El Nino is a weather event involving the eastward migration of a mass of warm water normally found in the western equatorial Pacific Ocean. Periodically (usually 3-7 years) the easterly trade winds in the Pacific weaken and allow the pool of warm water to drift from Australia to the western coast of South America, often triggering heavy rains there. This vast pool of warm water is thought to set off a chain reaction that can affect jet stream and weather patterns around the world, especially in the winter months in the northern hemisphere. El Nino is sometimes referred to as ENSO for El Nino Southern Oscillation. The Southern Oscillation is a sea saw of air pressures on the eastern and western halves of the Pacific.
http://www.almanac.com/elnino/
To understand the changes that accompany El Nino,
you have to think about the normal situation first. The sun heats the equatorial
regions more strongly than the rest of the globe, so a lot of heated air
tends to rise from the surface there, to be replaced by air from the subtropics.
These winds moving towards the equator are affected by the Coriolis Force,
turning them to the right in the Northern Hemisphere and the left in the
Southern Hemisphere. This turning of the winds results in a convergence
along the equator forming the tradewind belt which runs from East to West
over the width of the tropical Pacific along the equator. These powerful
winds tend to push the surface water of the ocean towards the west, so
the sea level in Indonesia in usually about 50 centimeters higher than
that of the South American coast. As this water is pushed westward
away from South America, cooler water from deeper, more nutrient richer
layers in the ocean are drawn up creating a surface temperature that is
about ten degrees Fahrenheit cooler than in the west. The thermocline,
the drastic change in temperature of sea water with depth, is affected
by the tradewinds. The thermocline is slanted with the high end where the
upwelling of cooler water along the South American coast, and the lower
end near Indonesia. I used the animation of SST with the thermocline depth
to illustrate the normal and changing conditions during El Nino.
But what is El Nino, and why is it called
El Nino? El Nino is the weakening of these westerly blowing tradewinds.
Every two to seven years, these strong trade winds subside, and warm water
slowly moves back eastward across the Pacific, like water shifting in a
giant bathtub. I illustrated this with the animation of Sea Surface Temperate
(SST) with Height. The warm water and shifting winds interrupt the upwelling
of cool, nutrient-rich water. With the slowing of the upwelling, the east-to-west
contrast in temperature is reduced, and so the tradewinds weaken even further
leading to a positive feedback loop and a complete collapse of the normal
cycle with an essentially flat thermocline, and even SST condition across
the entire Pacific.
The name El Nino (referring to the Christ
child) was originally given by Peruvian fishermen to a warm current that
appeared every year around Christmas. What we now call El Nino seemed to
them like a stronger version of the same event, and the usage of the term
evolved over time until it only referred to the irregular strong events.
It wasn't until the 1960s that people started realizing this was not just
a local Peruvian occurrence, but was associated with changes over the entire
tropical Pacific and beyond. In effect, El Nino was too big to be seen
as the huge event that it is. It just seemed like a lot of unconnected
unusual weather events around the world at coinciding times.
The name El Nino as scientists and others
now use it refers to the warming in the water and atmosphere of the Pacific
region. The complete phenomenon is known as the El Nino/Southern Oscillation,
abbreviated ENSO. The warm El Nino phase typically lasts for approximately
eight to 10 months, while the entire ENSO cycle usually lasts about
three to seven years. The cycle often includes a cold phase (known as La
Nina) that may be similarly strong, as well as some years that are neither
abnormally warm nor cold. However, the cycle is not a regular oscillation
like the change of seasons because it can be highly variable in strength
and timing.
The Southern Oscillation was named by Sir
Gilbert Walker in 1923, who noted that "when pressure is high in
the Pacific Ocean it tends to be low in the Indian Ocean from Africa to
Australia." This was the first recognition that changes across the tropical
Pacific and beyond were not isolated phenomena but were connected as part
of a larger oscillation. The comparison of pressure differences is also
now used for the SOI. The first modern scientific description of the mechanics
of El Nino/Southern Oscillation was made by Professor Jacob Bjerknes of
the University of California, Los Angeles in 1969.
The SOI is the most widely used indicator
of the ENSO phenomenon to determine the strength of an event, or for the
use of comparing one event to the other. The SOI is based on the atmospheric
surface pressure difference between Tahiti and Darwin, Australia, on opposite
sides of the Pacific. It was noted as far back as the 1920's that these
two stations were anticorrelated, so that when Tahiti pressure is high,
Darwin pressure is usually low. Sir Gilbert Walker was the first to notice
the correlation between pressures, sometime in the 1920's. This pressure
difference reflects the very large scale of the phenomena, since one would
not usually expect such a close relation between such faraway places. When
Tahiti pressure is high, it indicates that winds are blowing more strogly
than normal towards the west, and when the pressure is low, the winds are
blowing less strongly (ENSO). A major advantage of the SOI is that time
series at these two locations extend back to the 1880's, so we can see
the distribution of El Nino events back much farther than we can see in
records of ocean temperatures. The SOI is given in normalized units of
standard deviation, which can be used as an intensity scale. There are
problems with the SOI though. For example, the SOI values for the 82-83
El Nino were about 3.3 standard deviations. By this standard, the 97-98
El Nino was not as strong as the 82-83 event, but we know now after experiencing
it, that the effects were just as strong. The SST anomaly of the 97-98
event is larger than that in the 82-83 event, and some experts say that
is a more important measure. This shows that there is no single number
that summarizes the intensity of events.
The following chart is an example of a
SOI with the 7 strongest El Nino's plotted. Its easy to see the difference
between the 82-82 event and the most recent 97-98 event at the top of the
scale.
After studying this graph and some of the SST animations I raised a question to myself. The 97-98 El Nino "double peaked", and now has dropped below the level of the 82-83 event. What I want to know, is will the La Nina event also "double peak"? We already saw a flush a cold water follow El Nino and then recede. Will another flush of cold water come, just as a second flush of warm water came in El Nino? Time will only tell.
This animation is showing how the thermocline's
height in relation to the surface changes as the 97-98 El Nino occurs.
As the warm pool of water shifts from near Indonesia towards Central America,
the thermocline is pushed deeper, leaded to less upwelling of the cooler,
more nutrient rich water.
http://www.pbs.org/wgbh/nova/elnino/anatomy/images/thermocline.gif
This animation shows how the Sea Surface Height
(SSH) varies as the trade winds weaken during the 97-98 El Nino. The normal
pattern is for the SSH to be higher in the Western Pacific from the Westerly
provailing tradewinds constantly "pushing" the water there. When these
winds weaken during El Nino, the water sloshes back towards S. America.
http://www.cdc.noaa.gov/~jjb/anim.html
Effects of El Nino on Significant World Regions
Africa- There is a relationship between the warm-ocean phase of El Nino and drought in South Africa. Warm episodes in May and April lead to drought. There are tendencies for wetter weather in East Africa.
Australia- There are drought conditions in eastern Australia due to El Nino. The heavier the El Nino (SOI) the farther south Australia is affected by drought.
Canada- There are drier than normal conditions form British Columbia to the Great Lakes. Along with reduced amounts of rain and snowfall. This comes along with the higher temperatures that the region experiences.
US (Midwest)- This region
of the United States experiences warm winters during El Nino. This leads
to reductions of snowfall from 10-20 inches in:
-northern Illinios and Indiana
-western Michigan and the upper peninsula of Michigan
-west central Minnesota
-southeast North Dakota
-northeast South Dakota
-eastern edges of Lakes Erie and Ontario
US (West)- On the winter in the US southwest, the experienced weather patterns of El Nino are that the area experiences mildly wet days more frequently and very wet days are even more frequent in typical El Nino years. High streamflow values are much more frequent in typical El Nino years. Streamflow patterns are accentuated versions of precipitation patterns.
Winter in Pacific Northwest and the Rockies- In typical El Nino years the Pacific Northwest region experiences mildly wet days and very wet days less frequently than in normal years. The high streamflow values are much less frequent.
This information can be found at
http://www.wrcc.sage.dri.edu/enso/enso.html
This web site shows the US temps during el nino
http://nic.fb4.noaa.gov/products/analysis_monitoring/ensostuff/verif/jfmtvrfweb.html
The next two are on WA and CA and the precip they recieved during El Nino.
http://nic.fb4.noaa.gov/products/analysis_monitoring/ensostuff/dist/jfmpstat/waO.gif
http://nic.fb4.noaa.gov/products/analysis_monitoring/ensostuff/dist/jfmpstat/caO.gif
Washington recieved about 10% less rain than the yearly average during
El Nino. This is because El Nino redrew the jet stream for Washington.
The bottom arm of the jet stream, the subtropical storm track, directed
a good amount of storms to the Califrornia coast. This explains how
California received all of its rain. They not only recieved a lot
of our rain but because El Nino raised the temperature of the water of
California's coast it allowed incoming storms to pick up more rain before
they hit.
