What
Is La Niña, And Will It Cause Serious Climate Disruption?
By Steve Connor
03 March, 2007
The
Independent
Why are we asking
this question now?
American scientists have
warned that a weather phenomenon in the South Pacific known as "El
Niño", which results in warm Pacific Ocean currents, is
dying out and will be replaced by its opposite number, "La Niña".
The US National Oceanic and Atmospheric Administration (NOAA) said that
cooler-than-normal water temperatures have developed at the surface
in the east-central equatorial Pacific.
It means that La Niña
conditions are likely to develop this spring and are set to strengthen
over the next few months and could last for up to two years. This naturally
occurring phenomenon is likely to reach peak intensity in December to
February and then begin to weaken next March to May.
Does this have any
wider implications?
One of the greatest concerns
is that La Niña is associated with an increase in Atlantic hurricanes.
It can also cause drier-than-usual conditions in the southern United
States, which experienced a serious drought during the last La Niña
some seven years ago. However, Australia and Indonesia, which often
have droughts during El Niño years, can be wetter during a La
Niña phase.
Conrad Lautenbacher, NOAA's
administrator, warned that the next La Niña may mean a stormier
yet drier time ahead for the United States: "Although other scientific
factors affect the frequency of hurricanes, there tends to be a greater-than-normal
number of Atlantic hurricanes and fewer-than-normal number of eastern
Pacific hurricanes during La Niña events," he said. "During
the winter, usual La Niña impacts include drier and warmer-than-average
conditions over the southern United States."
How often does La
Niña occur?
On average, El Niño
and La Niña occur every three to five years. However, the actual
interval between them has varied from two to seven years. La Niñas
began this century with the first in 1903, followed by events in 1906,
1909, 1916, 1924, 1928, 1938, 1950, 1954, 1964, 1970, 1973, 1975, 1988,
and 1995. The last La Niña began in 1998 and ended in 2001. These
events typically continued into the following spring, although they
can last longer. Since 1975, La Niñas have been only half as
frequent as El Niños.
What causes La Niña?
Cooler-than-normal subsurface
water in the tropical Pacific builds up, as eastern-moving winds and
waves help to bring cold, deep water to the surface through a complex
set of events that are still under study. Sea surface temperatures can
fall to at much as 4C below normal. The warm body of water associated
with El Niño - which expands to cover the tropics during this
phase of the Pacific cycle - begins to shrink to the west during La
Niña.
What is the difference
between La Niña and El Niño?
Both terms refer to large-scale
changes in sea-surface temperature across the central and eastern tropical
Pacific. Usually, sea-surface readings off South America's west coast
range from 15C to 20C, while they exceed 26C in the "warm pool"
located in the central and western Pacific. This warm pool expands to
cover the tropics during El Niño but shrinks to a small enclave
in the west during La Niña.
La Niña events sometimes
follow on the heels of El Niño conditions, although not always.
Sometimes there is a "neutral" phase that is neither one nor
the other. The scientific name for the phenomenon is the El Niño/Southern
Oscillation (ENSO), and it describes the coupled ocean-atmosphere process
that includes both El Niño and La Niña.
Does global warming
influence La Niña or El Niño?
Scientists are still not
sure about this. They want to know whether a warmer world is likely
to produce more El Niños and La Niñas or cause more intense
cycles to occur. According to NOAA: "At this time we can't preclude
the possibility of links but it would be too early to definitely say
there is a link." What is pretty clear, however, is that global
warming is likely to increase the risk of extreme weather events associated
with La Niña and El Niño. In other words, global warming
is likely to make matters worse.
What is the origin
of the name 'La Niña'?
Fishermen off the west coast
of South America recognised the periodic appearance of warm water in
their fishing grounds, which they called El Niño, which means
"The Little Boy" or "Christ child" in Spanish. They
called it this because the warm body of water usually appeared around
Christmas time.
When scientists recognised
that there was an opposite event in the southern Pacific oscillation,
they dubbed it La Niña, which means "The Little Girl"
in Spanish. La Niña is sometimes called El Viejo (Old Man), anti-El
Niño, or simply "a cold event" or "a cold episode".
There has been a confusing range of uses for the terms El Niño,
La Niña and ENSO by both the scientific community and the general
public.
Are there any environmental
benefits of La Niña?
Coral bleaching - when coral
reefs eject the microbes that give them their distinctive colours -
appears to be worse during an El Niño phase, when Pacific Ocean
temperatures get warmer. Coral bleaching was particularly pronounced
during 1997-98 because a very strong El Niño occurred that year.
The increase of sea temperatures caused by that El Niño was superimposed
on the sea temperature warming trend observed in some parts of the Pacific
and Indian Oceans, caused by global warming. Coral seem very sensitive
to rising sea temperatures, so the cooler phase created by La Niña
should in theory benefit corals, albeit temporarily given that ocean
temperatures globally are tending to rise.
Is it true that La
Niña and El Niño can affect the spin of the Earth?
Yes, but the effect is very
small. El Niño results in a decrease in the earth's rotation
rate, an increase in the length of day. La Niña tends to have
the opposite effect.
El Niño is associated
with a weakening of the tropical Pacific trade winds, and with a strengthening
of the mid-latitude westerlies both at the surface and at higher altitudes.
To balance these changes in atmospheric winds, the earth's rotation
rate decreases in order to conserve total angular momentum of the earth-atmosphere
system. Conservation of angular momentum is a basic physical principle
which operates, for example, when a ballerina brings her arms closer
to her body to spin faster. The change, however, is only about 1 millisecond
at the peak of a strong El Niño. There are 86,400 seconds in
a day, so this change represents one part in 100 million. Such a change
will have little effect on normal activities on a human scale, such
as flying an aeroplane.
© 2007 Independent News
and Media Limited