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Temporal structure of ENSO in 20th Century Climate simulations
Antonietta CapotondiNOAA/Earth System Research Laboratory
Collaborators: Andrew Wittenberg, Simona Masina, Clara Deser, Mike Alexander, Yuko Okumura
ENSO in coupled climate models 2Evolution of the Niño3.4 index
Observations
CCSM3 (NCAR)
GFDL
NASA GISS
Spectral characteristicsNino3.4 index (5°S-5°N, 170°W-120°W)
Capotondi, Wittenberg and Masina 2006
What determines the ENSO timescale?
• Guilyardi et al. (2004) have shown that atmospheric model resolution can be important. Higher atmospheric resolution leads to more realistic (longer) periods.
• Fedorov and Philander (2001) have emphasized the importance of the mean state (intensity of the winds, depth and strength of the equatorial thermocline) as a controlling factor for ENSO properties.
• Dewitte et al. (2007) have stressed the importance of thermocline depth. A shallow thermocline in the central-west Pacific may favor fast equatorial modes, and lead to a shorter timescale.
• Simple oscillator models: Delayed oscillator, Recharge oscillator, Western Pacific oscillator, Advective-Reflective oscillator. Unified theory provided by Wang (2001).
What determines the ENSO timescale?Studies based on intermediate coupled models emphasize the
importance of the spatial structure of the anomalous wind stress
Kirtman (1997) used an intermediate complexity model and surface wind stresses of different meridional scales.
What determines the ENSO timescale?Studies based on intermediate coupled models
An and Wang (2000) examined the causes for the longer ENSO period after the 1976-77 climate regime shift, and examined the changes in the pattern of the wind stress.
Important aspects of anomalous wind stress pattern:
Meridional width: increases adjustment time through extra-equatorial Rossby waves
Longitudinal position: Controls whether anomalous advection of zonal mean temperature gradients promotes ENSO growth or phase transition.
Regression of τx upon the Niño3.4 indexNCEP/NCAR Reanalyses
‘Center of mass’ of τx
168°W
Regression of τx upon the Niño3.4 index
CCSM3
GFDL-CM2.0
IPSL-CM4
NCEP
Regression of τx upon the Niño3.4 index
GISS-EH
PCM
MRI
Regression of τx upon the Niño3.4 index
UKMO-HadCM3
CSIRO
CNRM
Dependency of period upon structure of anomalous wind stress
T vs. Ly
T vs. C
T vs. Tp(Ly,C)
CCSM3 vs. HadCM3
Influence of meridional width of τx
Regression of the curl(τ) upon the Nino3.4 index
Curl(τ) vs. Standard Deviation of pycnocline transportMeridional transport between the base of the mixed layer and the 26σθ isopycnal, zonally averaged from the eastern edge of the WBC and the eastern ocean boundary
POP simulation forced with COARE climatology
Thermocline VariabilityDepth of 15°C isotherm (Z15)
Meinen and McPhaden 2000
EOF2EOF1
INGV ocean analysis
Influence of meridional width of τx
EOF2 of thermocline depthINGV
CCSM3 HadCM3
10°N
10°S
Phase relationship between the Z15 modes
INGV
CCSM3
UKMO-HadCM3
PC2-PC1 lag-correlation
CCSM4
Niño3.4 time series
Observations HadISST
CCSM4- Spectra
CCSM4: Wind Stress and Curl(τ)
CCSM4
CCSM4: Thermocline variability
INGV
CCSM4
Seasonal Footprinting mechanism: CCSM4 vs. obs
Seasonal Footprinting mechanism: CCSM3 vs. obs
Asymmetry in the duration of El Niño and La Niña (Okumura and Deser 2010)
HadISST 1900-1947 HadISST/NCEP 1948-2008 CCSM4
El Niño
La NiñaDec0
Dec+1
Dec0
Dec+1
Open Questions•Relative importance of wind forcing vs. mean upper-ocean stratification in determining the ENSO timescale
•SST variations appear strongly correlated with thermocline variability in the CMIP3 models. What about S-modes?
•Can models reproduce the SFM? SFM is a precursor of a large fraction of ENSO events. What is its connection with the Western Wind Events (WWE)?
•Is the difference in duration of El Niño and La Niña events an important metrics to include in evaluating models?
• Can the models reproduce the observed ENSO diversity, including amplitude and frequency modulation, and longitudinal position of the warming? What are the mechanisms?