Axel Timmermann

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ENSOs sensitivity to past and future climate
change

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

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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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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
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ECHO-G simulation 140ka B.P. 20ka A.P.
Annual cycle
ENSO
ka
Zonal SST gradient obliquity cycleACY and
ENSOamplitude precessional cycle
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ECHO-G simulation 140ka B.P. 20ka A.P.
Meridional SST gradient precessional cycleACY
and ENSOamplitude precessional cycle
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ECHO-G simulation 140ka B.P. 20ka A.P.
ACY strength is driven by meridional SST
gradientmeridional SST gradient varies with
precessional cycleWHY?
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Emergence of an annual mean precessional cycle
Annual cycle of cloud albedo
Annual cycle of cloudiness
gt 0
lt
gt ?0
lt
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ENSO response to orbital forcing
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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
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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
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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
Model Atmosphere Ocean Forcing CO2
GFDL_CM2.1 2x2.5, L24 1/3-1x1, L50 (MOM4) fresh water 286
HadCM3 2.5x3.75, L19 1.25x1.25, L20 (pre-MOM) virtual salt 290
CCSM2 T42, L26 1x1, L40 (POP) fresh water 355
ECHAM5/MPI-OM T31, L19 3x3 virtual salt 280
Monthly SST, Z20, wind stress (precipitation,
geopotential height)
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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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AMOC weakening a paradigm for LIA-MCA
Gulf Stream 10 weaker Caribbean 2C colder ITCZ
south Cariaco Galapagos wet Reduced Indian
monsoon Wetter in Southwest US Palau dry Warm
Santa Barbara basin Higher Peru river
discharge Central Chile wet Cold MD81 Stronger
ENSO Palmyra Pallcacocha Huascaran .
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Mechanisms
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Impacts
Hurricanes? Wildfires Dust storms Productivity Ext
remes Indian Monsoon Sahel
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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
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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
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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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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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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

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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.
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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