Onset and termination mechanisms of El Nio events in a Coupled General Circulation Model

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Onset and termination mechanisms of El Niño
events in a Coupled General Circulation Model
Internal coupled processes vs Stochastic forcing

• Matthieu Lengaigne, IRD/LOCEAN, France

Collaboration J.P. Boulanger, P. Delecluse, C.
Menkes, J. Slingo, G. Vecchi....
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The 1997/98 El Niño event
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Outline

• Influence of intraseasonal wind variability in El
  Niño onsets
• Influence of the seasonal cycle in El Niño
  terminations

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Influence of WWEs on the evolution of the
ocean-atmosphere coupled system

• Tool Hadam3-OPA CGCM (HadOPA)

• Lengaigne et al., Triggering of El Niño by WWEs
  in a CGCM. Clim. Dyn., 2005.

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Influence of WWEs on the evolution of the
ocean-atmosphere coupled system
Evolution of sea surface temperature in the
equatorial Pacific
Ensemble mean
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The ensemble dispersion

• CRE ensemble
• 1 strong warming associated with a WWEs sequence
  in early 62
• 1 moderate warming
• CWE ensemble
• 4 strong warmings
• 4 moderate warmings

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The intense El Niño events
The strong warming cases
Strong warming (simulated)
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Sensitivity experiments
Influence of the WWEs generated by the model
after the imposed WWE on the tropical Pacific
evolution
CRE Reference ensemble CWE inserted WWE in
mid-February CWN WWE in mid-February, no WWEs
after May
WWE
CRE - Weaker warming compared to CWE - Less
strong El Niño triggering
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Sensitivity experiments
WWE influence on tropical Pacific Warm pool
displacement vs eastern Pacific warming
CRE Reference ensemble CWE WWE added in
mid-February CEP Eastern Pacific warming in March
CEP imposed SST Anomaly
WWE
CEP - Weaker warming compared to CWE - Less
strong El Niño triggering
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Seasonal dependance of the WWE impact
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Summary (I)
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Outline

• Influence of intraseasonal wind variability in El
  Niño onsets
• Influence of the seasonal cycle in El Niño
  terminations

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El Niño termination the interaction with the
seasonal cycle

• What makes the thermocline shoal at the end of El
  Niño events?

A robust feature of El Niño events is a late-year
southward shift of the near-Dateline zonal wind
anomalies
Harrison (1987)
Harrison and Vecchi (1999)
Vecchi and Harrison (2003)
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Termination of strong El Niño events

• Realisation of a 10-member ensemble experiment
  (STR) initialised in September of an intense El
  Niño event (gt2C)

Lengaigne et al., 2006 Influence of the
seasonal cycle in the termination of El Niño
events, J. Clim.
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Mechanism of the the equatorial westerly wind
decay
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Mechanism of the the equatorial westerly wind
decay Sensitivity experiment

• Realisation of a sensitivity experiment (FIX)
  where insolation is fixed to September conditions

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Unusual termination of extreme El Niño events
Composite evolution of observed El Nino events
(SST Precipitation anomalies)
Moderate events
Extreme events
Year1
Year 0
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HadOPA coupled model results


Moderate events
Extreme events
Year1
Composite evolution of HadOPA El Nino events
(SST Precipitation anomalies)
Year 0
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Mechanisms involved in the prolonged warming
during extreme El Niños (HadOPA results)
Eastern Pacific variability (120W-90W)
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Extreme El Niño events in the IPCC-AR4 database

• 3 of the 23 IPCC models are able to simulate
  extreme El Niño events.

SSTA
PrecA
CNRM Model
MPI Model
GFDL-1 Model
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Conclusions (II)
Courtesy of G. Vecchi
These coupled results are in agreement with
forced oceanic and atmospheric experiments of
Vecchi et al. (2007) and Vecchi (2007)
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Sensitivity to ENSO phase
Insertion of a WWE during a cold phase of the
ENSO cycle
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Sensitivity experiments
Influence of the WWEs generated by the model
after the imposed WWE on the tropical Pacific
evolution
CRE Reference ensemble CWE WWE in mid-February
CWN WWE in mid-February, no WWE after May
WWE
WWE
WWE