1. Whither ENSO? Assessing El Niño/Southern Oscillation risks for the coming decades Andrew Wittenberg NOAA/GFDL

2. Earth's dominant year-to-year climate fluctuation: NOAA/CPC El Niño Normal

3. How will ENSO behave in the coming decades? CMIP5 & AR5 – new focus on near-term (30yr) projections. ENSO drives/confounds global SI-to-decadal variability. If we knew the next decade would have: - mega-ENSO → insure, invest, prepare, monitor & model - no ENSO → different investments, e.g. rebuild habitats

4. Both historical & paleo records suggest past modulation of ENSO Vecchi & Wittenberg (WIREsCC 2010) Historical SSTA (ERSST.v3) Palmyra corals (Cobb et al., Nature 2003)

5. CMIP3/AR4 model projections (Meehl at al., IPCC-AR4 2007) 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. - Decadal ENSO risk: volcanoes, intrinsic modulation, memory.

6. 4000-year pre-industrial control run 1860 atmospheric composition, insolation, land cover 220yr spinup from 20th-century initial conditions big investment: 2 years on 60 processors Delworth et al., Wittenberg et al., Merryfield et al., Joseph & Nigam (JC 2006), Wittenberg (GRL 2009) Zhang et al. (MWR 2007); van Oldenborgh et al. (OS 2005); Guilyardi (CD 2006); Reichler & Kim (BAMS 2008) Kug et al. (JC 2010), Vecchi & Wittenberg (WIREsCC 2010), Collins et al. (Nature Geosci. 2010) 1990 control (300yr), 2xCO 2 (600yr), 4xCO 2 (600yr) 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 (ESM2M, ESM2G, CM3, CM2.5)

7. What sort of ENSO do we simulate? These are from a single run with unchanging forcings. strong, skewed, long period, eastward propagating (1980s & late 1990s) weak, biennial, “Modoki” (early 1990s & 2000s) regular & westward propagating (1960s & 70s)

8. 20 centuries of simulated NINO3 SSTs annual means & 20yr low-pass Wittenberg (GRL 2009)

9. Modulation of NINO3 SST power spectrum 2000yr mean (e.g. satellites, TAO)‏(e.g. reconst SST)‏ Wittenberg (GRL 2009)

10. Centuries of weak or strong ENSOs

11. CM2.1 mean state hardly differs between active/inactive ENSO centuries

12. Will forecasts capture anything at all? How predictable are decades of extreme ENSO? Will forecast ensembles differ between epochs? °C°C °C°C Will forecasts capture the intensity of the epoch?Will forecasts capture events, if not their timing?Will forecasts track the control?Tiny perturbation: +0.0001C at one gridcell (equator, 180W, top 10m) NINO3 SSTA

13. “Perfect” ensemble reforecasts Some members resemble the control. (forecasts with minimum NINO3 SST RMS error over each decade) °C°C °C°C

14. “Perfect” ensemble reforecasts Other members look nothing like the control. (forecasts with maximum NINO3 SST RMS error over each decade) °C°C °C°C

15. “Perfect” ensemble reforecasts These are what perfect forecasts look like! (perfect model, near-perfect initial conditions, 40 members) °C°C °C°C

16. “Perfect” ensemble reforecasts Summarize the ensemble PDF with quartiles. 25th and 75th percentiles of NINO3 SSTA, from 40 members °C°C °C°C

17. “Perfect” ensemble reforecasts Quartiles “forget” initialization after a few years. gray: 95%-bands for control quartiles, from 5000 resampled 40-ensembles °C°C °C°C

18. Ensemble spread 40-member interquartile range (IQR), with 95%-band from control °C°C °C°C “Perfect” ensemble reforecasts

19. Decadal statistics of ENSO Can we predict the epoch-mean amplitude? Absolute value of NINO3 SSTA (degC) °C°C °C°C

20. Decadal statistics of ENSO Smoothed measure of ENSO activity NINO3 SSTA amplitude, smoothed with 4yr running mean °C°C °C°C

21. Decadal statistics of ENSO Ensemble forecasts of ENSO activity smoothed NINO3 SSTA amplitude from 40 members °C°C °C°C

22. Decadal statistics of ENSO Activity distributions also “forget” the ICs {10,50,90}-percentiles of smoothed amplitude, with 95%-bands from control °C°C °C°C

23. Long-term memory? median 6yr 10% >15yr 5-year wait most common recharge delay consistent with Poisson But beyond 10 years? Distribution of inter-event wait times suggests that NINO3 SSTA might have some memory beyond 5 years. Even a purely memoryless ENSO would give occasional waits of 20 years or more, as seen in CM2.1. Wittenberg (GRL 2009)‏ 3822yr / 495 events = 7.7yr mean wait

24. ENSO events and their nearest neighbors strong events more isolated weak EN, 3yr after strong EN strong EN, 4yr after weak EN

25. Best hope for long-term ENSO predictability? NINO3 memory might last 5yr, following strong warm events.

26. Best hope for long-term ENSO predictability? NINO3 memory might last 5yr, following strong warm events.

27. Given enough years, we can say...

28. CM2.1 ENSO is too strong

29. CM2.1 ENSO is very sensitive to some parameters

30. Last Glacial Maximum (20ka)‏ tropical SST cools 3°C TC deeper & more diffuse

31. Mid-Holocene (6ka)‏ perihelion shifts from Jan -> Oct; less SH seasonality seasonal/ENSO confounding in paleo proxies?

32. Pre-industrial range of 100yr spectra

33. 1990: ENSO strengthens, spectrum narrows

34. 2xCO2: slightly shorter period than 1990

35. 4xCO2: ENSO weaker than at 2xCO2

36. Can we extrapolate ENSO projections 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

37. Summary 1. CMIP3/AR4 projections were ambiguous for ENSO: a. Diverse responses to anthropogenic forcings b. ENSO modulation in models, historical/paleo records 2. 4000-year run of pre-industrial CM2.1: a. Strong intrinsic modulation of ENSO b. Extreme ENSO centuries: not due to climate shifts c. Extreme ENSO decades: - Multidecadal lulls consistent with memoryless ENSO - ENSO memory up to 5 years after strong warm events - After that, even perfect forecasts don't beat a memoryless PDF 3. With long enough ENSO records, we can still detect: a. Model biases & sensitivities to some parameters/forcings b. CO2 impacts (barely detectable with 100yr record) c. CO2 optimum for ENSO - A source of disparate model sensitivities? - How close is the optimum, and which side are we on?

38. Projecting ENSO risks for the coming decades 1. What is ENSO capable of on its own? - long runs, large ensembles - historical/paleo reanalyses, pseudoproxies - impacts of extreme events 3. Understand ENSO's sensitivities - primary controls, feedbacks & nonlinearities - diverse tests: forecasts, volcanoes, paleo, idealized - model diversity + physical understanding -> extrapolation to reality 4. Decadal forecasts - does precise initialization matter for ENSO? - intrinsic modulation may dominate ENSO behavior over our lifetimes 2. Improve models, understand & convey their uncertainties - metrics: robust, grounded in theory (ICMs), community-wide - AR5: new feedbacks, better resolution/physics -> different projections?

39. Reserve Slides

40. Spectrum of NINO3 SST Wittenberg (GRL 2009)

41. less phase-locking of cold events & weak warm events strong warm events peak in SON warm events are stronger & rarer than cold events CM2.1 ENSO peaks vs. calendar month abs(NINO3) > 1 stddev

42. CM2.1 mean state hardly differs between active/inactive ENSO centuries 100yr-mean SST & trades are robust diagnostics for CM2.1

43. Inactive centuries have slightly warmer water in the west Pacific

44. Active/inactive centuries show no systematic difference in the scaled anomaly patterns

45. Active/inactive centuries show no systematic difference in the scaled anomaly patterns

46. Warm pool intraseasonals are slightly more variable during active-ENSO centuries

47. Two extreme epochs Activity spread 40-member IQR of 4yr-smoothed amplitude, with 95%-band from control °C°C °C°C