Origins 10(1):41-43 (1983).
Unusual weather patterns were the rule from Adelaide to Quito.
Australia experienced killing droughts and subsequent fires devastated large sections of
land. In Equador, areas that normally receive 13 mm of rain during a dry-season month were
inundated with 583 mm. Ocean temperatures along major portions of the west coasts of the
Americas were in some areas 10º warmer than usual. These warm waters caused a marked drop
in primary production of microorganisms, and as a result the usually enormous anchovy
population disappeared. Animals which depended on the fish either starved or hunted
elsewhere for food. On Christmas Island, for example, an estimated 17 million eggs and
unfledged young of sea birds were abandoned because the food in surrounding waters
literally disappeared.
The cause of these seemingly unrelated phenomena is blamed on El Niño
(The Christ Child). El Niño historically has been a periodic warm-water current that
starts moving along the coast of Peru near Christmastime. As coastal water is driven west
by trade winds, a current of warm water from the north moves southward usually around
February or March and covers the colder water along the coast. With the nutrient-rich
water now very deep, fish which depend on the plankton and other organisms for survival
move to other locations. In times past this yearly warming marked the end of the fishing
season around February.
Currently, the term El Niño is much more limited in its use and
presently describes the interannual catastrophic events which destroy much life caused by
massive amounts of warm water flowing in from the mid-Pacific instead of the more normal
northerly warm current. These events have a 6-8 year cycle, in contrast to the more mild
yearly warming events described above.
Just what causes the El Niño event is somewhat like the
chicken-or-the-egg question. Under normal conditions nearly constant westerly trade winds
in combination with the Coriolis effect (phenomena related to the rotation of the earth)
push the coastal surface water in the equatorial latitudes towards Asia. The water warms
significantly as it moves across the Pacific.
Only recently have scientists begun to unravel why some seasonal
changes are more devastating than others. It is now realized that the El Niño is coupled
to another atmospheric event in the eastern Pacific known as the Southern Oscillation.
In the Southern Oscillation major shifts in barometric pressure take
place between two areas in the Pacific. One centers in Malaysia with a measuring station
situated in Darwin, Australia. The other is in Tahiti. If the pressure is high in Darwin,
it will be low in Tahiti. The converse is similarly true. Over a period of several years
the pressure difference seesaws back and forth with periods varying from 2-8 years.
In the low-pressure area, a circular air current, known as the Walker
Circulation, also develops. Water-laden air dumps its moisture as it rises into the upper
atmosphere. After moving across large distances in the upper atmosphere, it then returns
to sea level at the other end of the pressure cell. The location of the rising column of
air controls where the heaviest rain fall will take place. The low-pressure area is
additionally important because it attracts the westerly trades and determines their
general direction of movement.
In the El Niño event of 1982-83, the low-pressure area, which should
have been located at Tahiti, moved farther east. Record pressure differences were recorded
between Darwin and Tahiti. This low-pressure area then drew the westerlys. To the west of
this atmospheric basin, the normally westerly winds reversed themselves and blew in the
opposite direction, probably attracted by the low-pressure area.
The current El Niño event was anomalous in that a second cell of warm
water developed in the western Pacific in addition to the warm water which the wind pushed
in from the coast of South America. As the water along the equator heated, the intensity
of the trades decreased and ultimately stopped. This, combined with a greater-than-average
heating, produced water temperatures at the equator much above normal. Conditions were now
set for what has been called a once-in-a-century event.
The warm water, which was originally derived from South America,
combined with the anomalous warm cell and started to return eastward to the South American
coast because it was no longer held back by the westerly winds. Several months were
required for the water to reach the South American coast. In some oceanic islands the sea
level changed a foot or more as the warm water sloshed eastward.
Upon reaching the continental coast, the current divided into branches
moving north and south along the coast and did its destructive work. Record high ocean
temperatures were recorded as far north as Washington. Eventually, with the mixing of
colder waters and the resumption of the trade winds now blowing toward Darwin again, the
El Niño faded.
The anomalous weather patterns in the central Pacific affect
significantly weather patterns in far distant portions of the globe. Known as
teleconnections, these are statistically consistent changes that occur in El Niño years,
causing large weather shifts in the northern latitudes and perhaps being partly
responsible for the strange weather experienced on several continents.
When the next El Niño event will arrive is not known, of course, but
one can be certain that the mammoth interannual weather shifts will be here sooner than
some would wish. The El Niño event should also give one pause to consider that world-wide
disturbances can be caused by seemingly small events such as the shifting of a
low-pressure area or the elevation of water temperature by a few degrees. The earth is
perhaps more fragile than most would suspect.
PERTINENT LITERATURE
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Geoscience Research Institute. All rights reserved.
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