Looking for a particular
question?
What
is La Niña?
La Niña
is defined as cooler than normal sea-surface temperatures in the central
and eastern tropical Pacific ocean that impact global weather patterns.
La Niña conditions recur every few years and can persist for as
long as two years.
What
is the difference between La Niña and El Niño?
El Niño and La Niña are extreme phases of a naturally occurring
climate cycle referred to as El Niño/Southern Oscillation. Both
terms refer to large-scale changes in sea-surface temperature across the
eastern tropical Pacific. Usually, sea-surface readings off South America's
west coast range from the 60s to 70s F, while they exceed 80 degrees F
in the "warm pool" located in the central and western Pacific. This warm
pool expands to cover the tropics during El Niño, but during La
Niña, the easterly trade winds strengthen and cold upwelling along
the equator and the West coast of South America intensifies. Sea-surface
temperatures along the equator can fall as much as 7 degrees F below normal.
Why
do El Niño and La Niña occur?
El Niño and La Niña result from interaction between the
surface of the ocean and the atmosphere in the tropical Pacific. Changes
in the ocean impact the atmosphere and climate patterns around the globe.
In turn, changes in the atmosphere impact the ocean temperatures and currents.
The system oscillates between warm (El Niño) to neutral (or cold
La Niña) conditions with an on average every 3-4 years.
What
causes La Niña?*
Typically, a La Niña is preceded by a buildup of cooler-than-normal
subsurface waters in the tropical Pacific. Eastward-moving atmospheric
and oceanic waves help bring the cold water to the surface through a complex
series of events still being studied. In time, the easterly trade winds
strengthen, cold upwelling off Peru and Ecuador intensifies, and sea-surface
temperatures (SSTs) drop below normal. During the 1988- 89 La Niña,
SSTs fell to as much as 4 degrees C (7 degrees F) below normal. Both La
Niña and El Niñq tend to peak during the Northern Hemisphere winter.
What's
the difference between La Niña and El Niño?*
Both terms refer to large-scale changes in sea-surface temperature across
the central and eastern tropical Pacific. Usually, sea-surface readings
off South America's west coast range from the 60s to 70s F, while they
exceed 80 degrees F in the "warm pool" located in the central
and western Pacific. This warm pool expands to cover the tropics during
El Niño but shrinks to the west during La Niña. The El Niño/Southern
Oscillation (ENSO) is the coupled ocean-atmosphere process that includes
both El Niño and La Niña.
What
are the global impacts of La Niña?
Both El Niño and La Niña impact global and U.S. climate
patterns. In many locations, especially in the tropics, La Niña
(or cold episodes) produces the opposite climate variations from El Niño.
For instance, parts of Australia and Indonesia are prone to drought during
El Niño, but are typically wetter than normal during La Niña.
What
are the U.S. impacts of La Niña?
La Niña often features drier than normal conditions in the Southwest
in late summer through the subsequent winter. Drier than normal conditions
also typically occur in the Central Plains in the fall and in the Southeast
in the winter. In contrast, the Pacific Northwest is more likely to be
wetter than normal in the late fall and early winter with the presence
of a well-established La Niña. Additionally, on average La Niña
winters are warmer than normal in the Southeast and colder than normal
in the Northwest.
What
is the forecast for sea surface temperatures for the remainder of 1998
and into winter 1999 and how strong will the event be?
The latest National Oceanic and Atmospheric Administration (NOAA) coupled-model
forecast (an ocean-atmosphere model) (July 1998) indicate strengthening
cold episode conditions in the tropical Pacific during the remainder of
1998.
Other
statistical and coupled-model
forecasts indicate a similar evolution. The consistency among the available
predictions together with the evolution of oceanic and atmospheric conditions
since early May indicate that a cold episode is developing and will likely
continue through the northern 1998-99 winter.
The current forecasts indicate that the 1998/99 La Niña will be
a moderate to strong episode.
What
is the forecast for U.S. weather patterns for the remainder of this year
and into 1999?
NOAA's Climate Prediction Center issues monthly long lead climate outlooks
that extend for the next 13 months. The forecast issued on July 16, 1998,
calls for continued above normal temperatures throughout most of the Southwest
U.S. and southern Florida in the late summer and fall. Into the fall,
warmer than normal temperatures are forecast for much of the Southwest
including Texas, New Mexico, Arizona, and portions of Colorado. These
conditions will extend across the Southeast during the winter months.
Cooler than normal temperatures are expected for the Pacific Northwest
in the winter. Temperatures are expected to be cooler than normal across
the Great Lakes and Northeast later in the winter into spring.
Drier-than-normal conditions will persist in west Texas, New Mexico and
Arizona through August and into October.
The late fall and early winter
forecasts indicate continued dry conditions throughout much of the southern
United States and into portions of the Midwest. Greater than normal precipitation
is predicted for the Pacific Northwest throughout the fall and into the
winter months and for the Ohio and Tennessee River Valley in the winter.
Temperature and precipitation patterns in areas that are not specifically
addressed by the forecast are not predictable at this time.
Does
a La Niña typically follow an El Niño?
No, a La Niña episode may, but does not always follow an El Niño.
Is
there such a thing as "normal", aside from El Niño and
La Niña?*
Over the long-term record, sea-surface temperatures in the central and
eastern tropical Pacific diverge from normal in a roughly bell-curve fashion,
with El Niño and La Niña at the tails of the curve. Some
researchers argue there are only two states, El Niño and non-El
Niño, while others believe either El Niño or La Niña
is always present to a greater or lesser degree. According to one expert,
NCAR's Kevin Trenberth, El Niños were present 31% of the time and
La Niñas 23% of the time from 1950 to 1997, leaving about 46% of
the period in a neutral state. The frequency of El Niños has increased
in recent decades, a shift being studied for its possible relationship
to global climate change.
How
often does La Niña occur?
El Niño and La Niña occur on average every 3 to 5 years.
However, in the historical record the interval between events has varied
from 2 to 7 years. According to the National Centers for Environmental
Prediction, this century's previous La Niñas began in 1903, 1906,
1909, 1916, 1924, 1928, 1938, 1950, 1954, 1964, 1970, 1973, 1975, 1988,
and 1995. These events typically continued into the following spring.
Since 1975, La Niñas have been only half as frequent as El Niños.
How
long does a La Niña last?
La Niña conditions typically last approximately 9-12 months. Some
episodes may persist for as long as two years.
How
do scientists detect La Niña and El Niño and predict their
evolution?
Scientists from NOAA and other agencies use a variety of tools and techniques
to monitor and forecast changes in the Pacific Ocean and the impact of
those changes on global weather patterns. In the tropical Pacific Ocean,
El Niño is detected by many methods, including satellites, moored
buoys, drifting buoys, sea level analysis, and expendable buoys. Many
of these ocean observing systems were part of the Tropical Ocean Global
Atmosphere (TOGA) program, and are now evolving into an operational El
Niño/Southern Oscillation (ENSO) observing system.
NOAA also operates a research
ship, the KA'IMIMOANA, which is dedicated to servicing the Tropical Ocean
Atmosphere (TAO) buoy network component of the observing system. Large
computer models of the global ocean and atmosphere, such as those at the
National Centers for Environmental Prediction, use data from the ENSO
observing system as input to predict El Niño. Other models are
used for El Niño research, such as those at NOAA's Geophysical
Fluid Dynamics Laboratory, at the Center for Ocean-Land-Atmosphere Studies,
and other research institutions.
Why
is predicting these types of events so important?
Better predictions of the potential for extreme climate episodes like
floods and droughts could save the United States billions of dollars in
damage costs. Predicting the onset of a warm or cold phase is critical
in helping water, energy and transportation managers, and farmers plan
for, avoid or mitigate potential losses. Advances in improved climate
predictions will also result in significantly enhanced economic opportunities,
particularly for the national agriculture, fishing, forestry and energy
sectors, as well as social benefits.
What
is the relationship between El Niño/La Niña and global warming?
The jury is still out on this. Are we likely to see more El Niños
because of global warming? Will they be more intense? These are the main
research questions facing the science community today. Research will help
us separate the natural climate variability from any trends due to man's
activities. We cannot figure out the "fingerprint" of global warming if
we cannot sort out what the natural variability does. We also need to
look at the link between decadal changes in natural variability and global
warming. At this time we can't preclude the possibility of links but it
would be too early to definitely say there is a link.
Is
this a "La Niña" hurricane/tropical storm/drought/fire/flood/winter
storm?
It is inaccurate to label individual
storms or events as a La Niña or El Niño event. Rather,
these climate extremes affect the position and intensity of the jet streams,
which in turn affect the intensity and track of storms. During La Niña,
the normal climate patterns are enhanced. For example, in areas that would
normally experience a wet winter, conditions would likely be wetter than
normal.
How
is La Niña influencing the Atlantic and Pacific hurricane seasons?
Dr. William Gray at the Colorado State University has pioneered research
efforts leading to the discovery of La Niña impacts on Atlantic
hurricane activity, and to the first and, presently only, operational
long-range forecasts of Atlantic basin hurricane activity. According to
this research, the chances for the continental U.S. and the Caribbean
Islands to experience hurricane activity increases substantially during
La Niña.
What
impacts do El Niño and La Niña have on tornadic activity
across the country?
Since a strong jet stream is an important ingredient for severe weather,
the position of the jet stream determines the regions more likely to experience
tornadoes.
Contrasting El Niño
and La Niña winters, the jet stream over the United States is considerably
different. During El Niño the jet stream is oriented from west
to east over the northern Gulf of Mexico and northern Florida. Thus this
region is most susceptible to severe weather. During La Niña the
jet stream extends from the central Rockies east- northeastward to the
eastern Great Lakes. Thus severe weather is likely to be further north
and west during La Niña than El Niño.
How
are sea surface temperatures monitored?
Sea surface temperatures in the tropical Pacific Ocean are monitored with
data buoys and satellites. NOAA operates a network of 70 data buoys along
the equatorial Pacific that provide important data about conditions at
the ocean's surface. The data is complimented and calibrated with satellite
data collected by NOAA's Polar Orbiting Environmental Satellites, NASA's
TOPEX/POSEIDON satellite and others.
How
are the data buoys used to monitor ocean temperatures?
Observations of conditions in the tropical Pacific are essential for the
prediction of short term (a few months to one year) climate variations.
To provide necessary data, NOAA operates a network of buoys that measure
temperature, currents and winds in the equatorial band. These buoys transmit
data that are available to researchers and forecasters around the world
in real time.
Why
has the public not heard much about La Niña before now?*
For many decades, scientists have known about the oscillation in atmospheric
pressure across the tropical Pacific at the heart of both El Niño
and La Niña. However, La Niña's effects on fisheries along
the immediate coast of South America, where El Niño was named,
are benign rather than destructive, so La Niña received relatively
little attention there. Research on La Niña increased after its
wider impacts (often called teleconnections) were recognized in the 1980s.
Text for
this page courtesy of NOAA Public Affairs
* Text from NCAR press release 11/13/98
|