Ants Leetmaa Geophysical Fluid Dynamics Laboratory Princeton, NJ 08540 March 29, 2005

1. Results from GFDL IPCC Simulations: Climate of the 20th Century Opportunities for NOAA Climate Program Ants Leetmaa Geophysical Fluid Dynamics Laboratory Princeton, NJ 08540 March 29, 2005

2. Challenge • Establish physics-based framework for nature and origins of decadal to centennial trends during the 20th century of the coupled ocean-atmosphere system • Understand interactions between ENSO and warming trends • Improve seasonal forecasts • Establish credible projections for the next 10 - 30 years • Tools • Ocean and atmospheric observational data sets (not collected for the purposes of detecting small trends) • Coupled models forced with changes to radiative forcings resulting from natural and anthropogenic causes • Atmospheric models forced with observed sea surface temperatures • How far can we push this approach?

3. Climate Forcing Scenarios Used in IPCC 2007 for Attribution Studies and Projections of Possible Future Changes Projections with large uncertainty Atmospheric CO2 concentrations Focus on Climate of 20th Century for Attribution Studies reconciling models and data – results most certain A U.S. focus on 2000-2050 for exploring if..then technological mitigation scenarios as part of CCSP/CCTP – more certain - continuation of existing trends

4. Need good agreement to be a “player” • However, “goodness of fit” does little to convince critics • Need to do regionality+

5. Going Beyond Global to “Regional” Trends – What is (are) the Underlying Physics Which Links These? Understood Physics Trumps Simulations Other examples also exist: Sahel rainfall, global stratospheric and tropospheric temperature, global ocean salinity …

6. Major Processes Inducing Regional Changes – a Conceptual Picture “Observed radiative change” at top of atmosphere Changes in radiative forcing remote Local – North America Land processes Oceans (tropical?)

7. New This IPCC: Good Agreement for Global Mean Tropospheric Temperature Trends: MSU Channel 2 vs GFDL GCM Linear Trends (1981-1999) MSU: 0.14 K/decade GCM: 0.16 +/- 0.02 K/decade Model forced with observed SSTs and greenhouse gases, solar, volcanic and aerosols Model forced with just SSTs (e.g. past history of forcings) gives same level of agreement! How is the SST communicated to the troposphere – a dominant role for the tropics? Is there more to the trend than just unequal numbers of El Ninos/La Ninas/volcanoes?

8. Existence of Spatial Structures to (20yr) Trends in T2 Model – specified SST & all forcing MSU Enhanced ridges Model – specified SST Coupled Model “forecast” – all forcings Enhanced ridges El Nino variability too high (Radiative forcings add signal) Topics too warm What other evidence is there for this off-equatorial ridging? Reanalyses? Ocean changes?

9. Z200 Trends: Reanalyses and Climate of 20th Century Simulation Trend over last 40 years DJF simulation good, but yearly mean needs improvement 20 year trends similar but weaker

10. Zonal Mean Z200: ERA40 Reanalysis and Model forced with observed SSTs Model/SSTs Reanalysis Trend (?) plus events Events coincide with neutral & La Nina conditions Possibly getting stronger with time Model overdoes near equatorial height increase trend Model “events” weaker

11. Anomalous Ocean – Atmosphere Structures of 1998-2002 Event Warming pattern getting stronger with time – not equatorially confined

12. Comparison of (1988-1990) and (1998-2002) Events: all anomaly fields stronger for recent event (1984-86) responses weakest – no Indo-Pacific warming

13. Diagnostic Study for 1998-2002: On the Origin of Mid-latitude Ridging

14. GOGA EPOGA IWPOGA Types of Model Experiments for SST-based Attribution Studies Experiments forced with: Observed anomalies prescribed for global ocean Observed anomalies prescribed for ENSO region Observed anomalies prescribed for Indo-Pacific region

15. Standardized SST Anomaly for 1998 - 2002 June 1998 – May 2002 Indo - Pacific Eastern Pacific A Persistent Warming Trend Impacts result from combination of La Nina and Indo-Pacific Warming

16. Attribution of Z200 Changes during 1998 - 2000 Observations Model forced with observed global SSTA’s La Nina – a weak contributor Forcing with Pacific SSTA Major contributor – seasonally almost invariant structures Forced with Western Pacific – Indian Ocean SSTA

17. Simulation of Air Temperature Anomalies - 1998-2004 GOGA Observed EPOGA IWPOGA Indo-Pacific SSTA of dominant importance La Nina a minor player

18. Yearly Attribution Studies Based on Multi-Model Ensembles are Becoming an Integral Part of the Climate Program: Looks pretty good! (but perhaps right answer for wrong reason!) Study for 2003

19. Attribution Study for 2003: Surface climate simulation “looks” reasonable but major disagreement for atmospheric circulation response! Getting off equatorial interaction “correct” perhaps key to understanding model deficiencies Somehow all models (in AMIP mode – including GFDL) have “deficient” response to specified SST anomalies

20. Eastward shift of mean rainfall Gradual warming of tropics Equatorial Trends in Climate of 20 Century (C20C) Runs (observed T and model 8 member mean T & P) Is the eastward rainfall shift real? Agreement with “related” fields suggests that it is, e.g. a real change in the forcing of global circulations has occurred.

21. Global Precipitation Trends: How well do the GFDL models do? “blues” are drying trends Shift into western Pacific not obvious from land observations However, trends captured best in eastern hemisphere suggestive of this region is closes to where the “action” is

22. Ocean combined with land data give coherent pattern of global rainfall changes Salinity (top 500 meters) –model C20C runs trends trends Observations (500m) Obs .12 psu/70 yrs Model .10* psu/70 yrs Obs .04 psu/70 yrs Model .05* psu/70 yrs Obs .08 psu/70 yrs Model .10* psu/70 yrs Obs .06 psu/70 yrs Model .06* psu/70 yrs * approximate Similarity in spatial structure and amplitude of trends suggest that the C20C runs are simulating real shifts in hydrologic cycle – more rainfall in western tropical Pacific/Pacific

23. Simulations and Diagnostic Studies are Leading to an Understanding of the Origins of Central African Drought Simulations with observed SSTs A “prediction” starting in 1860 observed observed One ensemble member Spread from 10 member ensemble • Diagnostic studies show the trends to be driven by warming trends in the Indo-Pacific region and a contribution from the Atlantic • These simulation suggest an anthropogenic impact; previously the observed trends were thought to be result of natural variability. • Idealized runs show similar drought development (1% per year increase of CO2)

24. C20C Runs Suggest North American Trends are Related to Changes in Radiative Forcing and “driven” mostly by redistributions of tropical rainfall U.S. 48-state average surface temperature “Forecast” from 1860 with specified radiative forcing Model observations 1960 to 1980 1980 to 2000 Model slightly overdoes warming trend Rapid warming of the past 20 years well simulated

25. Decadal Average Wintertime Temperature Anomaly for U.S. (deg C.) Observed - determined from atmospheric reanalysis Simulated - model forced with observed ocean temperatures Predicted - model forced with greenhouse gases, volcanoes, solar fluctuations from 1860 to present Coupled simulations seem to do better than those forced with observed SSTs!

26. Seasonality of Long Term Temperature Trends

27. Seasonality of Model Projections • Seasonality and spatial structure of warming (but not cooling) over US similar in model runs and observations • Interestingly simulations also get cooling in N.Pacific and N.Atlantic – features usually attributed to natural variability

28. Summary Models are starting to be capable of explaining decadal and regional climate variability. This enables more credible attribution studies and should lead to enhancements in skill of climate forecasts. A likely significant factor in global trends is the redistribution of tropical rainfall associated with a warming of the tropics – in the observations this is mostly in the Indo-Pacific region There are suggestions that this warming is changing the nature of global teleconnections during ENSO (especially during cooler than normal conditions) and perhaps introducing other kinds of air-sea interactions in this region Routine “attribution” studies need to become a more important part of the climate program to assess: a) limitations of models (coupled vs. uncoupled; fidelity of responses to SSTA); b) better define physical mechanisms; c) define observing and process studies, a key component of an end-to –end program.

29. Terrestrial Carbon Modeling - dominant role of variability Biomass NEP - dry year -1988 precip mm/day Pg Carbon Net Ecosystem Production (NEP) NEP - wet year - 1990 precip Pg Carbon mm/day Inversion range

30. Earth System Model at GFDL: taking pulse of planet • GFDL’s global climate variability and change model plus: • ocean biogeochemistry and biology • CO2 (tracer) transport in atmosphere • Land physics and hydrology – river runoff to oceans • Dynamic vegetation and land use – nitrogen cycle being worked on • Soon – interactive atmospheric chemistry

31. 1860 1990 obs greenhouse NINO3 SST Spectrum Changes Period (years)