We continue our look at the effects of the ocean on our lives by examining the changes in Pacific sea surface temperatures … El
NINO!

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.

ENSO
The abbreviation stands for the two distinct aspects of the phenomenon:
E l N iño: the ocean current
S outhern O scillation: 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?

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.

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 .

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