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WHAT IS La Niņa ?


 

Do you know what is La Niņa?  Why is everyone talking about it?

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 tele-connections) were recognized in the 1980s.


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