Potential Mechanism for Response of ENSO Variability to Change in Land Surface Energy Budget

1. Potential Mechanism for Responseof ENSO Variabilityto Change inLand Surface Energy Budget • Zeng-Zhen Hu(1) • Edwin. K. Schneider (1,2) • Uma S. Bhatt (3) • Benjamin P. Kirtman (1,2) • COLA, Calverton, MD 20705 • (2) SCS, George Mason University, Fairfax, VA 22030 • (3) IARC/Frontier, U. of Alaska, AK 99775

2. Questions Q1: Is there any influence of the land surface property change on ocean and on air-sea coupling, including the mean state of tropical climate and ENSO variability ? Q2: What is the mechanism behind the influence ?For example,what are the relative contributions of the changed mean state and ‘atmospheric noise’ on the tropical climate and how is the sensitivity change?

3. Models • AGCM: • COLA spectral, T30 (5˚ lon by 4˚ lat), L18 • Mass flux convection scheme • Predictive clouds • Prognostic land surface, Simplified SiB • Deep convection parameterized • OGCM: • Princeton GFDL MOM2 • 3˚ by 3˚ (+ enhanced tropical resolution), 20 levels (4000m) • Realistic boundaries & bottom topography • Sub-grid eddies not parameterized • Thermodynamic sea ice • CGCM: • No flux corrections • Air-sea exchanges every 24 hours • Remove climate trends by demean, Lanczos filter (20 yr) lower frequencies, linearly de-trend Model Details in Dewitt and Schneider (1999), Mon. Wea. Rev., 127, 381-395.

4. Experiments Soil wetness specification Simulations Surface zone layer 1: ~0.02m Root soil zone Layer 2: ~0.2-1.5 m Deep Soil Zone Layer 3: ~0.3-2.0 m COU(Control run) Fully coupled among land, atmosphere, ocean OCN (Sensitivity run) Layer 2-3 soil wetness fixed to COU annual cycle, fully coupled between ocean and atmosphere About the experiments:Bhatt, U. S., E. K. Schneider, and D. G. Dewitt, 2003: Influence of North American land processes on North AtlanticSST variability. Global and Planetary Change, 37,33-56. doi:10.1016/S0921-8181(02)00190-X.

5. Land Surface Becomes an Energy Sink in the Sensitivity Run(unexpected and unphysical)Surface Energy Budget: Sink over land, source over E. Tropical Pacific;SH: Sink over the land; LH: Source over the E. Tropical Pacific;

6. The Land Surface Energy Sinkmay be reinforced by unexpected feedback processes: permanent soil moisture source-more evaporation and cloud-less downward shortwave radiation- cooling land surface

7. Mean State Change of Tropical Climate A:TS: Reduces E-W temperature contrast;Precipitation: Shifts land/sea partition of P toward the oceans;SLP: Favors a negative phase of SO.

8. Mean State Change of Tropical Climate A:Ocean subsurface T: Reduces E-W temperature contrast;

9. Mean State Change of Tropical Climate B:(X-Z cross section along Equator)Temperature: Cools land & warms oceans;U: Modifies Walker cell;Div/Con: Div. over land & Con. over oceans, consistent with T/U change.

10. Mean State Change of Tropical Climate C:Ocean Wind Stress: Convergence in E. Tropical Pacific;W: Downwelling in E. Tropical Pacific;Thermocline Depth: Deepening in E. Tropical Pacific;

11. SST Variance Decreases in Central & Eastern Tropical Pacificin the Sensitivity Experiment

12. The ENSO becomes Less Energetic in the Sensitivity Experiment

13. The ENSO Frequencies do not be Changed muchin the Sensitivity Experiment

14. Changes of the Mean State of the Tropical Climate & ENSO variability Fixing deep soil moisture results in a unphysical surface energy sink over land, which may be reinforced by an unexpected feedback. The energy sink causes the following mean state and ENSO variability changes: A: Reduces SST contrast between the eastern and western tropical Pacific; B: Favors negative SO; C: Shifts the land/sea partition of precipitation toward the oceans; D: Changes the wind fields and their divergences; E: Causes downwelling & deepening the thermocline in the eastern tropical Pacific; F: Reduces SST variability in the central and eastern tropical Pacific; Makes the ENSO variability less energetic; G: Has a small influence on the ENSO frequencies.

15. Intermediate Coupled Model & Experiment Designs Model: Zebiak-Cane model modified by Kirtman and Shopf (1998). The model with self-sustained ENSO cycle of about 5 years. Experiments: Test the influence of‘atmospheric noise (1 mon -1 yr)’ and the mean state changes on the ENSO variability. Details of the model are given in: Kirtman & Shopf: 1998, J. Climate.

16. Possible Mechanism A:AtmosphericnoiseRole of atmosphericnoise (time scales < 1 year) is small.

17. Possible Mechanism B:Meanstate changeThe role of mean wind stress differences is crucial

18. Possible Mechanism C:Sensitivity change

19. Interpretation of the Sensitivity Chang • Definition of Sensitivity:Covariance of the wind stress and Nino3 SST divided by the variance of Nino3 SST. • Means of positive differences (OCN-COU) in the central and eastern tropical Pacific are:(a) Given the same wind stress anomaly, the OCN simulationwould have a weaker Nino3 SSTA than COU.(b) The Nino3 SST variabilitybecomes less sensitive to the zonal wind stress in OCNthan in COU. Therefore, from the view of sensitivity change,the Nino3 SST variability would become smaller in OCNthan in COU if the wind stress was the same. (c) But the wind stress is different. The westerly anomalies of wind stress of OCN-COUfurther reinforce the weakening in thesensitivity.

20. Possible Mechanism A: Mean state change plays a key role in determining the ENSO variance change. B: The change of the sensitivity of air-sea coupling favors the reduction of ENSO variance. C: The influence of amplitude change of atmospheric noise on ENSO variance change is minor.

21. Summary • Fixing deep soil moisture results in a unphysical and unexpected surface energy sink over land, which may be reinforced by unexpected feedbacks, cooling in the tropical land. • The cooling alters the mean state of tropical climate and makes the ENSO variability less energetic, but the influence on ENSO frequency is small. • Mean state change, not the decreased atmosphere noise amplitude, plays a crucial role in determining the ENSO variance change. • The unexpected land surface energy sink shows the potential impact of natural and anthropogenic induced change in the land-surface energy budget on ENSO variability and also on mean state of tropical climate. • To study tropical climate variability, should use a land-ocean-atmosphere fully coupled model.

22. Two Implications: • In global warming scenario: Warming over land is normally larger than that over ocean  ENSO variability may increase in global warming scenario. 2. Amazon deforestation : When the Amazonian tropical forests were replaced by degraded grass, there was an increase in tropical land Ts, which might lead to an increase in ENSO variability. This is true in our new experiments.

23. Further Information and Acknowledgements : Further Information : Web page:www.iges.org/pub/ctr/ctr_126.pdf Submission: J. Geophy. Res., 2004 E-mail: hu@cola.iges.org Acknowledgements : This research was supported by the Center for Land Atmosphere Ocean Studies (COLA), the Frontier Research System for Global Change through IARC/Frontier, NSF (ATM 95-20579, AMM 99-07915, & ATM 98-14295), NOAA (NA 96-GP0056), NASA (NAG 5-8202), and DOE (De-FG02-01ER63526). The authors are indebted to L. Bengtsson, P. Schopf, F.-F. Jin, B. Huang, L. Marx, P. Dirmeyer, D. Straus, and M. Zhao for their discussion and suggestions. We would like to thank Christopher Swingley (IARC/Frontier) for assistance with computer issues.

24. Summary • Fixing deep soil moisture results in a unphysical and unexpected surface energy sink over land, which my be reinforced by unexpected feedbacks, cooling in the tropical land. • The cooling alters the mean state of tropical climate and makes the ENSO variability less energetic, but the influence on ENSO frequency is small. • Mean state change, not the decreased atmosphere noise amplitude, plays a crucial role in determining the ENSO variance change. • The unexpected land surface energy sink shows the potential impact of natural and anthropogenic induced change in the land-surface energy budget on ENSO variability and also on mean state of tropical climate. • To study tropical climate variability, should use a land-ocean-atmosphere fully coupled model.

25. Atmospheric Noise Change

26. Implication I: Global warming Scenario (IPCC, 2001) • Warming over land is larger than that over ocean; • It amplifies the ENSO variability. Figure 9.10: The multi-model ensemble annual mean change of the temperature (color shading), its range (thin blue isolines) (Unit: °C) and the multi-model mean change divided by the multi-model standard deviation (solid green isolines, absolute values) for the CMIP2 scenarios at the time of CO2-doubling.

27. Implication II: Land use change(Nobre, Sellers, & Shukla, 1991, J. Climate) • When the Amazonian tropical forests were replaced by degraded grass, there was a significant increase in tropical Ts and in subtropical precipitation, and a decrease in tropical precipitation. • Based on the conclusion of the present study, cooling (warming) over the Amazon might result in a reduction (enhancement) of ENSO variability. • Thus, Amazon deforestation might lead to an increase in ENSO variability.

28. 1000 hPa Specific Humidity

29. 1000 hPa Relative Humidity

30. Long-wave and Short-wave Radiations

31. The Results of Kirtman and Shopf (1998, J. Climate, 2804-2822)Detailed change of surface wind stress is important !!

32. The Results of Kirtman and Shopf (1998, J. Climate, 2804-2822)