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DEATH BY
EL NINO!
We continue our look
at the effects of the
ocean on our lives by
examining the changes
in Pacific sea surface
temperatures … El
NINO!
El Niño / Southern Oscillation
A set of tropical weather and oceanic phenomena occurring in the
Pacific Ocean that affects the entire world.
Today, we'll explain:
What is it?
How does it work?
How often does it occur?
What areas of the Earth are most affected?
El Niño / Southern Oscillation variations can be devastating for
many parts of the world. Some areas and how they are influenced
include:
•Massive floods in Peru
••Economic troubles in Peru
(failure of the anchovy industry)
••Drought/Wildfires in Australia
••Drought in Brazel
••Heavy rains in the southern
United States
••Fewer Atlantic hurricanes
••Failure of the Southeast Asian
Monsoon
among other affected areas
around the globe.
The common abbreviation for the whole set
of phenomena that we will be discussing
today is
ENSO
The abbreviation stands for the two distinct aspects of the
phenomenon:
El Niño: the ocean current
Southern Oscillation: the atmospheric circulation
So what do these parts of ENSO do?
El Niño: A Spanish term meaning "The Child"—specifically, the
Christ Child, referring to the time of the year when El Niño is most
noticeable in South America around Christmas in the months of
December and January.
El Niño historically refers a massive warming of the coastal
waters off Peru and Ecuador. It is accompanied by torrential
rainfall, often resulting in catastrophic flooding. El Niño events
have been documented back to 1726 and there is other evidence
indicating occurrences for at least 1000 prior to that.
Paleoclimatic research has also suggested the possibility of El
Niño events back 5000 years.
Southern Oscillation: The subtropical circulation (air flow) that
exists in the southern Pacific Ocean, specifically between the
ocean off the coast of western South America and Australia.
The atmospheric component of ENSO, the Southern Oscillation,
is a more recent discovery. Although the term is sometimes used
to refer to the global complex of climatic variations, the Southern
Oscillation is specifically an oscillation in surface pressure (and
thus atmospheric mass) between the southeastern tropical Pacific
(the South Pacific subtropical high) and Australian-Indonesian
regions (the Indonesian low).
We have a better understanding of
how ENSO works than almost any
other global phenomena. So how
does it work?
The Theory of ENSO
Sir Gilbert Walker is considered the early
leader in describing relations between the sea
level pressure shifts (the Southern Oscillation)
and climate variations around the globe in the
early part of this century. He failed, however, to
address the oceanic element of the phenomenon,
El Niño. It was only in the 1960s that work
linking the oceanic and atmospheric elements of
ENSO began. That work was spearheaded by
Jacob Bjerknes.
Sir Gilbert Walker
Intriguingly, Walker started his work in an attempt to explain why
Captain Robert Scott died in Antarctica in 1912.
BUT FIRST … Important Terms:
Atmospheric Circulation Cell: A looping circle of air that
travels back to its starting point.
Upwelling: The rising of water toward
the surface in a body of ocean. In general,
upwelling is most prominent when winds
blow parallel to a coast. Upwelled water
is both colder and richer in nutrients then
the surface water.
Thermocline: A vertical
temperature gradient in a body of
water that is markedly greater
than the gradients directly above
and below it. Often the
thermocline serves to inhibit
movement through it.
Convection (or adiabatic uplift): The
uplift of air due to surface heating. This
process often creates thunderstorms and
rain. The summer thunderstorms in
Arizona are convection thunderstorms.
Now Back To Our Story: Bjerknes
recognized the fact that normally the SSTs
(sea-surface temperatures) at the eastern end of
the Pacific (near South America) are much
colder than one would expect given the
subtropical latitude. Since the western Pacific
(near Australia) is very warm, there is a large
ocean temperature gradient along the equator
in the Pacific.
As a result there is a direct thermal
circulation in the atmosphere along
the equator: the relatively cold, dry
air above the waters of the eastern
equatorial Pacific flows westward
along the surface towards the warm
west Pacific.
"There in the west Pacific," Bjerknes said, "after having been
heated and supplied with moisture from the warm waters, the
equatorial air can take part in large-scale, moist adiabatic
[convective] ascent." Some of the ascending air joins the poleward
flow at upper levels associated with the Hadley (or North-South)
circulation and some returns to the east to sink over the eastern
Pacific.
This air flow is related to a strong surface pressure gradient
associated with this equatorial circulation: high pressure in the east
(off the coast of South America) and low pressure in the west (over
Australia). Bjerknes named this circulation the "Walker
Circulation".
BEST SOURCE for more information:
Bjerknes, J., 1969: Atmospheric teleconnections from the
equatorial Pacific. Monthly Weather Review, 97, 163-172.
But what causes the temperature difference between the east
and west sides of the Pacific Ocean in the first place?
Bjerknes suggested three possible causes for unusually cold SSTs
in the east Pacific:
•
Horizontal advection. The easterly winds along the equator
drive currents that cause cold water to be pulled from Antarctica
along the South American coast.
•
Equatorial upwelling. The Coriolis force turns ocean
currents to the right in the Northern Hemisphere and to the left in
the Southern Hemisphere. Consequently, the surface flow at the
equator is deflected poleward and the poleward flow must be fed
by waters which upwell along the equator, water that are colder
than the surface.
•
Upward thermocline displacement. The tropical ocean can
usefully be viewed as a two-layer fluid, consisting of a shallow
warm ocean layer above the layer of cold abyssal waters. In the
real ocean the two are separated by the thermocline, a narrow
(50-100m) region of strong temperature change (10°C or more).
The easterlies along the equator push the waters of the warm
upper layer to the west and poleward, pulling the thermocline to
the surface in the east. As a result, the water upwelled there is a
colder than it would be if the upper layer waters were more evenly
distributed with longitude.
So WHICH one is it? Which one causes a colder Pacific Ocean?
Bjerknes was unable to determine which of these three factors is
most important. Even today there is considerable uncertainty as
to their relative roles, although it seems to be the case that none
are negligible.
Thus Bjerknes determined that the oceanic and atmospheric
circulations were mutually supportive in a "chain reaction"
where "an intensifying Walker Circulation also provides for an
increase of the east-west temperature contrast that is the cause of
the Walker Circulation in the first place.” This kind of event is
called a "La Niña"—the opposite of El Niño.
La Niña
But Bjerknes also noted that the interaction could operate in the
opposite sense: a decrease of the equatorial easterlies diminishes
the supply of the cold waters to the eastern equatorial Pacific (by
any of the 3 mechanisms); the lessened east-west temperature
contrast causes the Walker Circulation to slow down—in essence,
this is what causes El Niño.
So Bjerknes discovered that ENSO is fundamentally a huge positive
feedback phenomenon—the ocean/atmosphere components amplify
each other. But there are two distinct phases: El Niño (or Warm
Events) and La Niña (or Cold Events). What shifts ENSO from one
phase to the other?
Klaus Wyrtki adds the next piece to the puzzle.
He knew that El Niño occurred primarily due to
dynamic (air flow) rather than thermodynamic
(heating) changes. So rather than examine
SSTs, he looked at sea level changes. Through
collection of sea level data across the Pacific
Basin, Wyrtki found although SST changes
were primarily limited to the eastern basin, sea
level changes were basinwide.
And, importantly, he discovered that the
initial changes of the wind causing sea level
sloshing were in the central and western
Pacific—opposite from the locale of the SST
changes. Wyrtki suggested the cause of the
eastward propagation of the ENSO signal
from the west to the east was through the
medium of equatorial Kelvin waves.
What is a Kelvin wave?
The concept of an oceanic Kelvin wave is similar that of an
atmospheric wave (like the jet stream makes). However, Kelvin
waves is limited to the equatorial regions and operates in response
to energy/mass transfers by atmospheric waves. An atmospheric
wave called a Rossby wave involves energy transfer resulting
from a balance between Coriolis effect and pressure gradient force.
However, Rossby waves cannot operate within close proximity of
the equator.
Near-equatorial Rossby waves (or longwaves as they are
sometimes called) propagate energy westward (toward Australia).
Kelvin waves move eastward. Rossby waves move much slower
(by a factor of 3) than Kelvin waves. The Rossby waves transfer
mass & energy westward until they hit a boundary—at which time
the water carried by the Rossby waves is returned eastward in the
form of Kelvin waves.
Rossby Waves
Kelvin Waves
Rossby Waves
Early numerical modeling of ENSO indicated that the recharging of
the equatorial reservoir of warm water is a necessary precondition of
a warm event (El Niño). Why? The aftermath of a warm event
leaves the thermocline along the equator shallower than normal (cold
SSTs and low equatorial heat content). Over the next few years, the
equatorial warm water reservoir is gradually refilled. Only when
there is enough warm water in the equatorial band, can the Kelvin
waves move enough warm water to the eastern end of the equator to
initiate the next event.
This explains why generally El Niño is a periodic event of normally
about 7 years—a recharge time is needed.
Recent Past El Niño events:
1976-77, 1982-1983, 1987-1988
1990-1994 (the longest extended
period on record), 1997-8, 2002-3(?)
So a conceptual model of the El Niño phenomenon can be
created:
•
Wind changes are concentrated in the central equatorial
ocean while SST changes are concentrated in the East. This
addresses the fact that the strength of the winds will be a function
of the east-west temperature gradient (and the eastern
temperatures).
•
SSTs in the east are primarily controlled by thermocline
depth variations which in turn are driven by changes in the surface
wind stress. If the eastern SSTs are warm (thermocline high) then
the wind anomaly will be westerly forcing a Kelvin wave packet in
the ocean to further depress the thermocline in the east, amplifying
this situation.
•
The warm water in the east must be compensated by cold
water in the west. Energy from the cold water is transferred in the
form of equatorial Rossby wave packets moving westward. When
these Rossby waves reach the western boundary they are reflected
as faster-moving "cold" Kelvin waves, which propagate eastward
reducing the SSTs there.
•
Thus the original warm signal is eventually accompanied by
a cold signal—but with a delay. This delay accounts for the
turnabout from warm to cold states.
http://www.c
dc.noaa.gov/
map/clim/sst
_olr/el_nino_
anim.shtml
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