Aerosol, Interhemispheric Gradient, and Climate Sensitivity

1. Aerosol, Interhemispheric Gradient, and Climate Sensitivity Ching-Yee ChangDepartment of GeographyUniversity of California Berkeley Collaborators: John Chiang (UC Berkeley)Michael Wehner (Lawrence Berkeley Lab) Lawrence Livermore National Lab Seminar April 27, 2011

2. Sulfate Aerosols and Climate Direct forcing from anthropogenic sulfate forcing (Kiehl & Briegleb 1993) IPCC AR4 Difference in SAT caused by sulfate aerosol indirect effect (Rotstayn & Lohmann 2002)

3. Large uncertainty in the aerosol forcing IPCC AR4 Kiehl 2007

4. Outline • Sulfate aerosol control of Tropical Atlantic climate over the 20th century (Chang et al. 2011, in press for Journal of Climate) • Projection of the Interhemispheric Gradient in 21st century • Interhemispheric Gradient and Transient Climate Response

5. Sulfate aerosol control of Tropical Atlantic climate over the 20th century

6. Atlantic interhemispheric SST gradient over the 20th century • Interhemispheric SST index = South – North box • 21-yr running mean on annual mean data • Hadley SST, ERSST, Kaplan SST HADISST ERSST KAPLAN SST

7. Ensemble Empirical Mode Decomposition (EEMD) Modes from Ensemble Empirical Mode Decomposition (EEMD) analysis: Multidecadal (mode 4) and trend (mode 5) HADISST ERSST KAPLAN SST

8. The interhemispheric SST gradient and themeridional position of ITCZ Mode 1 of an MCA of SSTA and 10m winds, and regression of the SSTA mode 1 time series on precipitation (Chiang and Vimont, 2004)

9. Equatorial meridional winds ICOADS Winds June-July-August precipitation Smith et al. 2010: Reconstructed global precip south-north CRU TS 2.1 land precip.

10. CMIP3 models simulation of Atlantic Interhemispheric Gradient in 20th century climate experiment

11. 1st EOF of AITG indices from 71 model runs Variance explained by this EOF: 49% South Hemisphere warming more than North HemisphereThis trend mitigates after 1980 Projection of EOF1 onto each run Most of the projection coefficient are positive (Most models have a upward trend in the SSTA gradient indices)

12. Mean of 1900-1982 trend in SSTA gradient significantly different from preindustrial run Unit: 0.1K/100yr • T-test value=4.41 • p-value = 0.00001 • (assuming 71 d.o.f. for the 20th century runs, and 44 for the preindustrial)

13. Regression of SSTA onto model-index EOF1 Hadley SSTA Model ensemble averaged SSTA • Stronger warming in the South Atlantic

14. Regression of Precip. Anomaly onto model-index EOF1 CRU Precip. anomaly Model ensemble averaged Precip. anomaly • Southward shift of ITCZ

15. Attribution the cause of the trend • Trend behavior appears in model ensemble mean • Most likely to be externally forced • Single forcing runs

16. most resembles the 1st EOF of the indices of the 20C expt. Results from single forcing runs SST gradient index ITCZ index CCSM3 (2 members) PCM1 (4 members) GISS modelE (1 member)s)

17. CCSM3 sulfate aerosol emission forcing data CCSM3 simulated sulfate aerosol optical depth

18. EOF1 from different subsets of models AIE models: Models with both Aerosol Direct and Indirect Effect No-AIE models: Models with only aerosol direct, but no Indirect Effect X-axis unit: 0.1K/100yr • EOF1 from AIE models capture the turn of the trend better • AIE models simulate the AITG trend closer to observation

19. Modeled SSTA and Precip.A regression on model-index EOF1 Models with Aerosol Indirect Effect Models without Aerosol Indirect Effect Warming asymmetry and ITCZ southward shift stronger in AIE models

20. Summary I • Interhemispheric gradient of Atlantic SST found to have an upward trend before 1980, indicating stronger warming in the South Atlantic and southward shift of ITCZ • A similar positive trend is detected in the IPCC models. • This trend is likely due the north-south disparity in anthropogenic sulfate aerosol emissions

21. Projection of the Interhemispheric Gradient in 21st century Indices are defined as south box minus north box 3 different scenarios are examined A1B, A2, B1 Both Atlantic and Pacific sectors

22. Global mean of various anthropogenic forcing agents in future scenarios IPCC AR4 WG1, Fig.10.26

23. Pacific 95-year (2004~2098) trend statistics Atlantic A1B A2 B1 Most models project downward trend of the Pacific index in the 21th century in these 3 future scenarios => North Pacific warming stronger than South Less conclusive results on the projection of the Atlantic index trend X-axis unit: 0.1K/100yr

24. Pacific A1B Atmos. Sulfate burdenunit: 10e-6 kg/m2 (From miub_echo model) High-lat index (35~60) Tropical index (5~35) Atlantic Stronger change in the interhemispheric gradient of sulfate aerosol forcing across the equator in the Pacific sector It’s projected that most of the decrease of sulfate aerosol mainly comes from Asia More aerosol emission from Tropical Atlantic than from North Atlantic

25. Atmospheric Sulfate burden A1b A2 B1 All three scenarios have stronger change in sulfate aerosol forcing across the equator in the Pacific sector

26. 1%/yr to 2xCO2 experiment However, similar change of Pacific gradient is found in 1%/yr to double CO2 experiment, but with weaker magnitudes

27. Comparison:1%/yr to 2xCO2 and A1B experiments (Yr60~Yr80) – (Yr1~Yr20) (2079~2098) – (2005~2024)

28. Summary II • Most models project negative trend in the Pacific interhemispheric gradient – the rate of the warming in the north Pacific speeds up at the end of 21st century • Possibly related to the decrease of the aerosols in the north Pacific in the future, but GHG forcing or other factors may also contribute

29. Interhemispheric Gradient and Climate sensitivity

30. Trend Statistic from different subsets of models AIE models: Models with both Aerosol Direct and Indirect Effect No-AIE models: Models with only aerosol direct, but no Indirect Effect X-axis unit: 0.1K/100yr • AIE models simulate the AITG trend closer to observation

31. Kiehl 2007: Total forcing inversely correlated to climate sensitivity Large uncertainty in the aerosol forcing

32. Equilibrium Climate sensitivity and Transient Climate Response 600 ppm CO2 concentration T’ + Slab ocean AGCM 300 ppm T0 • Transient Climate Response CO2 concentration T’ 600 ppm + AGCM OGCM 300 ppm T0 IPCC AR4 Table 8.2 Equilibrium Climate Sensitivity

33. Climate sensitivity and Transient Climate Response IPCC AR4 Table 8.2

34. Atlantic SSTA Grad. Trend v.s. Climate Sensitivity • There seems to be a linear relationship between the gradient trend and the Transient Climate Response (TCR) among most of the models

36. Regional Transient Climate response Regional Transient Climate Responses (TCR) in the Tropical Atlantic regions are similar in the North and South Roughly a linear relationship between regional TCR and global TCR

37. A linear relationship between TCR and Interhemispheric Gradient Trend • Similar regional TCRs, in the Tropical Atlantic region across the equator • Roughly a linear relationship between regional TCR and global TCR • Models with higher TCR are models with stronger aerosol forcing, due to the constraint of the 20C global mean SAT change Stronger aerosol forcing with larger TCR => stronger SST gradient If we also constrain the models with observed interhemispheric gradient change?

38. Summary III • A linear relationship between the modeled Atlantic SST Interhemispheric Gradient and Transient Climate Response for most of the models • This relationship can be explained by the uncertainty of the aerosol forcings among the models • Further confirms that the important role of aerosol on the change of the Interhemispheric Gradient • Constraint on the simulation of Interhemispheric Gradient change (or trend) may be a way to confine the uncertainty of models’ climate sensitivity

39. Thank you for your attention

40. Comparison of SAT grad. change for 20C and 1%to2xCO2

41. Comparison of SAT grad. change for 20C and 1%to2xCO2

42. Climate sensitivity and total anthropogenic forcing 20 Century temperature change = climate sensitivity × Radiative Forcing In general, Let Smaller total anthropogenic forcing, larger climate sensitivity Larger total anthropogenic forcing, smaller climate sensitivity Consistent with Kiehl 2007

43. Regional TCR and interhemispheric gradient South – North Gradient change, △G Linear relationship btw.TCR and interhemispheric gradient change

44. Model internal averaged Gradient change

46. Pacific and AtlanticInterhemispheric Gradient Pacific HADISST ERSST(NOAA) KAPLAN SST Atlantic

47. 20C experiment EOF1 from all models Pacific Atlantic 49% 51% Projection of EOF1 on each run

48. Atlantic Interhemispheric SST Gradient A1B A2 B1

49. Pacific Interhemispheric SST Gradient A1B A2 B1