Axel Timmermann F.-F. Jin, J.-S. Kug & S. Lorenz Y. Okumura

1. Past and Future Amplitude Changes of ENSO Axel Timmermann F.-F. Jin, J.-S. Kug & S. Lorenz Y. Okumura

2. Is El Nino still a big story? • Unresolved questions: • What determines amplitude? • ENSO irregularity • Phase-locking to annual cycle • Skewness of ENSO • Bursting of ENSO • ENSO and TIWs • ENSO and ocean biology • Past ENSO variability • Response to GH warming

3. What controls the amplitude of ENSO? Nino 3 SSTA Noise level dT/dt=f (T,u..)+Σ(T)ζ Coupling strength ENSO variance maybe skewness Strength of annual cycle dT/dt=f (T,u..)+Asinωt Nonlinearities dT/dt=f (u'T', Σ(T)ζ) Background state dT/dt=f (T,u,h,v,w) External factors

4. Example 1:ENSO’s response to orbital forcing ENSO variance maybe skewness Strength of annual cycle dT/dt=f (T,u..)+Asinωt Background state dT/dt=f (T,u,h,v,w) External factors, Orbital forcing

5. ECHO-G simulation: 140ka B.P.– 20ka A.P. Annual cycle ENSO ka Zonal SST gradient: obliquity cycleACY and ENSOamplitude: precessional cycle

6. ECHO-G simulation: 140ka B.P.– 20ka A.P. Meridional SST gradient: precessional cycleACY and ENSOamplitude: precessional cycle

7. ECHO-G simulation: 140ka B.P.– 20ka A.P. ACY strength is driven by meridional SST gradientmeridional SST gradient varies with precessional cycleWHY?

8. Emergence of an annual mean precessional cycle Annual cycle of cloud albedo Annual cycle of cloudiness > ~0 < > ≠0 <

9. ENSO response to orbital forcing

10. Example 2:ENSO’s response to AMOC collapse ENSO variance maybe skewness Strength of annual cycle dT/dt=f (T,u..)+Asinωt Background state dT/dt=f (T,u,h,v,w) External factors, AMOC collapse

11. Tropical Pacific response to Heinrich I Pahnke et al. 2007 NADW McManus 2004

12. Tropical Pacific response to AMOC collapse GFDL CM2.1 Waterhosing Experiment Timmermann et al 2007 Stouffer et al 2006

13. Tropical Pacific response to Caribbean SSTA Linear moist baroclinic model coupled to tropical POP

14. CGCM Hosing Experiments (CMIP) Freshwater flux anomaly in N Atlantic (50-70N) (1Sv X 100 yrs; ~9m increase in sea level) 1Sv Year 100 200 Monthly SST, Z20, wind stress (precipitation, geopotential height)

15. Tropical Pacific response to AMOC shutdown 5 waterhosing experiments conducted as part of CMIP Weakening of annual cycle and Intensification of ENSO

16. Timmermann et al. (2005) Weakening of the AMOC Cooling of North Atlantic Caribbean anticyclone Cooling of northeastern tropical Pacific Timmermann et al. (2007) Intensification of Northeasterly trades In tropical Pacific Equatorial thermocline shoaling Weakening of Annual cycle in Equatorial Pacific Strengthening Of ENSO

17. Example 3: Noise-induced intensification of ENSO under greenhouse warming conditions Noise level dT/dt=f (T,u..)+Σ(T)ζ ENSO variance maybe skewness Background state dT/dt=f (T,u,h,v,w) External factors Greenhouse warming

18. Noise-induced intensification of ENSO Eisenman et al. 2005 WWB modulation by temperature for present-day climate

19. Noise-induced intensification of ENSO WWB modulation by temperature (BMRC MJO activity) Correlation/Regression between Nino3 SSTA and 20-60 day band-pass filtered wind variance

20. Noise-induced intensification of ENSO AR4 models simulate increased Intraseasonal variability WWB-ENSO interaction increased during the last 50 years

21. ENSO recharge model with state-dependent noise Coupling strength and noise may change slowly over time

22. ENSO recharge model with state-dependent noise Ensemble mean equation for ENSO State-dependent noise is “coupling” State-dependent noise is also “nonlinearity”

23. Past and future changes of ENSO amplitude • Control of ENSO amplitude is a complicated story: not only linear instability • We need better theory for annual cycle-ENSO interactions • We need better theory for WWB-ENSO interactions • We need more realistic representations of WWBs in CGCMs

24. Past and future changes of ENSO amplitude HADCM3 multi-model Ensemble: Relationship between Global climate sensitivity and Simulated NINO3 stdv Processes that amplify Global warming weaken ENSO ??? From Collins, pers. comm.

25. We see no statistically significant changes in amplitude of ENSO variability in the future, with changes in the standard deviation of the Southern Oscillation Index that are no larger than observed decadal variations. (Oldenborgh et al. 2005). From Oldenborgh