ENSO Glossary
Jan Null, CCM
Cool Phase - (or "cool event") refers to a La Niña with its anomalously cool water.

El Niño - 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 Niño Theme Page]

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

Intertropical Convergence Zone - (ITCZ) is the region of cloudiness near the Equator caused by the convergence of the Trade Winds. [Image of ITCZ]

La Niña - means "Infant Girl" in Spanish and is so named because in many ways it is the opposite of El Niño.  La Niña 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. [La Niña Theme Page]

Multivariate ENSO Index - (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 Niño events and negative values with to cool phase or La Niña events. [More on MEI]

Niño Regions - for data sampling of SSTs the tropical Pacific has been divided into a number of regions named Niño 1, 2,3, 4, and 3.4 (which is encompasses part of both region 3 and 4).  Niño 1 is the area defined by 80º-90ºW and 5º-10ºS, Niño 2 by 80º-90ºW and 0º-5ºS, Niño 3 by 90º-150ºW and 5ºN-5ºS, Niño 4 by 150ºW-160ºE and 5ºN-5ºS, Niño 3.4 by 120Wº-170ºW and 5ºN-5ºS.  [Map of Niño Regions]

Oceanic Niño Index - has become the de-facto standard that NOAA uses for identifying El Niño (warm) and La Niña (cool) events in the tropical Pacific.  It is the running 3-month mean SST anomaly for the Niño 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 Niño) events and at or below the -0.5 anomaly for cold (La Niña) events.  The threshold is further broken down into Weak (with a 0.5 to 0.9 SST anomaly), Moderate (1.0 to 1.4) and Strong (≥ 1.5) 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) is a key characteristic of El  and La Niña events.  SSTs are monitored from ship reports, buoys and satellite imagery.  [Current SST chart]

Sea Surface Temperature Anomaly - the departure of a given SST from the long term average temperature.  Positive anomalies are related to warm phase or El Niño events and negative anomalies are related to cool phase or La Niña events. [Current SST Anomaly chart]

Southern Oscillation - is the periodic change (i.e., oscillation) of sea level pressure differences across the tropical Pacific that have been found to correlate with El Niño and La Niña events.  First recognized by British scientist Sir Gilbert Walker in the 1920s by observing pressure differences between Darwin, Australia and Tahiti while stationed in India studying the monsoon.

Southern Oscillation Index - (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. [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.

Thermocline - is 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 Niño events when it deepens in the eastern Pacific. [Normal, El Niño, & La Niña examples]

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) are 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]

Upwelling - 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.

Walker Circulation - (named for Sir Gilbert Walker by Jacob Bjerknes in 1969) describes the three-dimensional circulation across the tropical Pacific and the displacement of this circulation by El Niño and La Niña events.  [Normal, El Niño, & La Niña examples]

Warm Phase - (or "warm event") refers to an El Niño with its anomalously warm water.


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