climate response of the equatorial Pacific to 2xCO2 in the IPCC AR4/CMIP3 models:

1. climate response of the equatorial Pacific to 2xCO2in the IPCC AR4/CMIP3 models: the role of Ocean Dynamics Trenberth et al., 2001 Pedro DiNezio Amy Clement, Brian Soden, Ben Kirtman, Sang-Ki Lee

2. motivation / objectives why the climate of the equatorial Pacific is important? • It is the time-mean state over which El Niño/ENSO events occur. • Climate scale SST anomalies can influence remote regions through ENSO teleconnections too. • Previous modeling studies have shown that the equatorial climate is very sensitive to Global Warming. • No consensus on the sign of the projected change in zonal SST gradient. • SST observations don’t show a clear signature (1880 – 2005). HadSST relation with Global Warming NOAA SST Vecchi, Clement and Soden, 2007

3. feedbacks in the SST equation Clement et al., 1997 ● not really a feedback

4. methodology / data IPCC AR4/CMIP3 model output from models: 4 different simulations: 1pctto2x picntrl

5. SST response 2xco2 1pctto2x 50-year climatology 50-year climatology 50-year climatology picntrl slabcntl 50-year climatology

6. positive into the ocean

7. positive into the ocean

8. positive into the ocean net surface heat flux ΔQ net ~ ocean divergence

9. weakening of the Walker circulation τx SLP ocean response uocn wocn

10. thermocline response D20

11. Qnet QU + QV = -ρcpH(uTx+vTy) Qw when the 2xCO2 climate reaches equilibrium… QSW + QLW + QLH + QSH + QU + QV + QW ~ 0

12. conclusions in GFLD CM2.1 and 5 more IPCC AR4/CMIP3 models… Ocean Dynamics plays a central role in the 2XCO2 SST response both in the cold tongue and in the warm pool. The weakening of the Walker circulation plays influences the SST response through the climatological Bjerknes feedback. Different counteracting feedbacks determine the SST response in the warm pool than in the cold tongue. Cold tongue: ocean dynamical thermostat vs weaker-Walker (through Bjerknes feedback) Warm pool: SW-feedback vs weaker-Walker (through Bjerknes feedback) The cooling signature of the Ocean thermostat is not detected in the SST response, however it remains the main negative feedback controlling the SST response in the cold tongue. Less robust role for LH-feedback across models.

13. Observational implications:the response to 2xCO2 in the equatorial Pacific should be easier to detect in thermocline depth, sea level, or sea level pressure rather than SST. e.g. ―Kaplan ―Hadley Centre - - NCEP ship-based obs. Vecchi et al. (2006)

14. future work • Does the effectivity of the ocean thermostat change in time? • explore transients in the climate response associated with long-term (cenennial) ocean transients. • Are ocean feedbacks realistically represented in the models? • quantify the contribution of each feedback to the SST response on the IPCC AR4/CMIP3 models. Compare with observations. • Is the weakening of the Walker circulation an externally driven process or is it coupled?

15. Annual-mean net heat into the ocean in W/m2 (ERA40 Atlas)