ENSO Glossary
Jan Null, CCM

Cool Phase -

  • Or "cool event") refers to a La Niņa with its anomalously cool water.

El Niņo -

  • Comes from the Spanish term for "the Christ Child" because of the occurrence of warmer than normal waters that disrupted fishing along the coast of Ecuador and Peru around Christmas.  Common usage of the term has expanded to refer to the large scale warming of the tropical Pacific Ocean at irregular intervals of between about 2 and 7 years and lasting for 1 to 3 years.
  • El Niņo is one of the two phases of the El Niņo-Southern Oscillation (ENSO) climate pattern that occurs in the tropical Pacific Ocean. During an El Niņo event, the surface waters of the central and eastern Pacific Ocean become unusually warm, which can cause changes in weather patterns around the world.

    Normally, trade winds blow from east to west across the Pacific Ocean, pushing warm surface water towards the western Pacific. This causes cooler water to rise to the surface in the eastern Pacific, which helps to maintain a stable climate in the region. However, during an El Niņo event, the trade winds weaken, or even reverse, allowing the warm water to spread eastward across the Pacific, leading to a warming of the surface waters in the eastern Pacific.


  • El Niņo/Southern Oscillation. ENSO is a general term used to describe both warm (El Niņo) and cool (La Niņa) ocean-atmosphere events in the tropical Pacific as well as the Southern Oscillation the atmospheric component of these phenomena.

Intertropical Convergence Zone (ITCZ) -

  •  The region of cloudiness near the Equator caused by the convergence of the Trade Winds. [Image of ITCZ]

La Niņa -

  • "Infant Girl" in Spanish and is so named because in many ways it is the opposite of El Niņo.  La Niņa is characterized by large scale cooling of the tropical Pacific Ocean and often begins during the summer at irregular intervals of between about 2 and 7 years and lasting for 1 to 3 years.

Modoki (or Central Pacific) El Niņo -

  • Modoki El Niņo is a type of El Niņo event that occurs when warm sea surface temperatures develop in the central tropical Pacific Ocean, rather than in the eastern Pacific as in a typical El Niņo. "Modoki" is a Japanese word meaning "similar but different," which reflects the fact that Modoki El Niņo events are similar in some ways to traditional El Niņo events, but different in their patterns of warming.

    During a Modoki El Niņo, the central Pacific experiences a warming of the ocean surface, while the eastern and western Pacific remain relatively cool. This warming can affect weather patterns around the world, causing changes in rainfall, temperature, and storm activity.

    Modoki El Niņo events were first identified in the early 2000s, although some researchers suggest that they may have occurred in the past but were not recognized at the time. The frequency of Modoki El Niņo events appears to be increasing in recent decades, possibly due to climate change.

Multivariate ENSO Index (MEI) -

  • (MEI) is a composite index using a number of variables to measure ENSO events.  The MEI uses sea surface temperatures, surface air temperatures, sea-level pressure, zonal (i.e., east-west) surface wind, meridional (i.e., north-south) surface wind and total amount of cloudiness.  Positive MEI values are related to warm phase or El Niņo events and negative values with to cool phase or La Niņa events. [More on MEI]

Niņo Regions

  • The Niņo regions, also known as the Niņo indices, are areas of the tropical Pacific Ocean that are used to monitor and measure the El Niņo-Southern Oscillation (ENSO) climate pattern. There are three Niņo regions, named Niņo 1+2, Niņo 3, and Niņo 4, each covering a different area of the Pacific Ocean.

    Niņo 1+2 covers a small area along the equator off the coast of South America, from 0° to 10°S latitude and 90°W to 80°W longitude. Niņo 3 covers a larger area along the equator, from 5°N to 5°S latitude and 150°W to 90°W longitude. Niņo 4 covers a wider area in the western and central tropical Pacific, from 5°N to 5°S latitude and 160°E to 150°W longitude.

    These regions are important for monitoring ENSO because they are areas where changes in ocean temperature can indicate the onset or end of an El Niņo or La Niņa event. For example, during an El Niņo event, the surface waters in the Niņo 3 and Niņo 4 regions become significantly warmer than normal, while during a La Niņa event, these regions become significantly cooler than normal. The Niņo indices are widely used by scientists and meteorologists to track and forecast ENSO events, and to predict their potential impacts on weather patterns and climate around the world.  [Map of Niņo Regions]

Oceanic Niņo Index (ONI)-

  • has become the de-facto standard that NOAA uses for identifying El Niņo (warm) and La Niņa (cool) events in the tropical Pacific.  It is the running 3-month mean SST anomaly for the Niņo 3.4 region (i.e., 5oN-5oS, 120o-170oW).  Events are defined as 5 consecutive months at or above the +0.5o anomaly for warm (El Niņo) events and at or below the -0.5 anomaly for cold (La Niņa) events.  The threshold is further broken down into Weak (with a 0.5 to 0.9 SST anomaly), Moderate (1.0 to 1.4),  Strong (1.5 to 1.9), and Very Strong (≥ 2.0) events.  For the purpose of this report for an event to be categorized as weak, moderate or strong it much have equaled or exceeded the threshold for at least 3 months. [Data and Plot]

Sea Surface Temperature (SST) -

  • A key characteristic of El  and La Niņa events.  SSTs are monitored from ship reports, buoys and satellite imagery.  [Current SST chart]

Sea Surface Temperature Anomaly (SSTA)-

  • The departure of a given SST from the long term average temperature.  Positive anomalies are related to warm phase or El Niņo events and negative anomalies are related to cool phase or La Niņa events. [Current SST Anomaly chart]

Southern Oscillation (SOI) -

  • Is the difference between the seasonally normalized sea level pressures of Darwin, Australia and Tahiti.  A negative SOI (also called "low index") usually relates to a weakening of the Trade Winds and a resulting warm event.  Conversely a positive SOI ("high index") usually relates to an increase in the Trades and a resulting cool event. First recognized by British scientist Sir Gilbert Walker in the 1920s by observing pressure differences between Darwin, Australia and Tahiti while stationed in India and studying the monsoon. [Current SOI Table]

TAO Array -

  • Tropical-Ocean-Atmosphere Array  (renamed the TAO/TRITON array ) consists of approximately 70 moored buoys in the tropical Pacific Ocean that telemeter oceanographic and meteorological data in real-time.   [Project Page]

Teleconnections -

  • A statistical relationship between conditions in one part of the world affecting the weather in a different part of the world. For example, there appears to be a teleconnection between the ENSO events and rainfall in California.


  • The boundary layer between the warmer well-mixed surface water and colder deep ocean water.  The depth of the thermocline is another indicator of an El Niņo events when it deepens in the eastern Pacific. [Current]

TOPEX/Poseidon -

  • (TOPography EXperiment for Ocean Circulation) is a joint US/French satellite launched in 1992 to map ocean surface topography.  [Mission page]

Trade Winds - (also Trades or Tropical Easterlies)

  • The steady winds resulting from the circulation of air from the Subtropical High pressure regions toward the Equator and deflected to the west by the Coriolis Force.   The convergence of these winds along the Equator is the Intertropical Convergence Zone (ITCZ).   Because of the dependability of these winds they were used by sailing ships of the 17th and 18th centuries to establish "trade" with the New World. [Image of Trade Winds]


  • The upward motion of sub-surface water toward the surface of the ocean which is often a source of cold, nutrient-rich water. Significant upwelling occurs along the equator under the influence of persistent trade winds and also along coastlines. The disruption of upwelling by the warmer than normal waters during El Niņo events disrupts fishing along the coast of Ecuador and Peru.

Walker Circulation -

  • Named for Sir Gilbert Walker by Jacob Bjerknes in 1969, it describes the three-dimensional circulation across the tropical Pacific and the displacement of this circulation by El Niņo and La Niņa events.  See Walker Circulation.

Warm Phase -

  • Or "warm event", refers to an El Niņo with its anomalously warm water.


Search this Site
Questions, comments or suggestions.  Email jnull@ggweather.com
Copyright Š 2023, Golden Gate Weather Services