Noiseinduced instability of ENSO F.F. Jin, Axel Timmermann

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Noise-induced instability of ENSOF.-F. Jin,
Axel Timmermann J.-S. Kug
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What controls the amplitude of ENSO?
Nino 3 SSTA
Noise level dT/dtf (T,u..)S(T)?
Coupling strength
ENSO variance maybe skewness
Strength of annual cycle dT/dtf (T,u..)Asin?t
Nonlinearities dT/dtf (u'T', S(T)?)
Background state dT/dtf (T,u,h,v,w)
External factors
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Noise-induced intensification of ENSO under
greenhouse warming conditions
Noise level dT/dtf (T,u..)S(T)?
ENSO variance maybe skewness
Background state dT/dtf (T,u,h,v,w)
External factors Greenhouse warming
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Slingos strawman, revisited
????
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Some early work on ISO-ENSO interactions
1994
Time evolution of 30-60bp filtered 200hPa Wind
variance in western equatorial Pacific
El Nino
El Nino
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Some early work on ISO-ENSO interactions
Li and Zhou 1993, 1994
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ISO-ENSO interactions, continued
From BMRC, correlation between ISO and ENSO not
smashing.
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Noise-induced intensification of ENSO
Eisenman et al. 2005
Correlation/Regression between Nino3 SSTA and
20-60 day band-pass filtered wind variance
WWB modulation by temperature for present-day
climate
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ENSO recharge model with state-dependent noise
Coupling strength and noise may change
slowly over time
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ENSO recharge model with state-dependent noise
Ensemble mean equation for ENSO
State-dependent noise is coupling State-dependen
t noise is also nonlinearity
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ENSO recharge model with state-dependent noise
Ensemble variance equation for ENSO
Deriving third order equations and 4th Order
closure We can obtain a closed set of
equations for the ensemble variance
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ENSO recharge model with state-dependent noise
period
Teast
Temperature variance
period/2
Variance growth rate
Time months
Instability of the second order moments
is associated with error dynamics
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A Fokker Planck perspective on ENSO with
multiplicative noise
Numerical solution of Fokker-Planck Equation for
B0, B1 Multiplicative noise generates Skewness,
nonlinearity
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Noise-induced intensification of ENSO
AR4 models simulate increased Intraseasonal
variability
WWB-ENSO interaction increased during the last
50 years
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Improving ENSO-MJO interactions in CGCMs
Lag Correlation between Noise Variance and ENSO
indices
NCEP/NCAR Coupled control simulation
SNU Coupled control simulation SNU with
Cumulus momentum transfer (CMT) CMT mixes down
westerly momentum Intensifies noise variance
probably by Rossby wave instability
Courtesy of Kug, 2007
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Summary

• Control of ENSO amplitude is a complicated story
  not only linear instability
• State-dependent noise IS coupling and generates
  nonlinearity
• Next generation of CGCMs with better
  representation of ISO/MJO/WWB might hold
  surprises with respect to ENSO amplitude changes
• We need more realistic representations of WWBs in
  CGCMs