Role of Stochastic Forcing in ENSO variability in a coupled GCM

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Role of Stochastic Forcing inENSO variability in
a coupled GCM

• Atul Kapur
• Chidong Zhang
• Javier Zavala-Garay

Acknowledgements Ben Kirtman, Amy Clement
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Introduction

• Stochastic Forcing (SF)
• Atmospheric variability uncoupled to the ocean

• Extent to which the ENSO in CGCMs is driven by SF
• Contributions of Madden Julian Oscillation (MJO)
  and non-MJO
• Dynamical regime of underlying coupled system
  Stable or Unstable

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Procedure
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Model and Data

• Bureau of Meteorology Research Center (BMRC) CGCM
  (Zhong et al. 2004)
• A 163-year run
• Realistic ENSO (Wu et al. 2002) and intraseasonal
  variability (Zhang et al. 2006)

CGCM
NCEP-2 Reanalysis (1979-2007)
Reanalysis

• Variant of Zebiak and Cane (1987) model
• Chaos switched off (Mantua and Battisti 1995)
• Admits daily SF Decorrelation time of tropical
  weather 3-8 days

CZZ model
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Procedure
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Stochastic Forcing

• Statistical model of u10 anomalies predicted by
  SST anomalies
• u10 A sst uResidual
• Wavenumber frequency spectra

(Hilbert EOF)
(CGCM)
Caveats Linear, Contemporaneous, Additive
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Procedure
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Simulations using NCEP-2 SFPower Spectra

• CZZ model able to reproduce spectrum
• ENSO statistics better for MJO than non-MJO
  forcing
• CZZ model performs best in weakly stable regime

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Simulations using NCEP-2 SFSeasonal Variance
Normalized variance

• Warm phase better simulated than cold phase in
  terms of seasonal variance

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Simulations using NCEP-2 SFSeasonal
Autocorrelation
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Procedure
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Simulations using CGCM SFPower Spectrum

• SF is able to reproduce even local peaks in power
  spectrum
• Results using MJO compare better to truth than
  non-MJO

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Simulations using CGCM SFSeasonal Variance
Norm. variance

• SF unable to reproduce the seasonal variance of
  ENSO exhibited by the BMRC CGCM
• Contribution of non-MJO appears to be higher than
  MJO

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Simulations using CGCM SFSeasonal Autocorrelation
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Procedure
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Conclusions

• Role of SF in BMRC CGCM ENSO
• At least the warm phase can be reasonably
  simulated using SF
• MJO contribution is higher than non-MJO
• Underlying dynamical state of coupled system
  appears to be weakly stable
• Seasonality of ENSO cannot be reproduced by SF
• Procedure can be implemented on any CGCM
• Even on runs with long temporal span