What Is La Nina?
Spanish for "little girl," La Niña is the name given
to the large-scale cooling of sea surface temperatures across the central and
equatorial Pacific Ocean.
It is one part of the larger and naturally occurring
ocean-atmosphere phenomenon known as the El Niño/Southern Oscillation or ENSO (pronounced "en-so") cycle.
La Niña conditions recur every 3 to 7 years and typically last
from 9 to 12 months up to 2 years.
One of the strongest La Niña episodes on record was that of
1988-1989 when ocean temperatures fell as much as 7 F below normal.
The last La Niña episode occurred during late 2016, and some
evidence of La Niña was seen in January of 2018.
La Niña vs. El Niño
A La Niña event is the opposite of an El Niño event.
Waters in the equatorial regions of the Pacific Ocean are
unseasonably cool.
The cooler waters affect the atmosphere above the ocean, causing
significant changes in climate, though usually not as significant as the
changes that occur during an El Niño.
In fact, the positive effects on the fishing industry make La
Niña less of a news item than an El Niño event.
Both La Niña and El Niño events tend to develop during the
Northern Hemisphere spring (March to June), peak during late fall and winter
(November to February), then weaken the following spring into summer (March to
June).
El Niño (meaning "the Christ child") earned its
name because of its usual appearance around Christmas time.
What Causes La Niña Events
You can think of La Niña (and El Niño) events as water sloshing
in a bathtub.
Water in the equatorial regions follows the patterns of the
trade winds. Surface currents are then formed by the winds.
Winds always blow from areas of high pressure to low pressure; the steeper the gradient difference in the pressure, the
faster the winds will move from highs to lows.
Off the coast of South America, changes in air pressure during a
La Niña event cause winds to increase in intensity.
Normally, winds blow from the eastern Pacific to the warmer
western Pacific. The winds create the surface currents that literally blow the
top layer of water of the ocean westward.
As the warmer water is "moved" out of the way by the
wind, colder waters are exposed to the surface off the western coast of South
America.
These waters carry important nutrients from deeper ocean depths.
The colder waters are important to fishing industries and the nutrient cycling
of the ocean.
How La Niña Years Differ
During a La Niña year, the trade winds are unusually strong,
leading to increased movement of water towards the western Pacific.
Much like a giant fan blowing across the equator, the surface
currents that form carry even more of the warmer waters westward.
This creates a situation where the waters in the east are
abnormally cold and the waters in the west are abnormally warm.
Because of the interactions between the temperature of the ocean
and the lowest air layers, the climate is affected worldwide.
Temperatures in the ocean affect the air above it, creating
shifts in climate that can have both regional and global consequences.
How La Niña Affects Weather and
Climate
Rain clouds form as a result of the lifting of warm, moist air.
When the air doesn't get its warmth from the ocean, the air
above the ocean is abnormally cool above the eastern Pacific. This prevents the
formation of rain, often needed in these areas of the world.
At the same time, the waters in the west are very warm, leading
to increased humidity and warmer atmospheric temperatures. The air rises and
the number and intensity of rainstorms increase in the western Pacific.
As the air in these regional locations changes, so too does the
pattern of circulation in the atmosphere, thereby affecting climate worldwide.
Monsoon seasons will be more intense in
La Niña years, while the western equatorial portions of South America may be
in drought conditions.
In the United States, the states of Washington and Oregon may
see increased precipitation while portions of California, Nevada, and Colorado
may see drier conditions.
Rachelle Oblack
Introduction
Rachelle
Oblack:
Is
a full-time science teacher.
Writes
for major textbook companies about climate and space.
Is
involved with the NASA Educator Astronaut Program, which motivates students and
educators to study science, technology, engineering, and mathematics.
Has
been listed in Who's Who Among America's Teachers.
Experience
Rachelle
Oblack is a former writer for ThoughtCo who contributed articles for five years
as well as a teacher and trainer who has written for major textbook companies
about climate and space. She has been involved with NASA's Educator Astronaut
Program, was a member of the Network of Educator Astronaut Teachers, and has
written for the PBS Newshour Extra.
Education
Rachelle
Oblack has a bachelor's degree in science and education from Wichita State
University, Kansas, and a master's in education. She is certified to teach
earth and space science, general science, biology, environmental science K-12,
chemistry, and physical science.
Awards and
Publications
Member
of the Delta Kappa Gamma Society International, an honor society for women
educators worldwide.
Listed
in Who's Who Among America's Teachers.
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Dotdash
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