ENSO sensitivity to change in stratification in CMIP3 Boris Dewitte Sulian Thual, Sang-Wook Yeh, Soon-Il An, Ali Belmadani CLIVAR Workshop, Paris, France, November 2010 New 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 Cf. Battisti and Hirst (1989)  ~6 months η~10-20 years  2 ~? k~? Understand the physical mechanism associated to the ‘rectification’ of ENSO variability/stability by the change in mean state? ?

Change in thermocline depth at decadal timescales On thermocline depth: small amplitude (Wang and An, 2001) Levitus data

T( ) D20 ( ) D20 ( ) (Moon et al., 2004; Dewitte et al., 2009) T( )-T( ) Change in mean temperature associated to the 1976/77 climate shift

T( )-T( ) The ‘Moon pattern’ indicates that change in mean state cannot be account for just one baroclinic mode..! (modes 1 to 3)

Sensitivity of ENSO to stratification Ocean dynamics perspective Shallow-water equations Stratification defined by (c, H) Multimode context Stratification defined by (c n, P n )

(Dewitte and Reverdin, 2000) Interannual variability of vertical displacements in a OGCM simulation ( ) 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

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-CM : IPSL-CM4 16 : MIROC3.2-HIRES 17 : MIROC3.2-MEDRES 18 : MIUB-ECHO-g 19 : MPI-ECHAM5 20 : MRI-CGCM2.3.2A 21 : NCAR-CCSM : UKMO-HadCM3 23 : UKMO-HadGem1 Belmadani et al. (2010) Mean circulation (, ) in CMIP3

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-CM : IPSL-CM4 16 : MIROC3.2-HIRES 17 : MIROC3.2-MEDRES 18 : MIUB-ECHO-g 19 : MPI-ECHAM5 20 : MRI-CGCM2.3.2A 21 : NCAR-CCSM : UKMO-HadCM3 23 : UKMO-HadGem1 Thermocline depth bias in CMIP3

Nonlinear Dynamical Heating Zonal Advective Feedback Thermocline Feedback Sensitivity of ENSO to stratification Thermodynamics perspective

y=y n y=0° Equator ~3°N Kelvin waves (he, ue) y=0°-> Rossby waves (hn) y=y n -> H mix hn=r E  hehe=r W  hn The Jin two- strip model (An and Jin, 2001)

 =1 ~4 yrs  =0 (basin mode) ~ 9 months α β Solution of the mode [X µ =X 0.e .t.cos(β.t +φ)] as a function of coupling efficiency  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 (P 1 ) Intensity, Sharpness (P n, n>1) Gent and Luyten (1985)

Decadal variability of P n – CNRM-CM3  D20<0  D20>0 180° 90°W  D20>0  D20<0 thermocline CNRM-CM3 N3VAR =0.5, =0.5, =0.2  P n (t) Dewitte et al. (2007)

Conceptual Model Ocean dynamics : Kelvin and Rossby wave on 3 baroclinic modes : K n, R n 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. Variables : (Thual et al., 2010)

Thermodynamical feedbacks Thermocline feedback Zonal advective feedback SODA dataset ( ) Adimentionalised feedback intensity

Stability Analysis Dominant eigenmode=ENSO modeEigenvectors of the ENSO mode (µ=1) Find eigenvalues (a+ ib) of from Each eigenmode (a,b) has the form

Sensitivity to Stratification Stratification acts as a coupling parameter, but with physical meaning. P 1 (1-δ), P 2 (1+δ/2), P 3 (1+δ/2) δ

Sensitivity of ENSO mode to stratification in the TD model Model parameters: P 1 (1-δ), P 2 (1+δ/2), P 3 (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

Yeh et al. (2010) Change in feedback processes 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

MRIGFDL 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) Change in feedback processes

Low frequency change of temperature (EOF1) in CMIP3 MIROC3_3_HIRESMIROC3_3_MEDRESMPI_ECHAM5 MRI_CGCM2_3_2ANCAR_CCSM3_0UKMO_HADCM3

CCCMA_CGCM3_1_t63CNRM_CM3CSIRO_MK3_5 GFDL_CM2_0INMCM3_0MIUB_ECHO_G CCCMA_CGCM3_1 FGOALSrun1GFDL_CM2_1 INVG_ECHAM4IPSL_CM4GISS_AOMrun1 Low frequency change of temperatu re (EOF1) in CMIP3