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?