ENSO sensitivity to change in stratification in CMIP3

Boris Dewitte

Sulian Thual, Sang-Wook Yeh, Soon-Il An, Ali Belmadani

CLIVAR Workshop, Paris, France, 17-19 November 2010New strategies for evaluating ENSO processes in climate models

Yeh et al. (2009) Dinezio et al. (2009)

Impact of climate change on the mean stratification in ensemble models

ΔT (2xCO2 – PI)

Conclusions/Perspectives

• The characteristics of the thermocline (depth, sharpness, intensity) needs to be taken into account for determining the stability of ENSO• SODA tells us that an increased stratification leads to more energetic and low-frequency ENSO (Climate change paradox..)• Need to understand the impact of stratification changes on ENSO non-linearities.

Motivation

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~6 monthsη~10-20 years2~?k~?

Understand the physical mechanism associated to the ‘rectification’ of ENSO variability/stability by the change in mean state?

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Change in thermocline depth at decadal timescales

On thermocline depth: small amplitude (Wang and An, 2001)

Levitus data

T(1960-2001)

D20 (1980-1997)

D20 (1960-1975)

(Moon et al., 2004; Dewitte et al., 2009)

T(1980-1997)-T(1960-1975)

Change in mean temperature associated to the 1976/77 climate shift

T(1980-1997)-T(1960-1975)

• The ‘Moon pattern’ indicates that change in mean state cannot be account for just one baroclinic mode..!

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Sensitivity of ENSO to stratification

• Ocean dynamics perspective

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Shallow-water equationsStratification defined by (c, H)

Multimode contextStratification defined by (cn, Pn)

(Dewitte and Reverdin, 2000)

Interannual variability of vertical displacements in a OGCM simulation (1985-1994)

A ‘finer’ representation of the thermocline allows for taking into account the ‘loss’ of energy associated to vertical propagation: Implication for ENSO energetics and feedbacks

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Nonlinear Dynamical Heating

Zonal Advective Feedback

Thermocline Feedback

Sensitivity of ENSO to stratification

• Thermodynamics perspective

1 : BCCR-BCM2.0

2 : CCCMA-CGCM3.1

3 : CCCMA-CGCM3.1 (t63)

4 : CNRM-CM3

5 : CSIRO-MK3.0

6 : CSIRO-MK3.5

7 : GFDL-CM2.0

8 : GFDL-CM2.1

9a : GISS-AOM (run1)

9b : GISS-AOM (run2)

11 : GISS-MODEL-E-R

12 : IAP-FGOALS1.0-g

13 : INGV-ECHAM4

14 : INM-CM3.0

15 : IPSL-CM4

16 : MIROC3.2-HIRES

17 : MIROC3.2-MEDRES

18 : MIUB-ECHO-g

19 : MPI-ECHAM5

20 : MRI-CGCM2.3.2A

21 : NCAR-CCSM3.0

22 : UKMO-HadCM3

23 : UKMO-HadGem1

Belmadani et al. (2010)

Mean circulation ( , ) in CMIP3WU

1 : BCCR-BCM2.0

2 : CCCMA-CGCM3.1

3 : CCCMA-CGCM3.1 (t63)

4 : CNRM-CM3

5 : CSIRO-MK3.0

6 : CSIRO-MK3.5

7 : GFDL-CM2.0

8 : GFDL-CM2.1

9a : GISS-AOM (run1)

9b : GISS-AOM (run2)

11 : GISS-MODEL-E-R

12 : IAP-FGOALS1.0-g

13 : INGV-ECHAM4

14 : INM-CM3.0

15 : IPSL-CM4

16 : MIROC3.2-HIRES

17 : MIROC3.2-MEDRES

18 : MIUB-ECHO-g

19 : MPI-ECHAM5

20 : MRI-CGCM2.3.2A

21 : NCAR-CCSM3.0

22 : UKMO-HadCM3

23 : UKMO-HadGem1

Thermocline depth bias in CMIP3

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Nonlinear Dynamical Heating

Zonal Advective Feedback

Thermocline Feedback

Sensitivity of ENSO to stratification

• Thermodynamics perspective

y=yn

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Kelvin waves (he, ue)

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The Jin two-strip model (An and Jin, 2001)

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Solution of the mode [Xµ=X0.e.t.cos(β.t +φ)] as a function of coupling efficiency

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The Jin two-strip model (An and Jin, 2001)

Stability of ENSO as a function of thermocline depth

Period

Growth rate

Federov and Philander (2001)

Increased thermocline depth -------->lower frequency stronger ENSO

• Defining thermocline…

• Depth (P1)• Intensity, Sharpness (Pn, n>1)

Gent and Luyten (1985)

Decadal variability of Pn – CNRM-CM3

D20<0

D20>0

180° 90°W

D20>0

D20<0

thermocline

CNRM-CM3

N3VAR

<P1>=0.5, <P2>=0.5, <P3>=0.2

Pn(t)

Dewitte et al. (2007)

Conceptual Model

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Ocean dynamics :

Kelvin and Rossby wave on 3 baroclinic modes : Kn, Rn

Thermodynamics :

Thermocline depth and zonal currents : H, U

Atmospherical component :

Statistical relationship (SVD) with a coupling coefficient µ.

comparable to the Jin two-strip model (Jin 1997b, An & Jin 2001) except for the ocean dynamics.

nxnnxnnnt KPKcK .)(

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),,,,,,(),( 321321 SSTRRRKKKtxX Variables :

(Thual et al., 2010)

Thermodynamical feedbacksSSTFUxFHxFSST DUHt )()(

dHdT

H wF

SSTF xU

Thermocline feedback

Zonal advective feedback

SODA dataset (1958-2008)

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Stability Analysis

Dominant eigenmode=ENSO mode Eigenvectors of the ENSO mode (µ=1)

XJX Find eigenvalues (a+ ib) of from J

Each eigenmode (a,b) has the form )),(cos()exp(),( txbtAattxX

Sensitivity to Stratification

Stratification acts as a coupling parameter, but with physical meaning.

P1(1-δ), P2(1+δ/2), P3(1+δ/2)

δ

Sensitivity of ENSO mode to stratification in the TD model

Model parameters:P1(1-δ), P2(1+δ/2),

P3(1+δ/2)

frequency

Growth rate

Pre-70s to Post-70s : Strong increase in stratification (δ =120%).

=> Stronger, lower frequency ENSO

The 1976/77 Climate shifts:

Data: SODA

Post-2000 : Slight decrease in stratification (δ =95%).

=> ENSO variability displaced toward the west. Processes ?

Data: SODA

The 2000 shifts:

Change in ENSO stability in the GFDL model

« Metrics » for the sensitivity to stratification change using the extended Jin’s two-strip model

f

Yeh et al. (2010)

EOF1 of low-passed filtered T(x,z,y=0) (PI runs)

MRI

GFDL

2xCO2 - PI

Sensitivity of ENSO to a warming climate: GFDL versus MRI

Change in feedback processes

Yeh et al. (2010)

Conclusions/Perspectives

• The characteristics of the thermocline (depth, sharpness, intensity) needs to be taken into account for determining the stability of ENSO• SODA tells us that an increased stratification leads to more energetic and lower-frequency ENSO (Climate change paradox.?.)• Need to understand the impact of stratification changes on ENSO non-linearities.

« Metrics » for the sensitivity to stratification change using the extended Jin’s two-strip model

f

MRI GFDL

Low frequency change of temperature (EOF1) in the MRI and GFDL models

Change in stratification tends to project on the high-order or « very slow » modes (n>3) impact Ekman layer physics

Change in stratification does project on the gravest modes (n=1,3) Impact ENSO stability

Yeh et al. (2010)

Change in feedback processes

Yeh et al. (2010)

Low frequency change of temperature (EOF1) in CMIP3

MIROC3_3_HIRES MIROC3_3_MEDRES MPI_ECHAM5

MRI_CGCM2_3_2A NCAR_CCSM3_0 UKMO_HADCM3

CCCMA_CGCM3_1_t63 CNRM_CM3 CSIRO_MK3_5

GFDL_CM2_0 INMCM3_0 MIUB_ECHO_G

CCCMA_CGCM3_1 FGOALSrun1 GFDL_CM2_1

INVG_ECHAM4 IPSL_CM4 GISS_AOMrun1

Low frequency change of temperature (EOF1) in CMIP3