What is El Niño and La Niña ?
Near the end of each calendar year, ocean surface temperatures warm along the coasts of Ecuador and northern Peru. In the past, local residents referred to this annual warming as “El Niño,” meaning “The Child,” due to its appearance around the Christmas season. The appearance of El Niño signified the end of the fishing season and the arrival of the time for Peruvian fishermen to repair their nets and maintain their boats. Every two to seven years much stronger warming appears along the west coast of South America, which lasts for several months and is often accompanied by heavy rainfall in the arid coastal regions of Ecuador and northern Peru. Over time the term El Niño began to be used in reference to these major warm episodes. During the 1960s, scientists began to link the abnormally warm waters along the west coast of South America with abnormally warm waters throughout the central and east-central equatorial Pacific. In addition, the warmer than average waters were shown to be closely related to a global atmospheric pressure oscillation known as the Southern Oscillation.
The term El Niño now refers to the coupled ocean-atmosphere phenomenon characterized by:
- Abnormally warm sea surface temperatures from the dateline (180W) east to the South American coast
- Changes in the distribution of tropical rainfall from the eastern Indian Ocean east to the tropical Atlantic
- Changes in sea level pressure throughout the global Tropics (low-index phase of the Southern Oscillation)
- Large-scale atmospheric circulation changes in the Tropics and portions of the extratropics in both
Other terms commonly used for the El Niño phenomenon include “Pacific warm episode” and “El Niño/ Southern Oscillation (ENSO) episode.” In contrast to El Niño, La Niña is characterized by anomalously cool water in the central and east-central equatorial Pacific. It also results in changes in the intensity and distribution of rainfall in the Tropics and in changes in the patterns of sea level pressure (corresponding to the “high-index” phase of the Southern Oscillation) and atmospheric circulation that affect many areas worldwide. Another term commonly used for the La Niña phenomenon is “Pacific cold episode.”
The El Niño/ La Niña phenomena are the main sources of year-to-year variability in weather and climate for many areas of the world. El Niño and La Niña tend to alternate in an irregular cycle, which is often referred to as the ENSO cycle. The transition between El Niño and La Niña tends to be more rapid than the transition from
La Niña to El Niño. El Niño episodes tend to:
- Develop during the Northern Hemisphere spring season
- Occur every 3-5 years
- Usually last for 9-12 months.
- In contrast, La Niña may last 1-3 years; however, there
- is considerable event-to-event variability in the timing,
- intensity and evolution of both El Niño and La Niña.
- Periods, when neither El Niño nor La Niña is present, are
- referred to as ENSO-neutral.
What is the effect of La Niña?
La Niña conditions tend to influence wintertime atmospheric flow across the eastern North Pacific and North America, as shown in the figure below. La Niña episodes display considerable event-to-event variability and the overall effects tend to be less predictable than those for El Niño.
Seasonal precipitation impacts are generally opposite to those of El Niño. During La Niña winters, large portions of central North America experience increased storminess, and an increased frequency of significant cold-air outbreaks, while the southern states experience less storminess and precipitation.
La Niña There also tends to be considerable month-to-month variations in temperature, rainfall, and storminess across central North America during the winter and spring seasons, in response to the more variable atmospheric circulation throughout the period. In the eastern U.S., during the winter, there are generally
fewer coastal storms and more Alberta Clippers (fast eastward-tracking storms across the northern states) Then normal. In the summer and autumn, La Niña can influence hurricane development, often resulting in fewer eastern Pacific hurricanes and more Atlantic hurricanes.
What is the effect of La Niño?
El Niño conditions influence wintertime atmospheric flow across the eastern North Pacific and North America. There is considerable event-to-event variability in the
character of El Niño episodes and in some areas, impacts can vary substantially from one event to another. However, there are some sections of the United States where impacts are fairly consistent and predictable, especially when associated with strong El Niño episodes.
In general, El Niño results in increased precipitation across California and the southern tier of states, and decreased precipitation in the Pacific Northwest and in the Ohio and Tennessee Valleys. A milder Then normal winter across the northern states and western Canada is also a common effect. In the
eastern United States, El Niño episodes favor more coastal storms at the expense of Alberta Clippers (fast eastward-tracking storms across the northern states) in winter and early spring. During the warm season, El Niño influences hurricane development, resulting in more eastern Pacific hurricanes and
fewer Atlantic hurricanes.