El NioSouthern Oscillation ENSO

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El Niño-Southern Oscillation (ENSO)

• NOAA PMEL El Niño Theme Pagehttp//www.pmel.noaa.
  gov/tao/elnino/nino-home.html
• Southern Oscillation (atmosphere) andEl Niño
  (ocean) processes were identified separately,
  and subsequentlylinked
• Walker noted see-saw connection in barometer
  data from Tahiti and Darwinand coined the term
  Southern Oscillation
• Jacob Bjerknes connected SST, winds and SLP with
  atmosphere/ocean dynamics

Sir Gilbert Walker

• failure of monsoon rains (and famine) in 1899
• claimed Asian monsoon linked to drought in
  Africa and Australia, and mild winter in Canada
• Widely criticized, theory dismissed (no
  dynamical explanation)
• Never succeeded in predicting monsoon failures

http//library.thinkquest.org/20901/overview_2.htm
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SOI negative p.g. is weaker than
usualcorresponds to El Nino(or ENSO warm event)
Pdiff    (Tahiti MSLP) - (Darwin MSLP) monthly
averagedPdiffav    long term average of Pdiff
for the particular monthSD(Pdiff)  long term
standard deviation of Pdiff The multiplication
by 10 is a convention. Using this convention, the
SOI ranges from about 35 to about 35
Pdiff - Pdiffav SOI 10
x -----------------
SD(Pdiff)
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http//www.pmel.noaa.gov/tao/elnino/nino-home.html
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http//www.pmel.noaa.gov/tao/elnino/nino-home.html
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http//www.pmel.noaa.gov/tao/elnino/nino-home.html
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http//www.pmel.noaa.gov/tao/elnino/nino-home.html
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El Nino - warming first occurs subsurface in the
central Pacific because the thermocline is being
displaced downward.
http//meteora.ucsd.edu/pierce/elnino/en97/en97.h
tml
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Warming due to thermocline displacement occurs
all across. It is shallower in the east because
the thermocline itself is shallower
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Strong subsurface warming
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SST now increases, apparently starting from the
east, but largely due to what is happening
subsurface
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Loss of heat from the west has cooled the WPWP at
depth Warming travels up the N.Amer west coast
and spreads out into the Pacific very slowly
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El Nino contracts but waters are still cool to
the west. The WPWP must reset itself slowly
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Top Two-month mean rainfall rate (mm/month) for
Jan/Feb. Heavy rainfall appears over the South
Pacific Convergence Zone (SPCZ), South Indian
Ocean, and the South America. Bottom
Difference in the Jan/Feb mean rainfall 1999
minus 1998. During ENSO warm event (El Niño) in
1998 central Pacific rainfall is anomalously
high, and west Pacific rainfall is anomalously
low.
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http//www.pmel.noaa.gov/tao/elnino/nino-home.html
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http//sealevel.jpl.nasa.gov/gallery/tiffs/videos
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http//sealevel.jpl.nasa.gov/gallery/tiffs/videos
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• There is mounting evidence that a trigger for
  ENSO warm events might be westerly wind bursts in
  the western equatorial Pacific.
• A wind burst such as this sets in train wave
  motions that are characteristic of the equatorial
  region.
• The westerly wind burst causes
• converging Ekman transports (off equator) that
  increase sea level
• and depress the thermocline
• eastward geostrophic flow converges to the east
• and diverges to the west
• the pattern moves eastward
• Note that the same happens for an easterly wind
  burst the equatorial Kelvin waves only go east
  so the anomalous pattern cannot easily reset
  itself even if the WWB is followed by easterly
  wind anomalies.
• The equatorial Kelvin wave speed

is about 2.5 m/s (roughly 200 km/day) The
observed speed is about 10 20 faster than this
due to advection by the EUC
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http//www.pmel.noaa.gov/tao/elnino/nino-home.html
Present conditions
http//www.pmel.noaa.gov/tao/jsdisplay
http//iridl.ldeo.columbia.edu/maproom/.ENSO/.Time
_Series/SOI.html
NOAA ENSO Advisory http//www.cpc.ncep.noaa.gov
/products/analysis_monitoring/enso_advisory/ensodi
sc.html
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NOAA ENSO Advisory http//www.cpc.ncep.noaa.gov
/products/analysis_monitoring/enso_advisory/ensodi
sc.html
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Next upper ocean heat content
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