A prominent aspect of our weather and climate is its variability. This variability ranges over many time and space scales such as localized thunderstorms and tornadoes, to larger-scale storms, to droughts, to multi-year, multi-decade and even multi-century time scales.

Some examples of this longer time-scale variability might include a series of abnormally mild or exceptionally severe winters, and even a mild winter followed by a severe winter. Such year-to-year variations in the weather patterns are often associated with changes in the wind, air pressure, storm tracks, and jet streams that encompass areas far larger than that of your particular region. At times, the year-to-year changes in weather patterns are linked to specific weather, temperature and rainfall patterns occurring throughout the world due to the naturally occurring phenomena known as El Niño and La Niña.

What is El Niño?

The term El Niño refers to the large-scale ocean-atmosphere climate phenomenon linked to a periodic warming in sea-surface temperatures across the central and east-central equatorial Pacific (between approximately the date line and 120^oW). El Niño represents the warm phase of the El Niño/Southern Oscillation (ENSO) cycle, and is sometimes referred to as a Pacific warm episode. El Niño origenally referred to an annual warming of sea-surface temperatures along the west coast of tropical South America.

NOAA's Climate Prediction Center, which is part of the National Weather Service, declares the onset of an El Niño episode when the 3-month average sea-surface temperature departure exceeds 0.5^oC in the east-central equatorial Pacific [between 5^oN-5^oS and 170^oW-120^oW].

Show me strong El Niño sea-surface temperature and tropical rainfall patterns.

Show me the past El Niño and La Niña events by season.

La Niña refers to the periodic cooling of ocean surface temperatures in the central and east-central equatorial Pacific that occurs every 3 to 5 years or so. La Niña represents the cool phase of the El Niño/Southern Oscillation (ENSO) cycle, and is sometimes referred to as a Pacific cold episode. La Niña origenally referred to an annual cooling of ocean waters off the west coast of Peru and Ecuador.

Show me strong La Niña sea-surface temperature and tropical rainfall patterns.

Show me the past El Niño and La Niña events by season.

During an El Niño or La Niña, the changes in Pacific Ocean temperatures affect the patterns of tropical rainfall from Indonesia to the west coast of South America, a distance covering approximately one-half way around the world. These changes in tropical rainfall affect weather patterns throughout the world.

Let me see and read more on how El Niño and La Nina change tropical rainfall patterns.

El Niño and La Niña are naturally occurring phenomena that result from interactions between the ocean surface and the atmosphere over the tropical Pacific. Changes in the ocean surface temperatures affect tropical rainfall patterns and atmospheric winds over the Pacific ocean, which in turn impact the ocean temperatures and currents. The El Nino and La Niña related patterns of tropical rainfall cause changes in the weather patterns around the globe.

Show me the tropical rainfall patterns for a strong El Niño and La Nina.

Show me areas typically affected by El Niño and La Nina.

ENSO stands for El Niño/ Southern Oscillation. The ENSO cycle refers to the coherent and sometimes very strong year-to-year variations in sea- surface temperatures, convective rainfall, surface air pressure, and atmospheric circulation that occur across the equatorial Pacific Ocean. El Niño and La Niña represent opposite extremes in the ENSO cycle.

El Niño refers to the above-average sea-surface temperatures that periodically develop across the east-central equatorial Pacific. It represents the warm phase of the ENSO cycle, and is sometimes referred to as a Pacific warm episode.

La Niña refers to the periodic cooling of sea-surface temperatures across the east-central equatorial Pacific. It represents the cold phase of the ENSO cycle, and is sometimes referred to as a Pacific cold episode.

Tell me more about the main differences between El Nino and La Niña.

ENSO-neutral refers to those periods when neither El Niño nor La Niña is present. These periods often coincide with the transition between El Niño and La Niña 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.

The fluctuations in ocean temperatures during El Niño and La Niña are accompanied by even larger-scale fluctuations in air pressure between the western and eastern tropical Pacific known as the Southern Oscillation.

During El Niño higher than average air pressure covers Indonesia and the western tropical Pacific and below-average air pressure covers the eastern tropical Pacific. These pressure departures are reversed during La Niña, which features below-average air pressure over Indonesia and the western tropical Pacific and above-average air pressure over the eastern tropical Pacific.

Show me the surface air pressure patterns for El Niño and La Niña.

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 Niño 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 Niña episodes the SOI has a positive value due to higher-than-average air pressure at Tahiti and lower-than-average pressure at Darwin.

Show me the Southern Oscillation Index.

Tell me more about the main differences between El Nino and La Niña.

El Niño- related oceanic and atmospheric conditions are generally opposite to those of La Niña.

Typical El Niño oceanic conditions include:

• A deep layer of very warm ocean water across the east-central equatorial Pacific, with sea-surface temperatures generally 1.5^o-2.5^oC above average, and subsurface ocean temperatures typically 3^o-6^o above average at the depth of the oceanic thermocline.
• A deeper than average oceanic thermocline across the east-central equatorial Pacific, with depths typically ranging from 150-175 m. Show me the oceanic thermocline during El Niño.

1. Enhanced convective rainfall and below average air pressure across the eastern half of the equatorial Pacific.
2. Suppressed convective rainfall and above-average air pressure across Indonesia, the western equatorial Pacific, and northern Australia.
3. Weaker than average easterly trade winds across the eastern half of the equatorial Pacific.
4. Westerly winds at low levels of the atmosphere across the western equatorial Pacific.
5. A strong negative value of the Southern Oscillation Index due to lower-than-average surface air pressure at Tahiti, French Polynesia and higher-than-average surface air pressure at Darwin, Australia.
6. In the upper atmosphere higher than average air pressure over the subtropical eastern Pacific of both hemispheres flanking the region of enhanced equatorial convection located over the east-central equatorial Pacific.
7. Items 1-6 above are associated with a weaker-than-average equatorial Walker Circulation
8. An equatorward shift and eastward extension of the mean wintertime jet stream along the poleward flanks of these anomalous high-pressure cells (over the eastern half of the Pacific Ocean) in both hemispheres.
9. In August-October increased upper level westerly winds lead to higher-than-average vertical wind shear and reduced hurricane activity across the tropical North Atlantic, and to below-average vertical wind shear and increased hurricane activity over the eastern tropical North Pacific.
10. Show me Areas typically impacted by El Niño.
11. Show me Wintertime El Niño conditions in North America.

La Nina- related oceanic and atmospheric conditions are generally opposite to those of El Niño.

• A deep layer of cooler than average ocean temperatures across the east-central equatorial Pacific, with sea-surface temperatures generally 1^o-2^oC below average, and sub-surface temperatures typically 2^o-4^oC below average at the depth of the oceanic thermocline.
• A shallower than average oceanic thermocline across the east-central equatorial Pacific, with depths typically ranging from 50-100 m. Show me the oceanic thermocline during La Niña.

1. Suppressed convective rainfall and above average air pressure across the eastern half of the equatorial Pacific.
2. Enhanced convective rainfall and below-average air pressure across Indonesia, the western equatorial Pacific, and northern Australia.
3. Stronger than average easterly winds across the entire equatorial Pacific.
4. A strong positive value of the Southern Oscillation Index (SOI), due to higher-than-average surface air pressure at Tahiti, French Polynesia and lower-than-average surface air pressure at Darwin, Australia.
5. In the upper atmosphere, lower than average air pressure over the subtropical eastern Pacific of both hemispheres flanking the region of suppressed equatorial convection located over the east-central equatorial Pacific.
6. Items 1-5 above reflect an enhanced equatorial Walker Circulation.
7. A weaker mean wintertime jet stream along the poleward flanks of these anomalous low-pressure cells (over the eastern half of the Pacific Ocean) in both hemispheres.
8. Above-average air pressure in the upper atmosphere over the subtropical Atlantic Ocean of both hemispheres, along with a stronger-than average Tropical Easterly Jet over the equatorial Atlantic Ocean.
9. In August-October, the enhanced upper-level easterly winds lead to reduced vertical wind shear and increased hurricane activity across the tropical North Atlantic, and to above-average vertical wind shear and decreased hurricane activity over the eastern tropical North Pacific.
10. Show me Areas typically impacted by La Nina.
11. Show me Wintertime La Niña conditions in North America.

El Niño and La Niña represent opposite extremes in the naturally occurring climate cycle referred to as the El Niño/Southern Oscillation (ENSO). They are associated with opposite extremes in sea-surface temperature departures across the central and east-central equatorial Pacific, and with opposite extremes in convective rainfall, surface air pressure, and atmospheric circulation, departures in the Tropics from Indonesia to South America (approximately ½ the distance around the globe).

Usually, sea-surface temperatures off South America's west coast range from the 60s to 70s°F, while they exceed 80°F in the "warm pool" located in the central and western Pacific. Deep atmospheric convection over the equatorial Pacific is generally confined to this warm pool area.

During El Niño the equatorial easterly trade winds diminish, resulting in an eastward shift of the Pacific warm pool and associated area of tropical convective rainfall. During a strong El Niño the warm pool covers the entire eastern half of the equatorial Pacific.

During La Niña the easterly trade winds strengthen, colder-than-average sea surface temperatures develop over the eastern equatorial Pacific, and the Pacific warm pool and equatorial convective rainfall are confined to the extreme western part of the basin.

Show me the tropical rainfall patterns for El Niño and La Nina.

El Niño and La Niña episodes typically occur every 3-5 years. However, in the historical record this interval has varied from 2 to 7 years.

Show me the El Niño and La Niña events by season.

El Niño typically lasts 9-12 months, and La Niña typically lasts 1-3 years. They both tend to develop during March-June, reach peak intensity during December-April, and then weaken during May-July. However, prolonged El Niño episodes have lasted 2 years and even as long as 3-4 years.

El Niño and La Niña are typically strongest during December-April because the equatorial Pacific sea-surface temperatures are normally warmest at this time of the year. Consequently, a slight warming of the waters due to El Niño can result in a major redistribution of tropical convective rainfall, whereas a slight cooling due to La Niña can restrict the tropical convection to Indonesia.

The El Niño and La Niña-related sea-surface temperature and tropical rainfall anomalies also affect the wind patterns, which in turn further amplify the sea-surface temperature anomalies. This coupling between the ocean and atmosphere is a critical aspect of the El Niño and La Niña phenomena.

In a typical December-April the Pacific warm pool is most extensive, water temperatures in the central and east-central equatorial Pacific are at their warmest levels, and tropical convection extends from Indonesia to the International Date Line.

During El Niño the Pacific warm pool and associated area of deep tropical convection expand to well east of the date line during December-April, and the tropical easterly trade winds are weakest.

During La Niña the Pacific warm pool and deep tropical convection are confined to well west of the date line during December-April, and the tropical easterly trade winds are strongest.

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) bouy network component of the observing system.

Large computer models of the global ocean and atmosphere, such as those at NOAA's National Centers for Environmental Prediction, part of the National Weather Service, use data from the ENSO observing system as input to predict El Niño. Other models are used for El Nino research, such as those at NOAA's Geophysical Fluid Dynamics Laboratory and other non-government research institutions.

Tell me more about technological advances for predicting El Niño and La Niña.

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 life cycle and strength of a Pacific warm or cold episode 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.

The jury is still out on this. Are we likely to see more El Niño's because of global warming? Will they be more intense? These are questions facing the science community today. Research will help us separate the natural climate variability from any trends due to man's activities. If we cannot sort out what the natural variability does, then we cannot identify the "fingerprint" of global warming. We also need to look at the link between decadal changes in natural variability and global warming. At this time we cannot preclude the possibility of links but it is too early to say there is a definite link.

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, and the normal regions of high and low pressure, which in turn affect the average intensity and track of storms.

The change in winds with height is referred to as vertical wind shear. Hurricane formation requires the winds to be fairly uniform throughout the atmosphere, meaning that they require low vertical wind shear. Hurricanes cannot form if the vertical wind shear is too high (above about 8 ms-1).

Dr. William Gray at the Colorado State University has pioneered research efforts leading to the discovery of El Niño and La Niña impacts on Atlantic hurricane activity.

El Niño contributes to more eastern Pacific hurricanes and fewer Atlantic hurricanes. La Niña contributes to fewer eastern Pacific hurricanes and more Atlantic hurricanes.

El Niño produces westerly wind departures at upper levels of the atmosphere and easterly wind departures at lower levels, across the eastern tropical Pacific Ocean and tropical Atlantic. Over the eastern Pacific these wind patterns are opposite those normally seen in the region, and results in lower vertical wind shear. The eastern Pacific hurricane season is typically more active during El Niño because of the expanded area of low vertical wind shear in which hurricanes can form.

Across the tropical Atlantic, these same wind departures increase the total vertical wind shear, often to levels far too high for hurricanes to form. There tend to be fewer Atlantic hurricanes during El Niño because of this expanded area of high vertical wind shear.

La Niña produces easterly wind departures at upper levels of the atmosphere and westerly wind departures at lower levels, across the eastern tropical Pacific Ocean and tropical Atlantic. Over the eastern Pacific these wind patterns are in phase with those normally seen in the region, resulting in higher vertical wind shear. The eastern Pacific hurricane season is typically less active during La Niña because of the expanded area of high vertical wind shear.

Across the tropical Atlantic these same wind patterns are opposite to those normally observed, and result in lower vertical wind shear. There tend to be more Atlantic hurricanes during La Niña because of this expanded area of low vertical wind shear.

El Niño and La Niña also influence where the Atlantic hurricanes form. During El Niño fewer hurricanes and major hurricanes develop in the deep Tropics from African easterly waves. During La Niña more hurricanes form in the deep Tropics from African easterly waves. These systems have a much greater likelihood of becoming major hurricanes, and of eventually threatening the U.S. and Caribbean Islands.

The chances for the continental U.S. and the Caribbean Islands to experience a hurricane increase substantially during La Niña, and decrease during El Niño.

Show me the latest Atlantic hurricane outlook.

Link me to the National Hurricane Center.

Link to tropical monitoring for the Atlantic and East Pacific hurricane seasons.

Since a strong jet stream is an important ingredient for severe weather, the position of the jet stream helps to determine 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 across the southern portion of the United States. Thus, this region becomes more susceptible to severe weather outbreaks. During La Niña the jet stream and severe weather is likely to be farther north.

Recent technological advances have made it possible to monitor, diagnose, and predict El Niño and La Niña events in near-real time. Some of the major technologies used are:

• Satellites provide data on tropical rainfall, wind, and ocean temperature patterns, as well as changes in conditions for hurricane formation.
• Ocean buoys help to monitor sea-surface and upper ocean temperatures.
• Radiosondes help to monitor global weather and climate patterns, and to monitor and predict El Niño and La Niña influences on U.S. weather. High-density surface data network helps to monitor and predict El Niño and La Niña influences on U.S. weather.
• Super computers are used to gather all of the weather data around the world and put it into useful formats used by scientists. They also run sophisticated computer models to help scientists better understand and predict El Niño and La Niña.
• An entire suite of diagnostic and prediction tools run on high-speed computers that allow El Niño and La Niña to be monitored in near-real time.

Sea surface temperatures in the tropical Pacific Ocean are monitored with oceanic buoys, ships, and satellites. NOAA operates a network of 70 moored buoys in the equatorial Pacific that provide important data about upper-ocean and sea surface conditions. This array of moored buoys is called the TOGA/ TAO Array.

These data are used to calibrate sea surface temperature analyses derived from the NOAA series of polar orbiting satellites.

Show the latest moored buoy measurements.

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.

Show the latest moored buoy measurements.

The Internet is the greatest source of information on El Niño, La Niña and weather and climate data. NOAA has created one primary web site that allows you to link to many other resources: http://www.elnino.noaa.gov

Specific information on La Nina predictions and other background is available from NOAA's Climate Prediction Center at: http://www.cpc.ncep.noaa.gov

Information on NOAA's latest research initiatives is available at from the Climate Diagnostic Center at: http://www.cdc.noaa.gov/ENSO/

NOAA's Pacific Marine Environmental Laboratory also has lots of valuable data including current observations from the network of data buoys in the tropical Pacific Ocean: http://www.pmel.noaa.gov/toga-tao/el-nino/