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A Closer Look

El Niño and La Niña
he term El Niño was originally used by interacting with one another, most vis- • a warmer and drier-than-average
Tto describe an occurrence of warm, ibly in the tropical cyclone. The ocean sys- winter across the northern regions of
above-normal ocean temperatures off the tem supplies water vapor, latent heat, and Canada and the United States;
South American coast. Fishermen along condensation nuclei, which are the essen- • a variable effect on central regions of
this coast learned long ago to expect associ- tial elements of a tropical cyclone as well as the United States, ranging from reduced
ated changes in fishing patterns about every everyday weather changes and climate. The snowfall to no effect at all; and
three to seven years, which usually lasted atmosphere, on the other hand, drives the
• other changes in the worldwide global
for about eighteen months. They called this ocean with prevailing winds, moving warm
complex of ocean and weather events,
event El Niño, which is Spanish for “the or cool water to locations where they affect
such as droughts in normally wet
boy child” or “Christ child,” because it typi- the climate on the land. There is a complex,
climates and heavy rainfall in normally
cally occurred near Christmas. The El Niño interdependent relationship between the
dry climates.
event occurs when the trade winds along ocean and the atmosphere, and it is prob-
the equatorial Pacific become reduced or able that even small changes in one system One major problem in these predic-
calm, allowing sea surface temperatures to can lead to bigger changes in the other. tions is a lack of understanding of what
increase much above normal. The warm Normally, during non–El Niño times, causes many of the links and a lack of con-
water and changes in the circulation pattern the Pacific Ocean along the equator has sistency in the links themselves. For ex-
drive the fish to deeper waters or farther established systems of prevailing wind ample, southern California did not always
away from usual fishing locations. belts, pressure systems, and ocean cur- have an unusually wet season every time an
Today, El Niño is understood to be rents. In July, these systems push the sur- El Niño occurred and in fact experienced a
much more involved than just a warm face seawater offshore from South America, drought during one event.
ocean in the Pacific. It is more than a local westward along the equator and toward Scientists have continued to study
event, and the bigger picture is sometimes Indonesia. During El Niño times, the trade the El Niño since the mid-1980s, search-
called the El Niño-Southern Oscillation, or winds weaken and the warm water moves ing for patterns that will reveal consistent
ENSO. In addition to the warmer ocean of back eastward, across the Pacific to South cause-and-effect links. Part of the problem
El Niño, “the boy,” the term La Niña, “the America, where it then spreads north and may be that other factors, such as a vol-
girl,” has been used to refer to the times south along the coast. Why the trade winds canic eruption, may influence one part of
when the water of the tropical Pacific is weaken and become calm is unknown and the linkage but not another part. Another
colder than normal. The “Southern Oscilla- the subject of ongoing research. part of the problem may be the influence of
tion” part of the name comes from obser- Warmer waters along the coast of South unknown factors, such as the circulation of
vations that atmospheric pressure around America bring warmer, more humid air and water deep beneath the ocean surface, the
Australia seems to be inversely linked to the increased possibility of thunderstorms. track taken by tropical cyclones, or the en-
the atmospheric pressure in Tahiti. They Thus, the possibility of towering thun- ergy released by tropical cyclones one year
seem to be linked because when the pres- derstorms, tropical storms, or hurricanes compared to the next.
sure is low in Australia, it is high in Tahiti. increases along the Pacific coast of South The results so far have indicated that
Conversely, when the atmo spheric pres- America as the warmer waters move north atmosphere-ocean interactions are much
sure is high in Australia, it is low in Tahiti. and south. This creates the possibility of more complex than early theoretical models
The strength of this Southern Oscillation is weather changes not only along the west- had predicted. Sometimes a new model will
measured by the Southern Oscillation In- ern coast but elsewhere, too. The towering predict some weather changes that occur
dex (SOI), which is defined as the pressure thunderstorms reach high into the atmo- with El Niño, but no model is yet consis-
at Darwin, Australia, subtracted from that sphere, adding tropical moisture and creating tently correct in predicting the conditions
at Tahiti. Negative values of SOI are usu- changes in prevailing wind belts. These wind that lead to the event and the weather pat-
ally associated with El Niño events, so the belts carry or steer weather systems across the terns that result. All this may someday lead
Southern Oscillation and the El Niño are middle latitudes of North America, so typi- to a better understanding of how the ocean
obviously linked. How ENSO can impact cal storm paths are shifted. This shifting can and the atmosphere interact on this dy-
the weather in other parts of the world has result in namic planet.
only recently become better understood. Recent years in which El Niño events
• increased precipitation in California
The atmosphere is a system that re- have occurred are 1951, 1953, 1957–1958,
during the fall through spring seasons;
sponds to incoming solar radiation, the 1965, 1969, 1972–1973, 1976, 1982–1983,
• a wet winter with more and stronger 1986–1987, 1991–1992, 1994, 1997–1998,
spinning Earth, and other factors, such as the
storms along regions of the southern 2002–2003, 2005–2007, and 2009.
amount of water vapor present. The ocean
United States and Mexico;
and atmospheric systems undergo changes

586 CHAPTER 23 Weather and Climate 23-22