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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
)(...)(. 2
,
23
tNkTeTctTb
t
Ttt
Cf. Battisti and Hirst (1989)
~6 monthsη~10-20 years2~?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(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..!
M
n
nnTM dz
tzxdFtyxsltzyxT
1
1 )',,().,,(),,,(
(modes 1 to 3)
Sensitivity of ENSO to stratification
• Ocean dynamics perspective
02
0
0
tg
c
y
v
x
u
Hygyu
t
v
Hxgyv
t
u
y
x
0
.
.
2
tg
c
y
v
x
u
Py
gyut
v
Px
gyvt
u
nnnn
ynn
nn
xnn
nn
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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'.'.'
.'.'
.('
z
Tw
y
Tv
x
Tu
z
Tw
z
Tw
x
Tu
x
Tu
t
T
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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'.'
'.'.'
.'.'
.('
z
Tw
y
Tv
x
Tu
z
Tw
z
Tw
x
Tu
x
Tu
t
T
Nonlinear Dynamical Heating
Zonal Advective Feedback
Thermocline Feedback
Sensitivity of ENSO to stratification
• Thermodynamics perspective
y=yn
y=0°
Equator
~3°N
Kelvin waves (he, ue)
y=0°->emxe
ee hx
h
t
u
nee hhuavec
Rossby waves (hn)
y=yn-> nmn
Rn h
fCurl
x
hC
t
h
)( 2
2
2
2
~n
R y
c
f
cCavec
Hmix
2/exp),(),(
2
0
aee L
LyxTAyx
ee
ee T
z
Tw
x
Tu
t
T...
)(~ ee hfz
Twith
hn=rEhehe=rWhn
),(2
)/(/)(
)()(2
WE
yyxynnxnmt
xeexnemt
retrreflexionconditionsboundary
yyhh
hhh
n
The Jin two-strip model (An and Jin, 2001)
e
n
e
T
h
h
with X
XAX
t
=1
~4 yrs
=0(basin mode)
~ 9 months
α
β
Solution of the mode [Xµ=X0.e.t.cos(β.t +φ)] as a function of coupling efficiency
XAX
t
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
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
)(SSTfx
SSTFUxFHxFSST DUHt )()(
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 .)(
nxnnxn
nnt RPRc
R .3
)(
),,,,,,(),( 321321 SSTRRRKKKtxX Variables :
(Thual et al., 2010)
Thermodynamical feedbacksSSTFUxFHxFSST DUHt )()(
dHdT
H wF
SSTF xU
Thermocline feedback
Zonal advective feedback
SODA dataset (1958-2008)
Ad
imen
tio
nal
ised
fee
db
ack
inte
nsi
ty
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:
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.
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
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