La Niņa
is defined as cooler than normal sea-surface temperatures in the
central and eastern tropical Pacific ocean that impact global weather
patterns. La Niņa conditions recur every few years and can persist for
as long as two years.
What is the difference
between La Niņa and El Niņo?
El Niņo and La Niņa are extreme phases of a naturally occurring climate
cycle referred to as El Niņo/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 Niņo, but during La Niņa, 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.
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 Niņo but shrinks to the west
during La Niņa. The El Niņo/Southern Oscillation (ENSO) is the coupled
ocean-atmosphere process that includes both El Niņo and La Niņa.
Why do El Niņo and La Niņa
occur?
El Niņo and La Niņa 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 Niņo) to neutral (or cold La
Niņa) conditions with an on average every 3-4 years.
What causes La Niņa?
Typically, a La Niņa 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 Niņa, SSTs fell to as much as 4 degrees C (7 degrees F) below
normal. Both La Niņa and El Niņo tend to peak during the Northern
Hemisphere winter.
What are the global impacts
of La Niņa?
Both El Niņo and La Niņa impact global and U.S. climate patterns. In
many locations, especially in the tropics, La Niņa (or cold episodes)
produces the opposite climate variations from El Niņo. For instance,
parts of Australia and Indonesia are prone to drought during El Niņo,
but are typically wetter than normal during La Niņa.
What are the U.S. impacts of
La Niņa?
La Niņa 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 Niņa. Additionally, on average La
Niņa winters are warmer than normal in the Southeast and colder than
normal in the Northwest.
Does a La Niņa typically
follow an El Niņo?
No, a La Niņa episode may, but does not always follow an El Niņo.
How often does La Niņa occur?
El Niņo and La Niņa 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 Niņas 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
Niņas have been only half as frequent as El Niņos.
How long does a La Niņa last?
La Niņa conditions typically last approximately 9-12 months. Some
episodes may persist for as long as two years.
How do scientists detect La
Niņa and El Niņo 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 Niņo 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 Niņo/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 Niņo. Other models are used
for El Niņo research, such as those at NOAA's Geophysical Fluid Dynamics
Laboratory, at the Center for Ocean-Land-Atmosphere Studies, and other
research institutions.
What is the relationship
between El Niņo/La Niņa and global warming?
The jury is still out on this. Are we likely to see more El Niņos
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.
How is La Niņa 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 Niņa 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 Niņa.
What impacts do El Niņo and
La Niņa 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 Niņo and La Niņa
winters, the jet stream over the United States is considerably
different. During El Niņo 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 Niņa 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 Niņa than El Niņo.
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 Niņa before now?*
For many decades, scientists have known about the oscillation in
atmospheric pressure across the tropical Pacific at the heart of both El
Niņo and La Niņa. However, La Niņa's effects on fisheries along the
immediate coast of South America, where El Niņo was named, are benign
rather than destructive, so La Niņa received relatively little attention
there. Research on La Niņa increased after its wider impacts (often
called teleconnections) were recognized in the 1980s.
