1. The Role of ENSO in Regulating its Background State De-Zheng Sun Tao Zhang CU/CIRES/Climate Diagnostics Center &NOAA/Earth System Research Laboratory Boulder, Colorado http://www.cdc.noaa.gov/people/dezheng.sun

2. Background Both the oscillator theory and the stochastic theory of ENSO have the background state prescribed, leaving the question whether ENSO in turn plays a role in determining the background state unaddressed. Answering this question, however, is critical for a number of climatic issues including understanding the response of ENSO to global warming and diagnosing the causes of tropical biases in coupled GCM simulations.

3. Our Hypothesis The recurrent occurrence of El Nino and La Nina events--ENSO--may be a mechanism that prevents the time-mean state of the coupled tropical Pacific ocean-atmosphere from becoming substantially unstable.

4. The Methodology Conducting perturbation experiments in pairs with a coupled model; Turning off ENSO in one of the two experiments; Contrasting the differences in the response to the perturbations between the case with ENSO and the case without ENSO. The perturbations are enhanced tropical heating or enhanced extratropical cooling.

5. The Model Atmospheric component: empirical, F s ~SST p -SST,  x ~SST E -SST w Ocean component: The NCAR Pacific basin model Sun, D.-Z., 2003, J. Climate, 16, 185-205 Sun, D.-Z., T. Zhang, S.-I. Shin, 2004, J. Climate, 17, 3786-3798

6. A key parameter that measures the stability of the coupled tropical ocean-atmosphere system Tc Warm-pool Undercurrent Tw Warm-pool

7. Response of Tw-Tc with and without ENSO Tropical Heating Experiments

8. Response of the equatorial ocean temperature to tropical heating Without ENSOWith ENSO

9. Response of ENSO Amplitude to Tropical Heating

10. Equatorial Ocean Temperature Response During La Nina and El Nino

11. Destabilizing the tropics from the extratropics: the “ocean-tunnel” Water constituting the equatorial undercurrent and therefore the upwelling water in the equatorial Pacific comes from the subtropical/extra-tropical region (McCreary and Lu 1994, Pedlosky 1987)

12. Response in the upper ocean temperature to extratropical cooling Without ENSOWith ENSO

13. Response of ENSO amplitude to extratropical cooling

14. Response in the upper ocean temperature to extratropical cooling Without ENSOWith ENSO

15. Conclusion ENSO acts as a basin-scale heat “mixer” that prevents any significant increase from occurring in the time-mean difference between the warm-pool SST (Tw) and the temperature of the thermocline water (Tc).

16. A New Paradigm for Understanding How ENSO Responds to Global Warming CO2 2xCO 2 ENSOMean Climate 2xCO 2 Mean ClimateENSO Existing Paradigm: A Revised Paradigm:

17. Implications Climate models that do not have good simulations of ENSO may not give reliable predictions of the response of the mean climate to global warming. The excessive cold-tongue in the coupled GCM simulations of the time-mean tropical Pacific SST may be a consequence of the underestimate of the ENSO activity in these models. Our existing paradigm to understand the response of ENSO to global warming needs to be modified.

18. Response of Tw-Tc without and with ENSO Extratropical Cooling Experiments

19. Ocean Temperature Difference During La Nina and El Nino

20. 120 o E-160 o E 160 o E-210 o E 120 o E-160 o E

21. Meridional Structure of Ocean Temperature Response During La Nina and El Nino 120 o E-160 o E 160 o E-210 o E 210 o E-290 o E

22. Response in the upper ocean temperature to tropical heating Without ENSOWith ENSO

23. Mean temperature response to tropical heating: a meridional view

24. Temperature differences during La Nina: a meridional view 120 o E-160 o E 160 o E-210 o E 210 o E-290 o E

25. Temperature differences during El Nino: a meridional view 120 o E-160 o E 160 o E-210 o E 210 o E-290 o E

26. Ocean Temperature Response During La Nina and El Nino

27. Mean temperature response to subtropical cooling

29. Ocean Temperature Response During La Nina and El Nino