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El Nińo and La
Nińa,
the seesawing of Pressure and Sea levels
adapted from PH430 paper written by
a USP student for USP Physics Society quarterly publication "resonnance"
and is an article aimed at introducing El Nino and La Nina to
secondary school level students in the Pacific region.
El
Nińo and La Nińa... terms we’ve all heard before
associated with the extremities of weather phenomena that were
destructive over much of the globe back in early 1997. Little is
understood of the two phenomena by the general public though the
general idea was that many things could be blamed on whichever one
was occurring at the time.
We may have a
connection with both terms in that we understand that both don’t
always mean the most pleasant of weather during a given length of
time, but do we really understand what each is about or more
importantly what each means to Pacific Islanders? Understanding what
El Nińo and La Nińa is important for many reasons but
especially because the phenomena originates here in the Pacific, in
our backyard so to speak. This article attempts a little history,
together with the definitions of the phenomena and the simple
physics of the mechanics that drive El Nino and La Nina.
El
Nińo and La Nińa have occurred for as long as 2 million years
according to some field research estimates using samples from bored
corals in the El Nińo hotspots off the coasts of north west South
America. We have only begun recording these events scientifically
though for about 50 years after special note of its effect on the
monsoon seasons of India. The current cycles and patterns of what we
see and experience today may be vastly different though from what
they were those millions of years ago and perhaps even a hundred
years ago.
El
Nińo as we experience it today, occurs every 2-7 years, similarly
with La Nińa at an average of 4.5 years. To set the framework for
understanding the overall picture of El Nińo and La Nińa, the
current time period we are in is said to be within any given three defined
meteorological periods:
- Normal period
- El Nińo, and
- La Nińa period
These periods have
been clearly defined by past data records of occurrences of El Nińo
and La Nińa. Using past data with comparison to real time data
and analysis from global weather monitoring stations and satellites
determine with reasonable accuracy what period we are in presently.
This same data, through further analysis, is also used to calculate
a probability as to which period we may be in within 3 months, even
up to a year!
Definitions
Out
of the data collected over the years comes a definition of El Nińo
as accepted officially by the Australian and New Zealand scientific
community (and so used to characterize any given period we may
happen to be in).
An
El Nińo period is
defined as a period within which conditions exist that lead to and
contain the occurrence of a recognized El Nińo.
The
definition of El Nińo then is:
“A sustained warming, in excess of 1°C above normal, of the central and east tropical
Pacific ocean, typically centered around the NIŃO3 region. This
warming is accompanied by negative values of the SOI, a decrease in
the strength of the Pacific Trade Winds, and a reduction in rainfall
over east and northern Australia which often results in drought. The
most recent strong El Nińo began in autumn 1997 (around
April/May) and ended in Autumn 1998.”
A
Normal period is
defined as one where normal conditions are taking place as in Trade
Winds and east-west ocean currents along the equator throughout the
Pacific proceed as normal.
The
La Nińa period is one
that immediately follows an El Nińo period. The accepted
definition of La Nińa is as follows:
“A sustained cooling, in excess of 1°C below normal, of the central and east tropical
Pacific Ocean, typically centered around the Pacific NIŃO3
region. This cooling is usually accompanied by positive values of
the SOI, an increase of the strength in the Pacific Trade Winds, and
higher than normal rainfall over east and northern Australia,
sometimes with serious flooding. The most recent strong La Nińa
was in 1998/99; a weak La Nińa event which began in mid 1998
and ended late Autumn 1999.”
The
diagram above shows the NIŃO areas as referred to in the definitions
above. Nauru, Kiribati and Tuvalu are within the boundaries of the
NIŃO4 area while outlying some islands of French Polynesia and
Kiribati are within the NIŃO3 areas. Read on for the explanation of
the term SOI.
El Nińo and
La Nińa from a simple physics point of view are fuelled mainly
by one main factor: Pressure. By pressure we mean
atmospheric pressure.
The Normal Period
The
traditional areas of pressure readings were at Darwin and Tahiti for
the west and east end of the Pacific region respectively. During a
Normal Period a low pressure
area exists at the west
equatorial end of the Pacific (Darwin) and a high pressure area at
around the NIŃO3 and
NIŃO1+2 areas (Tahiti). These pressure points are very much
consistent during a
Normal period. The SOI refers to the Southern Oscillation Index and
is simply a number that indicates the ratio of the two pressure
points at any given time. A positive valued SOI means that
atmospheric pressure at Tahiti is higher than that at Darwin and is
expected during a normal period. A negative valued SOI is simply the
opposite.
The pressure
difference between these two points results in a gradient wind (wind resulting from
difference in pressure (aka pressure gradient) between
any two points). This gradient wind is the Trade Winds we know which
blows towards the west. As the Trade Winds move along the equator it
pushes along in front of it masses of water from the east Pacific
region. What results is an accumulation of water off the Australian
and Papua New Guinea coasts leading to a sea level height of much as
a meter compared to that of the east region.
The picture
above illustrates the Normal Period conditions while the schematic
below show the differences of atmospheric Pressure (P) and Sea Level
(S.L) at Darwin and Tahiti. The large P indicates the greater
pressure experienced at Tahiti or NIŃO3 region with its
corresponding small S.L. meaning the relatively lower sea level.
This
condition portrays simple physics where pressure and fluids
interact. If we imagine the Pacific Ocean as water in a container,
and a pressure or weight applied at one end being larger than the
other, then the corresponding effect is in the water level at the
lower pressure end higher.
We can imagine
this as an initial state where the sea level see-saw has the west
end higher and the east end lower. At the same time on the pressure
see-saw, the opposite initial state: lower on the west end and
higher at the east end.
El Nińo period
The chain of events that lead to an El Nińo as is now
accepted begin when the normally higher pressure at the NIŃO3
region drops below the Darwin pressure level. This results in the
Trade Winds slackening and dying down because the pressure gradient
and hence the SOI across the Pacific is now reversed. With the
pressure now higher at the Darwin side and lower at the Tahiti side,
the opposite of the normal conditions now occur. The accumulated
waters off the coasts of west Australia and the west Pacific region
now move to the lower pressure area of NIŃO3 and to a more
recent extent, NIŃO1+2, as shown in the diagram below.
Our
sea level see-saw has swung the opposite way, the west end now lower
than the east end. The pressure see-saw has also changed position
with the west end now higher than the east end.
The eastward
flow of the warmer west waters toward the cooler east NIŃO3
and NIŃO1+2 areas signify the start of the El Nińo season.
When these
waters have reached and accumulated at NIŃO3 and NIŃO1+2
areas, they have replaced the cooler waters present there during
Normal Conditions. Because the accumulated waters cover a large area
(practically the whole of the NIŃO1+2 area) and because of the
significant consistency of its warm temperatures, they directly
affect the surrounding atmosphere thus beginning the chain of
events that lead to the destructive worldwide weather effects that
cost the world wide economy more than US$15 billion in property
damage.
La Nińa Period
When
El Nińo begins to end, the pressure gradient is seemingly restored
and the accumulated warm waters in the NIŃO3 and NIŃO1+2 regions
begin to push back to the west Pacific region with the rejuvenated
Trade Winds. Although this may seem like a return to Normal
conditions (and it is possible to return to and remain at a Normal
period) initially what signifies a La Nińa period is the higher than
normal high pressure at the NIŃO3 and NIŃO1+2 regions, and the
extension of the cooler waters that of the NIŃO1+2 area into
the NIŃO3 area. These cooler temperature waters can extend far
into the
NIŃO3 region as it did in 1998, the presence of its unusual cool
temperatures again playing havoc with the surrounding atmosphere,
much as the warm waters of El Nińo did.
We can now begin to
understand how an El Nińo and La Nińa may occur and to a
certain degree, how to spot one in the making. What is yet to be
understood or found out is the reason why an El Nińo starts and
hence a La Nińa to follow. Organizations such as the NOAA
(National Oceanic and Atmospheric Administration), a leading ocean
and atmosphere monitoring and research organization, and governments
of countries the world over have pledged millions in funds to research into El Nińo
and the subsequent mayhem it brings with it. With more than US$15
billion in damages sustained in the last El Nińo/La Nińa
period, its no wonder why.
So for the time
being with more of an understanding of what each term now means, our
resorting to blaming seemingly “unusual” accidents on El Nińo
and La Nińa may now be limited. On the other hand, one now has
wisened up more to such excuses.
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