Axel Timmermann

1
Past and Future Amplitude Changes of ENSO

• Axel Timmermann
• F.-F. Jin, J.-S. Kug
• S. Lorenz
• Y. Okumura

2
Is El Nino still a big story?

• Unresolved questions
• What determines amplitude?
• ENSO irregularity
• Phase-locking to annual cycle
• Skewness of ENSO
• Bursting of ENSO
• ENSO and TIWs
• ENSO and ocean biology
• Past ENSO variability
• Response to GH warming

3
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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Example 1ENSOs response to orbital forcing
ENSO variance maybe skewness
Strength of annual cycle dT/dtf (T,u..)Asin?t
Background state dT/dtf (T,u,h,v,w)
External factors, Orbital forcing
5
ECHO-G simulation 140ka B.P. 20ka A.P.
Annual cycle
ENSO
ka
Zonal SST gradient obliquity 41k cycleACY and
ENSOamplitude precessional 21k cycle
6
ECHO-G simulation 140ka B.P. 20ka A.P.
Anomalous P-E
weak northern ITCZ
Strong southern ITCZ
Meridional SST gradient precessional cycleACY
and ENSOamplitude precessional cycle
weak southern ITCZ
Strong northern ITCZ
7
ECHO-G simulation 140ka B.P. 20ka A.P.
ACY strength is driven by meridional SST
gradientmeridional SST gradient varies with
precessional cycleWHY?
8
Emergence of an annual mean precessional cycle
Annual cycle of cloud albedo
Annual cycle of cloudiness
gt 0
lt
gt ?0
lt
9
ENSO response to orbital forcing
10
Example 2ENSOs response to AMOC collapse
ENSO variance maybe skewness
Strength of annual cycle dT/dtf (T,u..)Asin?t
Background state dT/dtf (T,u,h,v,w)
External factors, AMOC collapse
11
Tropical Pacific response to Heinrich I
Pahnke et al. 2007
NADW McManus 2004
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Tropical Pacific response to AMOC collapse
GFDL CM2.1 Waterhosing Experiment Timmermann
et al 2007 Stouffer et al 2006
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Tropical Pacific response to Caribbean SSTA
Linear moist baroclinic model coupled to tropical
POP
14
CGCM Hosing Experiments (CMIP)
Freshwater flux anomaly in N Atlantic
(50-70N) (1Sv X 100 yrs 9m increase in sea
level)
1Sv
Year
100
200
Monthly SST, Z20, wind stress (precipitation,
geopotential height)
15
Tropical Pacific response to AMOC shutdown
5 waterhosing experiments conducted as part of
CMIP
Weakening of annual cycle and Intensification of
ENSO
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Timmermann et al. (2005)
Weakening of the AMOC
Cooling of North Atlantic
Caribbean anticyclone
Cooling of northeastern tropical Pacific
Timmermann et al. (2007)
Intensification of Northeasterly trades In
tropical Pacific
Equatorial thermocline shoaling
Weakening of Annual cycle in Equatorial Pacific
Strengthening Of ENSO
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Example 3 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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Noise-induced intensification of ENSO
Eisenman et al. 2005
WWB modulation by temperature for present-day
climate
19
Noise-induced intensification of ENSO
WWB modulation by temperature (BMRC MJO activity)
Correlation/Regression between Nino3 SSTA and
20-60 day band-pass filtered wind Variance ENSO-p
recursor gt Mikes prediction model
20
Noise-induced intensification of ENSO
AR4 models simulate increased Intraseasonal
variability
Did the WWB/MJO-ENSO interaction increase during
the last 50 years
21
ENSO recharge model with state-dependent noise
Coupling strength and noise may change
slowly over time
22
ENSO recharge model with state-dependent noise
Ensemble mean equation for ENSO
State-dependent noise is coupling State-dependen
t noise is also nonlinearity
23
Past and future changes of ENSO amplitude

• Control of ENSO amplitude is a complicated story
  not only linear instability
• We need better theory for annual cycle-ENSO
  interactions
• We need better theory for WWB-ENSO interactions
• We need more realistic representations of WWBs in
  CGCMs

24
Past and future changes of ENSO amplitude
HADCM3 multi-model Ensemble Relationship
between Global climate sensitivity and Simulated
NINO3 stdv Processes that amplify Global warming
weaken ENSO ???
From Collins, pers. comm.
25
We see no statistically significant changes in
amplitude of ENSO variability in the future, with
changes in the standard deviation of the Southern
Oscillation Index that are no larger than
observed decadal variations. (Oldenborgh et al.
2005).
From Oldenborgh, 2005