Considering Climate Variability & Change TOGETHER

1. Considering Climate Variability & Change TOGETHER Lisa Goddard International Research Institute for Climate & SocietyThe Earth Institute of Columbia University goddard@iri.columbia.edu

2. OUTLINE • Possible ENSO-like changes in tropical Pacific mean state • Possible changes in ENSO variability under climate change Main Points • The tropical Pacific climate is controlled by coupled ocean-atmosphere dynamics. Getting both these aspects right is necessary for getting a realistic estimate of climate change. • Variability and change may have similarities, but they have many differences also. • Change in the mean state does not necessarily imply a change in variability, and vice versa. We are really into the realm of ‘active research’ here.

3. ENSO-like Changes in Mean Climate • Will it be La Niña-like? • Will it be El Niño-like? - What does theory say? - What do the models say? - What do the observations say?

4. Tropical Pacific Trend Pattern vs ENSO Variability Source: IPCC AR4 WG1, Chp 10

5. Example of Tropical Pacific SST ChangeversusComposite El Nino SST Anomalies Temperature Change (2071-2100)-(1971-2000) El Nino Composite (20th C) --------------- GFDL 2.0 --------------- --------------- GFDL 2.1 ---------------

6. ENSO-like Changes in Mean Climate • Will it be La Niña-like? • Will it be El Niño-like? - What does theory say? 2 Main theories: - Ocean thermostat mechanism - Weaker Walker Circulation mechanism

7. 1. Theoretical mechanism for radiative forcing of ENSO-likestates in the tropical PacificOcean Thermostat mechanism Cane, M.A., A.C. Clement, A. Kaplan, Y. Kushnir, D. Pozdnyakov, R. Seager, S.E. Zebiak, and R. Murtugudde, Twentieth-Century Sea Surface Temperature Trends, Science, 275, 957-960, 1997. (Courtesy Mike Mann)

8. Ocean ‘thermostat’ mechanism Mean modern conditions (Clement et al. 1996) (Courtesy Mike Mann)

9. Uniform heating (Courtesy Mike Mann)

10. Larger temperature response in the West Cooling by upwelling opposes forcing in the East, reducing temperature response (Courtesy Mike Mann)

11. Larger temperature response in the West Coupled interactions (i.e. the Bjerknes feedback) amplify the East/west temperature difference Cooling by upwelling opposes forcing in the East, reducing temperature response (Courtesy Mike Mann)

12. RESULT: A new Pacific mean state with a STRONGER EAST/WEST GRADIENT (Courtesy Mike Mann)

13. 2. Theoretical mechanism for radiative forcing of ENSO-likestate in the tropical PacificWeaker Walker mechanism Water vapor in lower troposphere increases as expected by Clausius-Clapeyron (in the models), but precipitation does not, due to atmospheric energy constraints. This implies that circulation must slow down. (Held and Soden, 2006, J. Climate;Vecchi and Soden, 2007, J. Climate)

14. 2. Theoretical mechanism for radiative forcing of ENSO-likestate in the tropical PacificWeaker Walker mechanism Some suggestion that such a slow down in the Walker Circulation is being observed… Linear Trend in SLP in Observations & Model (Vecchi et al, 2006, Nature)

15. Ocean Thermostat or Weaker Walker Mechanism ?What do the models say? Model complexity seemsto make a difference inwhat mechanism dominatesthe response to radiativeforcing. Contains active ocean dynamics but very simplified atmosphere Contains atmosphericenergy constraints butocean dynamics are fixed Contains both fully dynamicaland interactive ocean and atmosphere models. (Vecchi, Clement & Soden, 2008, EOS)

16. Ocean Thermostat or Weaker Walker Mechanism ?What do the observations say? The observations don’tanswer the question eitherbecause there is so muchuncertainty in the earlierdata. (Vecchi, Clement & Soden, 2008, EOS)

17. Changes in El Niño Variability • More or less? • Stronger or weaker? - Does oceanic component of ENSO change? - Do teleconnections (at least in the tropics) change?

18. Tropical Pacific Trend Pattern vs ENSO Variability Source: IPCC AR4 WG1, Chp 8

19. Coelho, C.A.S. and L. Goddard, 2009. El Niño-induced tropical droughts in climate change projections. J. Climate, DOI: 10.1175/2009JCLI3185.1 Motivating Questions 1) How well do coupled models simulate tropical droughts associated with El Niño during the 20th century? 2) How do ENSO-related drought patterns change in 21st century? 3) To what extent does the risk of presently defined drought during El Niño conditions change under climate change?

20. Observations OtherCoupled Models IPCCmodels ENSO Characteristics in IPCC Models Differences in Magnitude of ENSO events… … and their frequency Source: AchutaRao and Sperber, 2006

21. DATA IPCC Coupled Models: (20C3m; A2) • “WEAK” Models • CGCM3_T47, MIROC_mres, GISS-ER • “MODERATE” Models • NCAR_CCSM3, UKMO_HadCM3, GFDL_CM2.0 • “STRONG” Models • CNRM_CM3, MPI_ECHAM5, GFDL_CM2.1 Observations: • UEA-CRU Precipitation

22. “Good” ENSO Models • “MODERATE” Models • NCAR_CCSM3, UKMO_HadCM3, GFDL_CM2.0 • “STRONG” Models • CNRM_CM3, MPI_ECHAM5, GFDL_CM2.1 Guilyardi (2006) tropical Pacific climatology e.g. strength of seasonal cycle (winds & SST), seasonal cycle locking, Interannual coupling strength van Oldenborgh et al. (2005) coupled air-sea variability e.g. spatial structure and power spectrum of SSTa, wind stress & thermocline variability, air-sea coupling parameters

23. El Niño Events in IPCC Models Anomaliesrelative toevolvingcurrentclimatology Threshold based on 75th %-ile of climatology TOO WEAK MODERATE TOO STRONG (Coelho & Goddard, 2009, J. Climate)

24. El Niño Magnitude NOT Projected to Change Too much event-to-event variability to attribute change in any particular realization (Coelho & Goddard, 2009, J. Climate)

25. El Niño Frequency NOT Projected to Change Conclusions based on any single model is likely to contain substantial sampling bias (Coelho & Goddard, 2009, J. Climate)

26. ENSO FrequencySensitivity to Record Length (Wittenberg, 2009)

27. Motivating Questions 1) How well do coupled models simulate tropical droughts associated with El Niño during the 20th century? 2) How do ENSO-related drought patterns change in 21st century? 3) To what extent does the risk of presently defined drought during El Niño conditions change under climate change?

28. Note: 5 month lag between max. Nino 3.4 SSTA and extent peaks Tropical Drought and El Niño (B. Lyon, 2004, GRL)

29. IPCC C20C Model CompositesEl Niño (DJF) 30-year moving climatologyModels categorized by ENSO Variance IPCC models with reasonable ENSO variabilityreproduce reasonable El Nino teleconnectionscompared to seasonal forecast models. Strength of composite teleconnections scaleswith strength of ENSO events. (Coelho & Goddard, 2009, J. Climate)

30. IPCC Model Composites: 20C vs 21CEl Niño (DJF) 30-year moving climatologyModels categorized by ENSO Variance (Coelho & Goddard, 2009, J. Climate)

31. Precipitation ChangesDJF (2071-2100) – (1971-2000)Models categorized by ENSO Variance “MODERATE” “WEAK” “STRONG” No statistically significant tendency in patterns of mean precipitation changerelated to strength of a model’s ENSO variability. (Coelho & Goddard, 2009, J. Climate)

32. How to Make a Multi-Model (MM) Ensemble How to indicate robustness or confidence in the MME? • MM Mean • % models agreeing in sign of change • MM Signal > Inter-model Noise • Choosing (or weighting) a subset of models according to ??

33. Precipitation Changes (%)(2090-2099) – (1980-1999) Mean + % of models agreement White areas where less than 2/3 models agree in sign of change.Stippled areas where more than 90% of models agree in sign of change.

34. Precipitation Changes (mm/day)(2090-2099) – (1980-1999) Mean + (S/N > 1) Stippling denotes areas where the magnitude of the multi-model ensemble mean exceeds the inter-model standard deviation.

35. El Niño-related Drought Risk Based on 20th Century Observations: + Anthropogenic Precipitation Changes Based on IPCC Models for 21st Century: Concept of Layering  El Niño-related Drought Risk Projected for end of 21st Century

36. Concept of Layering

37. Drought Risk Now & In the FutureDJF: End 20th C vs End 21st C (Coelho & Goddard, 2009, J. Climate)

38. Drought Risk Now & In the FutureJJA: End 20th C vs End 21st C (Coelho & Goddard, 2009, J. Climate)

39. Using ENSO Fidelity to Vet Models DJF Much of thisencouraging resultis due to sampling, (i.e. only using4 models ratherthan 20+) JJA (Coelho & Goddard, 2009, J. Climate)

40. CONCLUSIONS: Tropical Pacific Mean State CONCLUSIONS: El Niño Variability(from CMIP3) • Changes in the tropical Pacific mean state appear to be controlled by both ocean dynamics, and energy constraints of the atmosphere • No discernible change in relative El Niño strength or frequency in IPCC models (according to individual models) • IPCC models, that do contain ENSO, exhibit realistic ENSO teleconnection patterns, which do not appear to change due to global warming. • Layering of information:trends from IPCC models + variability from observations (in the tropics) provides more meaningful guidance on drought risk than using the models alone. • Local confidence and specificity in such projections must be communicated carefully.