Some scientists use El Niņo and ENSO interchangeably. Others use the
phrase "warm event" to describe a warming of the surface
waters in the central and eastern equatorial Pacific, and "cold
event" to denote the opposite phase of the Southern Oscillation.
The glossary which follows is provided to help the reader sort out these
concepts.
El Niņo is a term originally used
to describe the appearance of warm (surface) water from time to time in
the eastern equatorial Pacific region along the coasts of Peru and
Ecuador. It was once suggested that minor El Niņo events occurred about
every two to three years and major ones about every eight to 11 years.
Today, scientists note that El Niņo has a return period of four to five
years. When an El Niņo event occurs, it often lasts from 12 to 18
months.
La Niņa refers to the appearance of
colder-than-average sea surface temperatures (SSTs) in the central or
eastern equatorial Pacific region (the opposite to conditions during El
Niņo). Many scientists do not like the use of the term and prefer to
call it a cold event (described below).
A warm event refers to the anomalous
warming of SSTs in the central and eastern equatorial Pacific. This term
is being used to avoid confusion over the use of other terms like ENSO
and El Niņo. A warming in the regions mentioned is accompanied by a
relative cooling in the western equatorial Pacific.
A cold event
is one where the SSTs become anomalously colder compared to the
long-term average for the central and eastern equatorial region. (It is
the opposite of a warm event in that region.) It has been referred to in
the past as anti-El Niņo and, more recently, as La Niņa. La Niņa,
however, unlike the restrictive view of El Niņo, is applied to Pacific
basinwide phenomena.
The Southern Oscillation is a
see-saw of atmospheric mass (pressure) between the Pacific and
Indo-Australian areas. For example, when the pressure is low in the
South Pacific high pressure cell and high over Indonesia and Australia,
the Pacific trade winds weaken, upwelling of cool water on the Pacific
equator and along the Peruvian coast weakens or stops, and SSTs increase
in these areas where the upwelling weakens.
The Southern Oscillation Index (SOI)
has been developed to monitor the Southern Oscillation using the
difference between sea level pressures at Darwin, Australia, and Tahiti,
although other stations have sometimes been used. Large negative values
of the SOI indicate a warm event, and large positive values indicate a
cold event (also referred to as La Niņa). It is important to note that
there is not a one-to-one correspondence between the occurrence of
Southern Oscillation events and El Niņo events, using the spatially
restrictive original definition of El Niņo.
ENSO is the term currently used by
scientists to describe the full range of the Southern Oscillation that
includes both SST increases (a warming) as well as SST decreases (a
cooling) when compared to a long-term average. It has sometimes been
used by scientists to relate only to the broader view of El Niņo or the
warm events, the warming of SSTs in the central and eastern equatorial
Pacific. The acronym, ENSO, is composed of El Niņo-Southern
Oscillation, where El Niņo is the oceanic component and the Southern
Oscillation is the atmospheric component of the phenomenon. The broader
definition of El Niņo has sometimes been used interchangeably with
ENSO, because ENSO is less well known in the popular electronic and
printed media.
Teleconnections can be defined as
atmospheric interactions between widely separated regions. They have
been identified through statistical correlations (in space and time).
Some of these correlations have been used to generate hypotheses about
geophysical processes related to teleconnections. Most countries in the
world are, or should be, interested in this aspect of the Southern
Oscillation. Some examples are provided in the following maps.
Five-month running mean of the sea level pressure anomalies at Darwin
(dashed) and Tahiti
(solid) (Climate Analysis Center, 1991: Climate Diagnostics Bulletin,
November.
Washington, DC: US Department of Commerce.
Selected extreme temperature events that persisted for a season or
longer in the
1982-84 period. (WMO, 1984: The Global Climate System: A Critical Review
of the Climate System During 1982-1984. Genever WMO.)
Selected extreme continental precipitation (wet and dry areas) that
persisted for a
season or longer in the 1982-84 period (WMO, 1984: see above.) |
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