Marine Science 320
El Niño Southern Oscillation
Equatorial upwelling chlorophyll from
SeaWiFS during June-August 1998
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El Niño-Southern Oscillation (ENSO)
• NOAA PMEL El Niño Theme Page
http://www.pmel.noaa.gov/tao/elnino/nino-home.html
• Southern Oscillation (atmosphere) and
El Niño (ocean) processes were
identified separately, and subsequently
linked
• Walker: noted ‘see-saw’ connection in
barometer data from Tahiti and Darwin
and 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
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http://library.thinkquest.org/20901/overview_2.htm
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When SOI is negative:
p.g. is weaker than usual
corresponds to El Nino
(or ENSO warm event)
[ Pdiff - Pdiffav ]
SOI = ----------------SD(Pdiff)
Definition of the
Southern Oscillation Index
Pdiff = (Tahiti MSLP) - (Darwin MSLP) monthly averaged
Pdiffav = long term (years) average of Pdiff for that month
SD(Pdiff) = long term standard deviation of Pdiff
Oct
2015
Sometimes SOI is given multiplied by 10 in which case it ranges
from about –35 to about +35 (i.e. +/- 3.5 std. dev.)
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http://iridl.ldeo.columbia.edu/maproom/.ENSO/.Time_Series/SOI.html
Equatorial upwelling
chlorophyll from Seawifs
during June-August 1998
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Chlorophyll from Seawifs
during Sep-Nov 1997
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http://www.pmel.noaa.gov/tao/elnino/nino-home.html
http://www.pmel.noaa.gov/tao/jsdisplay/
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Strong Trade winds
Cold upwelled water
Trades weaken in Feb/Mar
Upwelling slows and water warms
easterlies
westerlies 9
10
December 1997
Positive anomaly means
winds are more westerly
(toward the east) than
average
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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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Normal conditions
El Nino conditions
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El Nino - warming first occurs subsurface in the central Pacific
because the thermocline is being displaced downward.
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http://meteora.ucsd.edu/~pierce/elnino/en97/en97.html
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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ENSO conditions
Top right: Two-month mean rainfall rate (mm/mon) for Jan/Feb
1998. 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.
Negative means
much less rain in
1999 than 1998
Positive is more
rain in 1999
than 1998
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ADCP
Deep water
Assessment and
Reporting of Tsunamis
(DART) mooring.
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Consider what would happen next if any of the
following occurred:
1. The Trade winds weakened
2. Central Pacific SST warmed
3. Rainfall increased in the central Pacific
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Consider what would happen next if any of the
following occurred:
1. The Trade winds weakened …
* equatorial upwelling would slow down
* SST would rise in the central Pacific
* atmospheric convection would occur more quickly
* the region of precipitation would move to the east
closing the Walker cell sooner … weakening the
Trade winds in the west
A positive feedback that amplifies initial the
Trade wind weakening
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Consider what would happen next if any of the
following occurred:
2. Central Pacific SST warmed …
* atmospheric convection would occur more toward
the east
* the region of precipitation would move to the east
closing the Walker cell sooner
* weakening the Trade winds
* equatorial upwelling would slow down
* SST would rise in the central Pacific
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Consider what would happen next if any of the
following occurred:
3. Rainfall increased in the central Pacific …
* the Walker cell would close more toward the east
* weakening the Trade winds
* equatorial upwelling would slow down
* SST would rise in the central Pacific
* atmospheric convection and rainfall would occur
more toward the central than western Pacific
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There is some 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:
1. converging Ekman transports (off equator) that increase sea level
2. and depress the thermocline
3. eastward geostrophic flow converges to the east
4. and diverges to the west
5. 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
c

o
H  g H
(roughly ~200 km/day)
The observed speed is about 10 – 20% faster than this due to
advection by the EUC
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Present conditions:
http://www.pmel.noaa.gov/tao/jsdisplay
http://iridl.ldeo.columbia.edu/maproom/.ENSO/.Time_Series/SOI.html
SOI
-2 -1 0 1
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http://www.pmel.noaa.gov/tao/elnino/nino-home.html
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NOAA ENSO Advisory: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.html
EL NIÑO/SOUTHERN OSCILLATION `
DIAGNOSTIC DISCUSSION
issued by
CLIMATE PREDICTION CENTER/NCEP
8 October 2015
ENSO Alert System Status:
El Niño Advisory
Synopsis:
95% chance El Niño
will continue through NH
winter, gradually weakening
through spring 2016.
During September, sea
surface temperature (SST)
anomalies were well above
average across the central
and eastern Pacific Ocean
(Fig. 1)
Fig 1. Average sea surface temperature (SST) anomalies (oC) for the
week centered on 30 September 2015. Anomalies are computed with
respect to the 1981-2010 base period weekly means.
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NOAA ENSO Advisory: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.html
EL NIÑO/SOUTHERN OSCILLATION `
DIAGNOSTIC DISCUSSION
issued by
CLIMATE PREDICTION CENTER/NCEP
8 October 2015
ENSO Alert System Status:
El Niño Advisory
Synopsis:
95% chance El Niño
will continue through NH
winter, gradually weakening
through spring 2016.
The Niño indices generally increased,
although the far western Niño-4 index
was nearly unchanged (Fig. 2).
Next:
upper ocean heat content …
Fig 2. Time series of area-averaged sea surface temperature
(SST) anomalies (oC) in the Niño regions. Anomalies are
departures from the 1981-2010 base period pentad means.
EL NIÑO/SOUTHERN OSCILLATION `
DIAGNOSTIC DISCUSSION
issued by
CLIMATE PREDICTION CENTER/NCEP
8 October 2015
ENSO Alert System Status:
El Niño Advisory
Synopsis:
95% chance El Niño
will continue through NH
winter, gradually weakening
through spring 2016.
There are positive
subsurface temperature
anomalies in the central
and eastern Pacific (Fig. 3)
with largest departures
above 6oC (Fig. 4). There
were significant low-level
westerly wind anomalies
and upper-level easterly
wind anomalies persisting
from the western to the
east-central tropical
Pacific.
Fig 3. Area-averaged upper-ocean heat content anomaly (oC) in the
equatorial Pacific (5oN-5oS, 180o-100oW). The heat content anomaly is
computed as the departure from the 1981-2010 base period pentad means.
Fig 4. Depth-longitude section of equatorial Pacific upper-ocean (0-300m) temperature
anomalies (oC) centered on the pentad of 30 September 2015. The anomalies are averaged
between 5oN-5oS. Anomalies are departures from the 1981-2010 base period pentad means.
EL NIÑO/SOUTHERN OSCILLATION `
DIAGNOSTIC DISCUSSION
issued by
CLIMATE PREDICTION CENTER/NCEP
8 October 2015
ENSO Alert System Status:
El Niño Advisory
The traditional and equatorial Southern
Oscillation Index (SOI) values became more
negative (stronger), consistent with
enhanced convection over the central and
eastern equatorial Pacific and suppressed
convection over Indonesia
Synopsis:
95% chance El Niño
will continue through NH
winter, gradually weakening
through spring 2016.
Fig 5. Average outgoing longwave radiation (OLR) anomalies (W/m2) for the four-week
period 5-30 September 2015. OLR anomalies are computed as departures from the 19791995 base period pentad means
All models surveyed
predict El Niño to
continue into the
Northern Hemisphere
spring 2016, and all
multi-model averages
predict a peak in late
fall/early winter (Fig. 6).
Forecasts unanimously
favor a strong El Niño,
with peak 3-month SST
departures in the Niño
3.4 region near or
exceeding +2.0oC
Outlooks favor belowaverage temperatures
and above-median
precipitation across the
southern United States,
and above-average
temperatures and
below-median
precipitation over the
northern tier of the
United States.
Fig 6. Forecasts of sea surface temperature (SST) anomalies for the Nino 3.4
region (5oN-5oS, 120Wo-170oW). Figure updated 15 September 2015
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(a)
(b)
SOI
chlorophyll
(c) (d)
Equatorial thermocline
(e) temperature anomaly
(g)
(f)
(h)