El Nino Southern Oscillation (ENSO)

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El Nino Southern Oscillation (ENSO) 20 April
06 Byoung-Cheol Kim
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Why the ENSO Occurs?
http//www.atmos.albany.edu/
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• Outline
• Delayed Oscillator associated with ENSO
• - Kelvin wave
• - Rossby wave
• - Bjerknes hypothesis
• Summary

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Advanced ENSO Theory The Delayed Oscillator -
-Wakata and Sarachik 1991, Philander et al 1984,
and Zebiak and Cane 1987
lt Idealized Pacific Ocean basin domain gt
Assumption
1. No horizontal density and no temperature
variations 2. Only vertical temperature
variations (idealized thermocline)
Land area
Ocean
3. No horizontal pressure variations 4. Only
vertical pressure variations (balance
between changes pressure gradient force and the
gravity, dependent only on density)
Evolution of disturbances key processes
associated with delayed oscillator theory
Source IRI, http//iri.columbia.edu/index.html
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Perturbing the Ocean
lt Zonal wind-stress anomaly gt

• Motionless state
• perturbed by turning on an isolated patch of
• eastward wind stress near the equator
• (kept constant for 30 days)

Lat
Lon
ltZonal wind-stress anomaly (175W)gt
ltwind-stress curl anomaly (175W)gt
wind-stress - Max equator wind-stress curl -
Negative SH - Positive NH
Lat
Lat

• Response of a wind-stress forcing influenced
  by the Coriolis Force
• Near-surface ocean balance between wind stress
  forcing and the Coriolis
• Resulting in net transport to the right of
  the direction of wind stress (NH)

Source IRI, http//iri.columbia.edu/index.html
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Kelvin and Rossby Waves
Eastward wind stress forcing Acting to increase
depth of warm water layer (equator) mass surplus
Dispersing eastward Kelvin wave
(downwelling) downwelling deepen thermocline
mass-deficit area Propagate westward Rossby
wave (upwelling) upwelling shallow thermocline
Rossby wave
Kelvin wave
Lat
Lon
ltOcean surface height anomaly (cm)gt
180
140W
Kelvin wave
Rossby wave
Kelvin wave positive - Maximum at equator
Rossby wave negative - Symmetric and minimum
at equator
cm
cm
Lat
Lat
ltOcean surface height anomaly (cm)gt
Source IRI, http//iri.columbia.edu/index.html
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Evolution of Kelvin and Rossby Waves
Wave signals propagating at different
speeds Kelvin wave (70days) traveling eastward
(idealized case 2.9 m/s) extending from
120East to 80West Rossby wave
(210days) propagating westward traveling westward
at 1/3 speed of the Kelvin wave
Rossby wave
Kelvin wave
125days
25 days
50 days
175days
Lat
75 days
225days
100 days
275days
Lon
ltTime evolution for the idealized experimentgt
Source IRI, http//iri.columbia.edu/index.html
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Equatorial Waves

• Equatorial waves in the ocean play an important
  role in the onset and variability of ENSO
• Two types
• Kelvin waves (propagate eastward along the
  equator and along coasts)
• Rossby waves (long waves propagate westward)
• The relevant waves are baroclinic internal to
  the ocean, propagating along the density contrast
  of the thermocline
• Equatorial Kelvin waves travel 3 times faster
  than the equatorial Rossby waves

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Bjerknes hypothesis associated with ENSO
Western anomaly (Weak trade wind)
North
Warming (East)
Warm
Less entrainment cooling (eastern Basin)
Weaker trade
Equator
180o
Down
Weakend upwelling
Weakened upwelling
Deepening thermocline
feedback

• Bjerknes
• first to recognize the coupling between
  changes in the oceanic and
• atmospheric circulations during ENSO

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Summary

• Delayed Oscillator associated with ENSO
• Equatorial ocean waves show a mechanism to
  reverse
• phase of perturbations to the thermocline depth
• Without further wind forcing waves finally decay
• Thermocline depth perturbations influence SST in
  the
• upwelling regions of central / eastern
  equatorial Pacific
• gt coupling to atmosphere
• Equatorial waves and Bjerknes feedback
  (Positive)
• can generate a self-sustaining oscillation

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Questions?
Thank you