What is the El Niño–Southern Oscillation (ENSO) in a nutshell?
Go to any agency that is focused on
weather or climate forecasting and you’ll
hear scientists buzzing to one another
about “ENSO” (pronounced “en-so”).
After glancing at the stereotypical
scientist, you might immediately assume
“En-so” is a Star Wars character, but you
would be mistaken. ENSO is one of the
most important climate phenomena on
Earth due to its ability to change the
global atmospheric circulation, which in
turn, influences temperature and
precipitation across the globe. We also
focus on ENSO because we can often
predict its arrival many seasons in
advance of its strongest impacts on
weather and climate.
Though ENSO is a single climate
phenomenon, it has three states, or
phases, it can be in. The two opposite
phases, “El Niño” and “La Niña,” require
certain changes in both the ocean and the atmosphere because ENSO is a coupled climate
phenomenon. “Neutral” is in the middle of the continuum.
El Niño: A warming of the ocean surface, or above-average sea surface temperatures (SST), in the
central and eastern tropical Pacific Ocean. Over Indonesia, rainfall tends to become reduced while
rainfall increases over the tropical Pacific Ocean. The low-level surface winds, which normally
blow from east to west along the equator (“easterly winds”), instead weaken or, in some cases, start
blowing the other direction (from west to east or “westerly winds”).
La Niña: A cooling of the ocean surface, or below-average sea surface temperatures (SST), in the
central and eastern tropical Pacific Ocean. Over Indonesia, rainfall tends to increase while rainfall
decreases over the central tropical Pacific Ocean. The normal easterly winds along the equator
become even stronger.
Neutral: Neither El Niño or La Niña. Often tropical Pacific SSTs are generally close to average.
However, there are some instances when the ocean can look like it is in an El Niño or La Niña state,
but the atmosphere is not playing along (or vice versa).
So, by now, you might have noticed that while “ENSO” is a nice catchall acronym for all three
states, that acronym doesn’t actually have the word La Niña in it. Why is that? Well, that is a fluke
of history. Before La Niña was even recognized, South American fisherman noticed the warm up of
coastal waters occurred every so often around Christmas. They referred to the warming as “El
Niño,” (niño being Spanish for a boy child) in connection with the religious holiday.
Sir Gilbert Walker discovered the “Southern Oscillation,” or large-scale changes in sea level
pressure across Indonesia and the tropical Pacific. However, he did not recognize that it was linked
to changes in the Pacific Ocean or El Niño. It wasn’t until the late 1960s that Jacob Bjerknes and
others realized that the changes in the ocean and the atmosphere were connected and the hybrid
term “ENSO” was born. It wasn’t until the 1980s or later that the terms La Niña and Neutral gained
prominence.
What are El Niño and La Niña?
El Niño and La Niña are opposite phases of a natural climate pattern across the tropical Pacific
Ocean that swings back and forth every 3-7 years on average. Together, they are called ENSO
(pronounced “en-so”), which is short for El Niño-Southern Oscillation.
The ENSO pattern in the tropical Pacific can be in one of three states: El Niño, Neutral, or La Niña.
El Niño (the warm phase) and La Niña (the cool phase) lead to significant differences from the
average ocean temperatures, winds, surface pressure, and rainfall across parts of the tropical Pacific.
Neutral indicates that conditions are near their long-term average.
What happens during El Niño and La Niña?
During El Niño, the surface winds across the entire tropical Pacific are weaker than usual. Ocean
temperatures in the central and eastern tropical Pacific Ocean are warmer than average, and rainfall
is below average over Indonesia and above average over the central or eastern Pacific.
Rising air motion (which is linked to storms and rainfall) increases over the central or eastern
Pacific, and surface pressure there tends to be lower than average. Meanwhile, an increase in
sinking air motion over Indonesia leads to higher surface pressure and dryness.
Generalized Walker Circulation (December-February) anomaly during El Niño events, overlaid on
map of average sea surface temperature anomalies. Anomalous ocean warming in the central and
eastern Pacific (orange) help to shift a rising branch of the Walker Circulation to east of 180°,
while sinking branches shift to over the Maritime continent and northern South America.
During La Niña, it’s the opposite. The surface winds across the entire tropical Pacific are stronger
than usual, and most of the tropical Pacific Ocean is cooler than average. Rainfall increases over
Indonesia (where waters remain warm) and decreases over the central tropical Pacific (which is
cool). Over Indonesia, there is more rising air motion and lower surface pressure. There is more
sinking air motion over the cooler waters of the central and eastern Pacific.
Generalized Walker Circulation (December-February) anomaly during La Niña events, overlaid on
map of average sea surface temperature anomalies. Anomalous ocean cooling (blue-green) in the
central and eastern Pacific Ocean and warming over the western Pacific Ocean enhance the rising
branch of the Walker circulation over the Maritime Continent and the sinking branch over the
eastern Pacific Ocean. Enhanced rising motion is also observed over northern South America,
while anomalous sinking motion is found over eastern Africa.
Between the warm phase (El Niño) and cool phase (La Niña), scientists describe conditions as
“ENSO-neutral.” Neutral means that the temperatures, winds, convection (rising air), and rainfall
across the tropical Pacific are near their long-term averages.
Generalized Walker Circulation (December-February) during ENSO-neutral conditions.
Convection associated with rising branches of the Walker Circulation is found over the Maritime
continent, northern South America, and eastern Africa.
What causes El Niño and La Niña to occur?
The winds near the surface in the tropical Pacific usually blow from east to west. For reasons
scientists don’t yet fully understand, these relatively steady winds sometimes weaken or strengthen
for weeks or months in a row.
Weak winds allow warm surface waters to build up in the eastern Pacific. Sometimes, but not
always, the atmosphere responds to this warming with increased rising air motion and aboveaverage
rainfall in the eastern Pacific. This coordinated change in both ocean temperatures and the
atmosphere begins an El Niño event. As the event develops, the warmed waters cause the winds to
weaken even further, which can cause the waters to warm even more.
El Niño is often (but not always) followed by La Niña the following year, particularly if the El Niño
is strong. During La Niña conditions, the easterly trade winds near the equator get even stronger
than they usually are. Stronger winds push surface water into the western Pacific. Meanwhile, cool
water from deeper in the ocean rises up in the eastern Pacific. If the cooling persists, it can inhibit
rising air movement and rainfall in the eastern Pacific, beginning a La Niña event. As the event
develops, the cooled waters cause the winds to strengthen even further, which can cause the waters
to cool even more.
How long do El Niño and La Niña typically last?
El Niño and La Niña episodes typically last 9-12 months. They both tend to develop during the
spring (March-June), reach peak intensity during the late autumn or winter (November-February),
and then weaken during the spring or early summer (March-June).
Both El Niño and La Niña can last more than a year, but it is rare for El Niño events to last longer
than a year or so, while it is common for La Niña to last for two years or more. The longest El Nino
in the modern record lasted 18 months, while the longest la Niña lasted 33 months. Scientists aren’t
sure why the duration of the two types of events can be so different.