1. Geophysical Fluid Dynamics Laboratory Presented by Andrew Wittenberg Geophysical Fluid Dynamics Laboratory 17 October 2012 Tropical Climate Change and ENSO NOAA GOES-11 5 Oct 2011 1800 UTC

2. Geophysical Fluid Dynamics Laboratory The Tropics: Firebox of the global circulation 2 Thompson Higher Education

3. Geophysical Fluid Dynamics Laboratory Projected surface temperature changes 3 Strongest warming over land & equatorial Pacific More warming in calm areas, and where winds weaken Feedbacks from low clouds & ocean advection Vecchi et al. (2008) Vecchi & Wittenberg (2010) Collins et al. (2010) Xie et al. (2010)

4. Geophysical Fluid Dynamics Laboratory Projected water vapor changes 4 Warming pumps water vapor into the atmosphere Vecchi & Wittenberg (2010) Collins et al. (2010) Xie et al. (2010) Tropics today: ~40 kg of water vapor 2050: +4 kg

5. Geophysical Fluid Dynamics Laboratory Projected rainfall changes 5 Broadly: “the wet get wetter, the dry get drier”. Over tropical oceans: “the warmer get wetter”. Held & Soden (2006) Vecchi & Wittenberg (2010) DiNezio et al. (2010) Xie et al. (2010)

6. Geophysical Fluid Dynamics Laboratory Projected tropospheric temperature changes 6 Increased static stability of atmosphere Helps expand Hadley Cell Weakens convective mass fluxes & trade winds Held & Soden (2006) Vecchi et al. (2006) Frierson et al. (2007) Collins et al. (2010)

7. Geophysical Fluid Dynamics Laboratory Projected upper-ocean temperature changes 7 Tropical ocean more stratified Stronger, shallower, and flatter equatorial thermocline DiNezio et al. (JC 2009, EOS 2010) Collins et al. (2010)

8. Geophysical Fluid Dynamics Laboratory Earth's dominant interannual climate fluctuation 8 NOAA/CPC El Niño Normal

9. Geophysical Fluid Dynamics Laboratory Projected ENSO changes (CMIP3/AR4) 9 correl(SST trend of 1%/yr, SST.PC1 of PICTRL) 10S-10N, 120E-80W Yamaguchi & Noda (JMSJ 2006) std(SLP.PC1 of SRES.A2 (2051-2100)) / std(SLP.PC1 of 20C3M) 30S-30N, 30E-60W van Oldenborgh et al. (OS 2005) CM2.1 Weak/ambiguous near-term anthropogenic impacts on ENSO Intrinsic modulation Reviews: Meehl et al. (IPCC-AR4 2007) Guilyardi et al. (BAMS 2009) Vecchi & Wittenberg (WIREs CC 2010) Collins et al. (Nature Geosci. 2010)

10. Geophysical Fluid Dynamics Laboratory Intrinsic modulation of ENSO: Observed 10 Vecchi & Wittenberg (WIREs CC 2010) Historical SSTA (ERSST.v3) Palmyra corals (Cobb et al., Nature 2003) Multiproxy reconstructions: Emile-Geay et al. (2011abc, subm.)

11. Geophysical Fluid Dynamics Laboratory Intrinsic modulation of ENSO: Simulated 11 Wittenberg (GRL 2009)

12. Geophysical Fluid Dynamics Laboratory The most extreme ENSO epochs 12 Wittenberg et al. (in prep.)

13. Geophysical Fluid Dynamics Laboratory Initial conditions for “perfect” reforecasts 13 Wittenberg et al. (in prep.)

14. Geophysical Fluid Dynamics Laboratory 40 “perfect” reforecasts – best possible skill 14 Wittenberg et al. (in prep.)

15. Geophysical Fluid Dynamics Laboratory 1860: Spread of 100yr NINO3 SST spectra 15 286 ppmv Wittenberg (2009)

16. Geophysical Fluid Dynamics Laboratory 1990: Stronger annual cycle & ENSO 16 353 ppmv Wittenberg (2009)

17. Geophysical Fluid Dynamics Laboratory 2xCO2: A perfect climate for ENSO? 17 572 ppmv Wittenberg (2009)

18. Geophysical Fluid Dynamics Laboratory 4xCO2: Stronger annual cycle, weaker ENSO 18 1144 ppmv Wittenberg (2009)

19. Geophysical Fluid Dynamics Laboratory Competing changes in ENSO feedbacks 19 1. Amplifiers - stronger rainfall & wind stress responses to SSTAs - stronger thermocline, shallower mixed layer - weaker replenishment of surface waters from below 2. Dampers - stronger evaporative & cloud-shading responses - weaker upwelling -> surface less connected to thermocline - smaller dynamic warm pool -> less room for warming 3. Ambiguous effects - stronger intraseasonal wind variability Guilyardi et al. (BAMS 2009); Vecchi & Wittenberg (WIREs CC 2010) Collins et al. (Nature Geosci. 2010); DiNezio et al. (JC 2009; EOS 2010; JC 2011 subm.) Ongoing activities with CLIVAR Working Groups, D. Battisti, A. Atwood, M. Cane, C. Karamperidou, F.-F. Jin, J. Brown, F. Graham

20. Geophysical Fluid Dynamics Laboratory Summary 20 1. Projections of tropical climate change - tropics moisten, stratify, expand - circulation weakens; ocean thermocline shoals & flattens - SST: calm(er) get warmer; ocean advection changes - rainfall: wet get wetter; warmer get wetter - distinct from El Niño 2. Is ENSO changing? - diverse projections - competing feedbacks + optima + model biases -> uncertainty 3. Risk of intrinsic ENSO modulation - ENSO capable of wide swings in behavior on its own - interannual predictability only, except after a big event