By convention, when you hear the name el niño it refers to the warm episode of enso while the cool episode of enso is called la niña enso is primarily monitored by the southern oscillation index (soi), based on pressure differences between tahiti and darwin, australia.
Information on the el nino southern oscillation (enso), a periodic fluctuation in sea surface temperature and air pressure in the equatorial pacific ocean.
La niña the southern oscillation (so) is a variation in air pressure between the central and western tropical pacific these pressure changes alter the strength of the trade winds, affect surface ocean currents, and are related to el niño scientists often combine the terms, el niño and the southern oscillation, enso normal conditions are sometimes called la niña. For example, i consider our forecasts of the recent 2015-16 el nino and subsequent 2016-17 la nina to be pretty good in the sense that we knew the wintertime state of enso ~6 months in advance for the most recent 2017-18 la nina we issued a la nina watch (possibly arrival of la nina) in september 2017 and then declared its onset in november 2017.
The southern oscillation index (soi) is a standardized index based on the observed sea level pressure differences between tahiti and darwin, australia the soi is one measure of the large-scale fluctuations in air pressure occurring between the western and eastern tropical pacific (ie, the state of the southern oscillation) during el niño and la niña episodes. El niño events are associated with a warming of the central and eastern tropical pacific, while la niña events are the reverse, with a sustained cooling of these same areas these changes in the pacific ocean and its overlying atmosphere occur in a cycle known as the el niño–southern oscillation (enso.
The el niño–southern oscillation is a single climate phenomenon that periodically fluctuates between three phases: neutral, la niña or el niño la niña and el niño are opposite phases that require certain changes to take place in both the ocean and the atmosphere, before an event is declared. El nino-southern oscillation the correlation of el nino events with an oscillatory patters of pressure change is a persistent high-pressure cell in the southeastern pacific ocean and a persistent low-pressure cell over the east indies (this is an atmospheric event.
Conditions are updated weekly on the climate prediction center web site (el niño/la niña current conditions and expert discussions) forecasts are also updated monthly in the forecast forum of cpc's climate diagnostics bulletin additional perspectives and analysis are also available in an enso blog. A brief video from the nws in bismarck discussing what causes el nino and la nina ----- national weather service weather forecas.