|The coupled atmosphere-ocean phenomenon known as El Niño is frequently followed
by a period of normal conditions in the equatorial Pacific Ocean. Sometimes, but
not always, El Niño conditions give way to the other extreme of the El
Niño-Southern Oscillation (ENSO) cycle. This cold counterpart to El Niño is
known as La Niña, Spanish for "the girl child."
The Southern Oscillation
How La Niña Forms
During La Niña years, the trade winds are unusually strong due to an enhanced pressure gradient between the eastern and western Pacific. As a result, upwelling is enhanced along the coast of South America, contributing to colder than normal surface waters over the eastern tropical Pacific and warmer than normal surface waters in the western tropical Pacific.
next: The Effects of La Niña
|The Effects of La Niña|
|Changes in global atmospheric circulation patterns accompany La Niña and are
responsible for weather extremes in various parts of the world that are
typically opposite to those associated with El Niño. These patterns result from
colder than normal ocean temperatures inhibiting the formation of rain-producing
clouds over the eastern equatorial Pacific region while at the same time
enhancing rainfall over the western equatorial Pacific region (Indonesia,
Malaysia and northern Australia.) These patterns affect the position and
intensity (weakening) of jet streams and the behavior of storms outside of the
tropics in both the Northern and Southern hemispheres.
U.S. La Niña Impacts
Global La Niña Impacts
|NASA and NOAA Missions to Study La Niña|
|Over the years, several NASA missions have studied the effects associated with
La Niña and El Niño, such as changes in sea-surface temperature (SST) and cloud
cover. These studies are augmented by data from operational satellites of the
National Oceanic and Atmospheric Administration (NOAA).
Initial efforts at mapping SST and cloud cover were conducted using data from NASA's Nimbus series of satellites. The four-channel Advanced Very High Resolution Radiometer (AVHRR), flown on NOAA's TIROS-N weather satellite in 1978 and on the NOAA-6 satellite in 1979, greatly increased the accurate measurements of El Niño effects. ("Four channel" means that the instrument views in four different parts of the electromagnetic visible and infrared spectrum.)
Still further increases in accuracy resulted when a fifth channel was added to the AVHRR instrument flown on NOAA-7 in 1981, and on subsequent NOAA satellites. The fifth channel improved the measurement of SST by providing corrections for atmospheric water vapor that otherwise would have interfered with the temperature measurements.
The joint U.S.-French TOPEX/Poseidon mission was launched in 1992 and is providing global determinations of changes in ocean surface currents that are related to the La Niña and El Niño phenomena. Data retrieved from TOPEX/Poseidon is important because they provide measurements of the depth to which the cold or warm anomaly extends.
A NASA scatterometer called NSCAT flew on the Japanese Advanced Earth Observing System (ADEOS) spacecraft, which was launched in August 1996. NSCAT provided very high quality data on the speed and direction of ocean-surface winds worldwide. Unfortunately, after nine months in orbit, a spacecraft failure brought to an end the stream of NSCAT data. Recognizing the important contributions to Earth science made by NSCAT, NASA now plans to launch a copy of the new SeaWinds scatterometer in 1999 as part of a dedicated mission named QuikSCAT to bridge the gap remaining before launch of the Japanese spacecraft designated ADEOS II (planned for 2000), which will also carry a SeaWinds instrument.
In addition to the scatterometer measurements, which use active microwave radar systems to determine surface wind speeds and directions over the ocean, surface wind speeds are also being obtained from the Special Sensor Microwave Imager (SSM/I), a passive microwave sensor onboard a Department of Defense spacecraft.
Key sources of data related to El Niño have been retrieved from the five-channel AVHRRs flown on NOAA-7, 9, and 11. These historic data sets cover the period 1981 through 1992 and beyond and will permit more-accurate SST determinations than were previously available. These data are important to the development and testing of a new generation of computer models in which the interacting processes of the land, the atmosphere, and the oceans are coupled. These coupled models will lead the way to an increased understanding of phenomenon such as La Niña and the teleconnections that link La Niña with changes in weather patterns throughout the world.
NASA's SeaWiFS (Sea-viewing Wide Field of View Sensor) was launched on the OrbView-2 satellite in August 1997. The instrument onboard SeaWiFS is designed to detect ocean color, which is an indicator of microscopic plant life in the ocean. The growth of such plants (called phytoplankton) is affected by the changes in sea surface temperature that are related to La Niña and El Niño. SeaWiFS data enable scientists to compare and contrast El Niño's impacts on the marine biosphere with those of La Niña.
The Tropical Atmosphere Ocean (TAO) Array consists of nearly 70 moored buoys in the tropical Pacific designed by the National Oceanic and Atmospheric Administration (NOAA). These floating devices take real-time measurements of air temperature, relative humidity, surface winds, sea surfaces temperatures and subsurface temperatures down to a depth of 500 meters. Data from these moored buoys is processed by NOAA and then made available to scientists for collaborative research studies.
The joint U.S.-Japanese Tropical Rainfall Measuring Mission (TRMM), launched in November 1997, uses for the first time both active (radar) and passive microwave detectors from space to provide measurements of precipitation, clouds, and radiation processes in lower latitudes, including those portions of the Pacific Ocean where El Niño and La Niña occur. TRMM research team members have successfully retrieved sea-surface temperature data from the TRMM Microwave Imager (TMI) instrument onboard the spacecraft, giving them new insight into the complex evolution of the La Niña event. TMI is an all-weather measuring instrument that can see through clouds and is currently the only spaceborne microwave instrument observing sea-surface temperature in the tropics. Similar observations will be continued by the Advanced Microwave Scanning Radiometer (AMSR) to be flown on ADEOS-II and the AMSR-E instrument to be flown onboard Aqua, both of which will be launched in the year 2000.
With the launch of the EOS satellites, starting in 1999, we will have the means to collect and analyze the most comprehensive data set ever acquired for the development of coupled models. This data set will increase markedly our understanding of the causes and effects of such large-scale ocean-atmosphere phenomena as La Niña and El Niño.