI has a positive value due to
higher-than-average air pressure at Tahiti and
lower-than-average pressure at Darwin.
What does ENSO-neutral mean?
ENSO-neutral refers to periods when neither El Nino nor La Nina is present.
These periods often coincide with the transition between
El Nino and La Nina events. During ENSO-neutral periods
the ocean temperatures, tropical rainfall patterns, and
atmospheric winds over the equatorial Pacific Ocean are
near the long-term average.
Maps
of water temperatures during El Nino and La Nina episodes
In December 1993, the sea surface
temperatures and the winds were near normal, with warm water in the western
Pacific Ocean (in red on the top panel of the December 1993
plot), and cool water, called the "cold tongue" in the eastern Pacific Ocean
(in green and blue on the top panel of the December
1993 plot). The winds in the western Pacific are very weak (see the arrows
pointing in the direction the wind is blowing towards), and the winds in the
eastern Pacific are blowing towards the west (towards Indonesia). The bottom panel of the December 1993 plot shows
anomalies (the way the sea surface temperature and wind differ from a normal
December). In this plot, the anomalies are very small (yellow/green), indicating
a normal December.
December 1997 was near the peak of a
strong El Nino year. In December 1997, the warm water (red in the top panel
of the December 1997 plot) has spread from the western Pacific Ocean towards
the east (in the direction of South America), the "cold tongue"
(green color in the top panel of the December 1997 plot) has weakened, and
the winds in the western Pacific, usually weak and towards the west, are
blowing strongly towards the east, pushing the warm water eastward.
December 1998 was a strong La Nina (cold)
event. The cold tongue (blue) is cooler than usual by about 3� Centigrade.
The cold La Nina events sometimes (but not always) follow El Nino events.
Why
are El Nino and La Nina typically strongest during the winter and early
spring?
El Nino and La Nina are
typically strongest during the period from December to April because the
equatorial Pacific sea-surface temperatures are normally warmest at this time
of the year. A slight warming of the waters due to El Nino can result in a
major redistribution of tropical convective rainfall, whereas a slight
cooling due to La Nina can restrict the tropical convection to Indonesia.
How
do El Nino and La Nina affect weather patterns in the United States?
It is believed that El Nino may have
contributed to the 1993 Mississippi floods and the 1995 California floods. It is also believed that El Nino
contributes to a smaller chance of hurricanes in the North Atlantic, sparing Atlantic and
Gulf Coast states from the associated storm damage.
During winter, El Nino episodes (top map below)
feature a strong jet stream and storm track across the southern part of the United States,
and less storminess and milder-than-average
conditions across the North. This results in an exceptionally stormy winter
and increased precipitation across California and the southern U.S.
and less stormy conditions across the northern part
of the country. Also, there is an enhanced flow of marine air into western North America,
along with a reduced northerly flow of cold air
from Canada to the United States. These conditions result in a milder than normal
winter across the northern states.
La Nina episodes (bottom map below) feature
a more northward placement of jet stream flow over the United States and Canada, with colder and stormier than average conditions
across the North, and warmer and less stormy conditions across the South.
Large portions of central North America experience increased storminess,
increased precipitation, and an increased frequency of significant cold-air
outbreaks, while the southern states experiences less storminess and
precipitation.
Unfortunately, not all El Nino episodes are
the same, nor does the atmosphere always react in the same exact way from one
El Nino to another.
How do El Nino and La Nina
affect weather in the Lower Mississippi River Valley?
El Nino episodes generally mean wetter and cooler
weather during the winter and spring for the Deep South. During El Nino events, the jet stream is shifted
southward and an unusually deep trough builds over the central United States. One significant effect associated with this
pattern is cooler than average temperatures over the southeast United States. This pattern also increases the transport of
moisture from the Gulf of
Mexico, which results in
wetter than normal conditions over the Gulf Coast states.
During La Nina cycles, the jet lifts farther north
across the Great Lakes. As a result, the Deep South experiences drier and warmer conditions than would
normally be expected.
The connections between El Nino/La Nina
events and tornado development are not clear cut but preliminary research
indicates that La Nina corresponds to an especially active phase for
tornadoes over the Deep
South with a relatively
high frequency of cold-season outbreaks of F2 or stronger tornadoes.
El Nino events
generally suppress Atlantic hurricane activity. Fewer hurricanes than normal
form in the Atlantic during the peak of Atlantic hurricane season, protecting
the Gulf Coast states from hurricanes and their associated destruction. La Nina increases the number of
hurricanes that develop and allows stronger hurricanes to form.
How
do El Nino and La Nina affect the Atlantic hurricane season?
The change in winds with height
is referred to as vertical wind shear. Hurricane formation requires that
winds be fairly uniform throughout the atmosphere. In other words, hurricanes
cannot form if the vertical wind shear is too high.
El Nino produces stronger
westerly wind at upper levels of the atmosphere across the tropical Atlantic
than in normal non-El Nino seasons.
This increases the total vertical wind shear, basically shearing the tops
from developing storms before a healthy circulation can form. El Nino events
generally suppress Atlantic hurricane activity so fewer hurricanes than normal
form in the Atlantic during August to October, the peak of Atlantic hurricane
season.
During La Nina, westerly winds
high in the atmosphere weaken. This results in an expanded area of low
vertical wind shear, allowing more Atlantic hurricanes to develop during La
Nina events. La Nina increases the number of hurricanes that develop and
allows stronger hurricanes to form.
The chances for the continental
U.S. and the Caribbean Islands to experience a hurricane increase
substantially during La Nina and decrease during El Nino.
El Nino and La Nina also influence where Atlantic
hurricanes form. During La Nina, more hurricanes form in the deep Tropics
from weather disturbances that originate over North Africa. These systems have a much greater likelihood
of becoming major hurricanes, and of eventually reaching the U.S. and the Caribbean Islands.
The incidence of hurricanes is higher during the neutral phase (when
neither El Nino nor La Nina are in effect) than during El
Nino. Although hurricanes occur more often during La Nina
episodes, significant tropical weather events have occurred
during the neutral phase. For example, the record shattering
2005 hurricane season that included Katrina and Rita occurred
during the neutral phase. Hurricane Andrew, the most destructive
United States hurricane of record, made landfall along the
Gulf coast during a neutral phase of the El Nino/Southern
Oscillation in 1992.
How
do El Nino and La Nina affect tornado activity in the Southeast United
States?
The connections between El Nino/La Nina
events and tornado development over the Deep South are not clear cut. However, preliminary research indicates a
relationship between El Nino and the occurrence of strong to violent tornado outbreaks
over the Southeast
United States.
In the case of F2 and greater tornadoes, more appear to occur during La Nina
months. La Nina seasons are normally associated with warmer than average
temperatures in the South, producing a sharper frontal zone between warmer
air and the normally cold arctic air to the north. During this time,
Deep South weather is more unstable and colliding air masses set the stage
for more tornadoes. A study of the top 15 tornadic outbreaks (at least 40
tornadoes) showed that only one outbreak occurred during an El Nino event.
Six occurred during a La Nina event, while the remaining eight occurred
during neutral ENSO years.
Click here for the current
El Nino status and forecast from the National Weather Service Climate Prediction Center.
Click here for the monthly
El Nino/Southern Oscillation (ENSO) Diagnostic Discussion from the National Weather Service Climate Prediction Center.
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