CO2-induced changes in tropical climate

as simulated by the GFDL coupled GCMs

Andrew Wittenberg

NOAA/GFDL

CMIP3/AR4 results for future ENSO changes

1. Enhanced warming of cold tongue- dT/dy change more robust than dT/dx

- more stratified ocean & atmosphere

- evaporation, trades trump upwelling thermostat

- eastward shift of warm pool convection

2. Diverse changes in ENSO spectrum/pattern- hard to detect in short records

- increased damping opposes increased coupling

- "best" models show more amplification, eastward propagation

Guilyardi (CD 2005)

van Oldenborgh et al. (OS 2005)

Philip & van Oldenborgh (GRL 2006)

Held & Soden (JC 2006)

Merryfield (JC 2006)

Liu et al. (JC 2005)

Tanaka et al. (SOLA 2005)

Collins (CD 2005)

Jin et al. (GRL 2001)

Vecchi et al. (Nature 2006)

Guilyardi et al. (BAMS 2009)

GFDL CM2.1 global coupled GCM atmos: 2°x2.5°xL24 finite volume ocean: 1°x1°xL50 MOM4 (1/3° near equator) 2hr coupling; ocean color; no flux adjustments ENSO & tropics rank among top AR4-class models SI forecasts; parent of GFDL AR5 models

2000-year pre-industrial control run 1860 atmospheric composition, insolation, land cover 220yr spinup from 20th-century initial conditions substantial investment: 1 year on 60 processors

Delworth et al., Wittenberg et al., Merryfield et al., Joseph & Nigam (JC 2006)

Zhang et al. (MWR 2007); van Oldenborgh et al. (OS 2005); Guilyardi (CD 2006); Reichler & Kim (BAMS 2008)

1990 control (300yr), 2xCO2 (600yr), 4xCO

2 (400yr)

4xCO2 change in annual mean SST and wind speed

4xCO2 change in surface heat flux balance

4xCO2 change in atmospheric moisture

4xCO2 change in convection & trade winds

4xCO2 change in equatorial subsurface temperatures

4xCO2 change in seasonal cycle amplitude

Seasonal cycle changes in CM2.1

20 centuries of NINO3 SSTsannual means & 20yr low-pass

Pre-industrial range of 100yr spectra

1990: ENSO strengthens, spectrum narrows

2xCO2: slightly shorter period than 1990

4xCO2: ENSO weaker than at 2xCO2

ENSO atmospheric feedbacks and forcings

CM2.1 equatorial Pacific SST anomalies (ºC, 200yr)

1860 4xCO2

Relative to ECT

SSTs, the warm

pool contracts.

Relative to ECT

SSTs, cold water

moves closer to

the surface. subsurface

equator

Ecu

ador

Indo

nesi

a

SSTAs CO

2 increases:

Summary of greenhouse tropics in CM2.1

1. Tropical climate change differs from El Niño- cold tongue warms thermodynamically from above

- ocean & atmosphere more stratified & y-symmetric

2. Stronger seasonal cycle near equator- more ocean dynamical cooling south of NINO3

3. Centennial-scale modulation of ENSO- “natural” risks: have we observed Earth long enough?

- CO2-induced changes: barely detectable w/ 100yr of data

4. CM2.1 ENSO strongest near 2xCO2- strong wind coupling & noise, weak heat flux damping

- shallow NINO3 thermocline, large “relative” warm pool

- more eastward SSTA propagation as CO2 increases

Much to be done...

1. Improve models- long runs, paleo tests, pseudoproxies

- community metrics; rebirth of ICMs as diagnostics

2. Improve forcings- indirect effect & interactive aerosols (CM3)

- off-equatorial mixing & ocean color (ESM2M/G)

3. Understand natural modulation- what future risks does it imply? is it predictable?

4. Understand ENSO sensitivities & nonlinearities- reconcile theory with various model responses; ENSO-PMI?

- CMT, cloud feedbacks, wind noise; warm pool/cold well

- are we near an ENSO optimum?

- how to extrapolate reality from biased models?