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What is it?
During the past several months there has been a great deal of
discussion on the return of the El Niņo. El Niņo is a warming of the tropical Pacific ocean. It
is a warm ocean current setting south along the coast of
Ecuador, so called because it generally develops and
peaks just after Christmas. In exceptional years,
concurrently with a southward shift in the tropical rain
belt, the current may extend along the coast of Peru to
12° south. When this occurs, plankton and fish are killed
in the coastal waters and a phenomenon like
the red tide of Florida results.
El Niņo is part of a cyclical process made up of two parts
called ENSO (El Niņo Southern Oscillation). The
Southern Oscillation is a biennial event whereby the
stratospheric winds flip flop from west one year to east
the next. The ENSO is a natural phenomenon and there is
good evidence from cores of coral in the tropical Pacific
and glacial ice in the Andes that it has been going on for
millennia.
The ocean and atmospheric conditions in the tropical
Pacific are seldom "average", rather they fluctuate in an
irregular cycle alternating between El Niņo /the warm
phase/ and La Nina /the cold phase consisting of a
basinwide cooling of the tropical Pacific/. The preferred
period of one full cycle is about 3 to 6 years. The most
intense part of each phase usually lasts about a year.
A distinctive pattern of sea surface temperatures (SST) in
the Pacific Ocean sets the stage for an ENSO event. The
main features are a "warm pool" in the tropical western
Pacific with much cooler waters in the east Pacific, and a
cold tongue along the equator that is most pronounced
about October and weakest in March. Distinctive patterns
of SST are caused mainly by the winds.
How does El Niņo work?
During El Niņo, the warm waters from the western
tropical Pacific migrate eastward as the trade winds
weaken. This sets a chain of events in motion as this
shifts the pattern of tropical rainstorms, further
weakening the trade winds which further reinforces the
changes in SST. A complex feedback is set up in the
Hadley circulation pattern and hence the jet stream. It
should be noted that no two El Niņo events are exactly
alike. For typical El Niņo events, higher than normal sea
level pressure occurs over Australia, Indonesia, southeast
Asia, and the Philippines signaling drier than normal
conditions or even droughts. Dry conditions also prevail
in the Hawaiian Islands and extend west from Australia to
parts of Africa and across the Atlantic to the northeast
part of Brazil and Columbia. Excessive rains prevail over
the central and eastern Pacific, along the west coast of
South America near Uruguay, and southern parts of the
United States in winter.
What's the big deal?
From data gathered over the past several months, there is
no doubt that a strong El Niņo event is underway. The
current event developed earlier than all of the strong
events in recent history. It appeared that this event would
eclipse the 1982-83 event, which was the strongest in
recent history. Over the past two months, there as been
some decrease in the strength of the current event. Also
evident in the data are indications of its maturation as
well as the area of strongest above normal water
temperatures has begun to shift west. Warm episode
(ENSO) conditions continued through October and
remained very strong. Sea surface temperature (SST)
anomalies remained strong across the eastern and central
tropical Pacific Ocean during the early fall, with positive
SST anomalies greater than 1.0°C extending from near
180°W eastward to the South American coast. Anomalies
exceeding +2.0°C covered the tropical Pacific east of
170°W, while anomalies greater than +4.0°C were
observed along the equator east of 130°W. In many areas
of the eastern equatorial Pacific, SST's as of October
were already the largest observed in the last 50 years.
Atmospheric circulation anomalies consistent with those
generally experienced during warm episodes was already
evident as early as June. Drier than normal conditions
were observed over Indonesia, while wetter than normal
conditions were observed over the islands of the central
tropical Pacific. Also, a stronger than normal South
Pacific jet stream developed and extended further east
than normal. This resulted in a wetter than normal start of
the winter in central Chile and central Argentina,
Uruguay, and southern Brazil.
The 1997-98 event has been
stronger than any event on record, there is some question as
to whether it will end up as the strongest on record. Some of
the indicators are suggesting the 1997-98 event has peaked.
It is not uncommon for an El Niņo to rise unevenly with
more than one peak and this event could turn around and
rise above the 1982-83 event in the next couple months.
Though that is a possibility, it does appear the drop in the
past two months and the westward shift of the strongest
anomalies are telling us it may have indeed peaked. Either
way, this event is stronger than the 1982-83 event was at
this point in time. It is clear that a very strong El Niņo is
well underway.
Text taken from: http://www.crh.noaa.gov/dmx/el_nino.html
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