CO 2 -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), 2xCO 2 (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 SSTs annual 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)‏ 18604xCO 2

Relative to ECT SSTs, the warm pool contracts. Relative to ECT SSTs, cold water moves closer to the surface. subsurface equator Ecuador Indonesia SST As CO 2 increases:

Summary of greenhouse tropics in CM 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? - CO 2 -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 CO 2 increases

Much to be done 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?

Reserve Slides

Planet CM2 Wittenberg et al. (J. Climate, 2006)‏

Modulation of NINO3 SST power spectrum 2000yr mean (e.g. satellites, TAO)‏(e.g. reconst SST)‏

ENSO SST & rainfall changes

Changes in intraseasonal variability

Can we extrapolate to reality? weak ENSOs ? Merryfield (JC, 2006)‏ CM2.1 CM2.0 Future ENSO amplification Width of wind stress response The “most realistic” pre-industrial ENSOs show amplification at 2xCO 2

Key ENSO feedbacks SST noise u, w, mixing wind stress heat fluxes h

SST anomaly tendency equation Key to understanding the influence of background state on ENSO. dominant terms near equator

Regressions onto NINO3 SSTAs

4x (600yr)‏ 1860 (2000yr)‏ 2x (600yr)‏ 1990 (300yr)‏