CO 2 -induced changes in tropical climate as simulated by the GFDL coupled GCMs

1. CO2-induced changes in tropical climate as simulated by the GFDL coupled GCMs Andrew Wittenberg NOAA/GFDL

2. 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)‏

3. 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 1990 control (300yr), 2xCO2 (600yr), 4xCO2 (400yr)‏ 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)‏

4. 4xCO2 change in annual mean SST and wind speed

5. 4xCO2 change in surface heat flux balance

6. 4xCO2 change in atmospheric moisture

7. 4xCO2 change in convection & trade winds

8. 4xCO2 change in equatorial subsurface temperatures

9. 4xCO2 change in seasonal cycle amplitude

10. Seasonal cycle changes in CM2.1

11. 20 centuries of NINO3 SSTs annual means & 20yr low-pass

12. Pre-industrial range of 100yr spectra

13. 1990: ENSO strengthens, spectrum narrows

14. 2xCO2: slightly shorter period than 1990

15. 4xCO2: ENSO weaker than at 2xCO2

16. ENSO atmospheric feedbacks and forcings

17. CM2.1 equatorial Pacific SST anomalies (ºC, 200yr)‏ 1860 4xCO2

18. As CO2 increases: SST Relative to ECT SSTs, the warm pool contracts. Relative to ECT SSTs, cold water moves closer to the surface. Ecuador Indonesia subsurface equator

19. 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

20. 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?