Introduction to El Ni�o

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Introduction to El Ni�o Powered By Docstoc
					          El Niño




Based on a talk prepared by Ian Eisenman
Development of an El Nino event: a comparison
        of two major El Nino events:
Observations: El Nino is a coupled ocean-atmosphere
                    phenomenon
“ENSO”: El Nino
(warming of ocean)
and the Southern
Oscillation in
atmospheric pressure
are well correlated
Observations: The irregular variations between El Nino
           and La Nina limit predictability
                                                             Nino 3 SST
                         30

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• Period mostly in       28

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the 3-6 year range
                     C
                     o
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• Decadal variability
                         25

                         24

of El Nino               23
                          1900   1910   1920   1930   1940    1950   1960   1970   1980   1990   2000

Characteristics,
possibly due to          30
                                                             Nino 3 SST


interaction with         29


mid-latitudes            28

                         27
                     C
                     o




                         26

                         25

                         24

                         23
                          1900   1910   1920   1930   1940    1950   1960   1970   1980   1990   2000
Observations: while irregular, all El Nino episodes still look similar,
           they tend to peak at end of calendar year



Sea surface temperature
averaged over the
eastern equatorial pacific
during several El Nino
events, as function of
month (over two years):
              Why do we care: Global impacts
ENSO is currently the largest inter annual signal in global climate




          El Nino                           La Nina
Why do we care: ENSO’s Global climate impacts

                                      Floods
                             Lakeport, California (1998)




              Fires
          Australia (1998)
             Observations: system has two “modes”:
             El Nino (the child) & La Nina (the girl…)


El Nino:
     A     El Niño conditions




La Nina:
     B      La Niña conditions




The major players:
Easterly Trade Winds  Thermocline;
Thermocline  sea surface temperature.
Eastward propagating Kelvin waves, westward Rossby Waves
       Kelvin and Rossby waves, and the delayed
        oscillator mechanism (Schopf & Suarez, Battisti 1988)




1. An initial weakening of trades
2. Warm Kelvin wave, positive
   feedback, El Nino peaks
3. Cold Rossby waves making their
   way to the west pacific
4. Reflecting from western boundary,
   ending the event, start a cooling
5. All repeats with opposite signs…
Enough powerpoint for now
                   Observations: Summary
• Eastern equatorial Pacific ocean episodically warms every 3-6 years
(El Nino) in phase with a seesaw-like variation of atmospheric
pressure (Southern Oscillation). This coupled    4
                                                                                     nino3



atmosphere-ocean                                 2




                                            oC
phenomenon is                                    0



called ENSO.                                     -2
                                                 1950   1955   1960   1965   1970   1975     1980   1985   1990   1995   2000

                                                                                      SOI
                                                 4

                                                 2

                                                 0

                                                 -2

                                                 -4

                                                 -6
                                                 1950   1955   1960   1965   1970   1975 1980       1985   1990   1995   2000
                                                                                      year




                     • El Nino events are similar, but their
                     period, amplitude & characteristics are still
                     irregular and therefore difficult to predict.
                     • Dramatic impacts on global climate
                        Outline


 Fundamental questions about El Nino phenomenon


•What are the observational features of this tropical
Pacific variability?
•How are the observations made?
•What is the physical mechanism at play?
•Why is it irregular?
Making Observations: monitoring systems
Satellites                                   Moored buoys
SST (radiometer),                            temperature profile,
wind speed                                   wind speed, currents
(scatterometer),
SSH (altimeter)




                    Drifting buoys
                    SST & surface currents
                    “Lagrangian drifters”
Making Observations: TAO array




                                     NOAA
                                 KA’IMIMOANA
A large observational buoy
array maintained by a
dedicated research vessel,
international consortium
Equatorial Sea Surface
Temperature (SST) as
function of longitude &
time [from TAU data]
Same, but deviation of
SST from mean
                        Outline


 Fundamental questions about El Nino phenomenon


•What are the observational features of this tropical
Pacific variability?
•How are the observations made?
•What is the physical mechanism at play?
•Why is it irregular?
Outline for understanding El Nino’s
mechanism

•Variation of Coriolis force & the “beta plane”
•Rossby waves: the westward propagation of
pressure anomalies
•Kelvin waves: coastal, and then a qualitative
description of equatorial
•Wind forcing of thermocline anomalies
•SST forcing of wind
•The “delayed oscillator” mechanism of El Nino
•Irregularity and predictability
 El Nino Mechanism: thermocline & sea surface temperature
     A   El Niño conditions




     B    La Niña conditions




• Deepening of thermocline at east Pacific means cold water
  is far from surface and causes the warming during El Nino
• The East Pacific Thermocline becomes deep and shallow
  due to the propagation of Rossby and Kelvin waves
   Thermocline and Sea Surface Temperature during
                El nino and La Nina




Jan 1997 (La Nina)   Nov 1997 (El Nino)   Mar 1998 (El Nino ends)
El Nino mechanism: Sea surface temperature & trade winds
  Vertical air motion above equator during normal conditions:




West Pacific:                                    East Pacific: cool,
                      Easterly surface
warm, air rises,                                 air sinks, high
                      wind from high to
low pressure                                     pressure
                      low pressure
Physical mechanism: How does El Nino start? (Bjerknes 1960 mechanism)

  Stronger Easterly trade winds moves warm water westward
   thermocline gets shallower in east, deeper in west Pacific
   East-west Sea Surface Temperature gradient increases
   atmospheric pressure gradient increases
   enhanced trade winds
   … positive feedback between winds and SST; an explosive
  surface ocean warming

  East Pacific SST               Easterly Trade winds weaken
       warms


                                         Warm Water above
Thermocline deepens
                                     thermocline shifts eastward
   in East Pacific
Mechanism of El Nino: How does it End?
Ocean Wave preliminaries:

• Internal waves: wavy
  motions of thermocline
• Amplitude: 10s of meters
• Wavelength: 100s-1000s of km
• Period: hours to weeks
• Two classes of waves:
  – Kelvin waves: at equator, travel eastward only, cross
    Pacific Ocean in 2 months
  – Rossby waves: a few degrees off equatorial, travel
    westward only, cross Pacific Ocean in 8 months
                     How does El Nino End?
      A growing El Nino contains the seeds of its own destruction.
1. Hit ocean with a westerly
wind stress pulse




2. Warm Kelvin waves
propagate east along equator
 Event starts!
3. Cold Rossby waves
propagate west at higher lat
and reflect back eastward as
Kelvin waves  Event ends
                                             http://iri.columbia.edu/climate/ENSO/theory/
                  models and Prediction
• Prediction: 6-12 months in advance
• Two main Prediction methods:
   • statistical methods: comparing
   the current ocean-atmosphere state
   to previous similar ones & their
   evolution
   • dynamical models: run
   mathematical models from known
   present state into the future, like
   weather prediction
• Both approaches are doing equally
badly right now: the difficulty of
predicting the present
                        Outline


 Fundamental questions about El Nino phenomenon


•What are the observational features of this tropical
Pacific variability?
•How are the observations made?
•What is the physical mechanism at play?
•Why is it irregular?
            Reasons for irregularity: Chaos or stochastic forcing
             Why can’t we predict El Nino well in advance??

                            Two competing paradigms
   Chaos: the butterfly effect               Noise (Stochastic forcing):
•Irregularity from deterministic chaos in   •Irregularity from
interannually varying components of         weather forcing on
coupled system                              interannually varying
                  •ENSO oscillator          components
                  irregularly jumps         •Nonlinearity in
                  between nonlinear         ENSO models is not
                  resonances with the       crucial. Regular oscillations are
                  seasonal cycle.           randomized by stochastic forcing
•reproduces major ENSO features             •Models reproduce major ENSO features
•Deterministic system but sensitive to      •Insensitive to initial conditions, but
initial conditions – 9 month predictions    stochastic forcing effects add up –
                                            9 month predictions
    What’s happening right
            now?




http://www.esrl.noaa.gov/psd/map/clim/sst_olr/sst_anim.shtml
                              Summary
• Observational features of El Nino:
Eastern tropical Pacific warms and trade winds slacken for nearly a
year every 3-6 years; affects weather worldwide: fires in Australia,
floods in south America, droughts in Africa, weather in the US
• Making observations:
Large array of stationary buoys, satellites, drifting buoys, ships
• The physical mechanism at play:
El Nino starts due to feedback between trade winds and Sea Surface
Temperature.
El Nino ends due to delayed ocean adjustment through Coriolis-related
waves on thermocline.
• Why is it irregular and difficult to predict?
Debate about the importance of the roles played by weather and chaos

				
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posted:9/29/2012
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