The Zoo of Mechanisms for Tropical Rainfall Variability and Change

1. The Zoo of Mechanisms for Tropical Rainfall Variability and Change A Sample J. D. Neelin*, C. Chou**, B. Lintner*, M. Munnich*, H. Su*, J. Meyerson*, C. Holloway*, K. Hales*, & O. Peters* *Dept. of Atmospheric Sciences & Inst. of Geophysics and Planetary Physics, U.C.L.A. **Academica Sinica, Taiwan

2. Outline • Illustration of model precipitation sensitivity • Global warming/El Niño • Sahel bafflement • Some principles: • Widespread warming/ local precipitation balances • Moist static energy budget • A few mechanisms: upped-ante, rich-get-richer, ventilation • Role of ventilation: Mid-Holocene example • Prototype for convective margins

3. Precipitation Change under global warming(CMIP3 a.k.a. IPCC 4th Assessment report models) SRES A2 scenario (heterogeneous world, growing population,…) for greenhouse gases, aerosol forcing Precip change: HadCM3, June-Aug., 2070-2099 avg minus 1961-90 avg. 4 mm/day model climatology black contour for reference mm/day Neelin, Munnich, Su, Meyerson and Holloway , 2006, PNAS • Data: LLNL Prog. on Model Diagnostics & Intercomparison; • Thanks: Clivar Working Group on Coupled Modeling+groups

4. GFDL_CM2.0 JJA Prec. Anom.

5. NCAR_CCSM3 JJA Prec. Anom.

6. CCCMA JJA Prec. Anom.

7. CNRM_CM3 JJA Prec. Anom.

8. GFDL_CM2.1 JJA Prec. Anom.

9. CSIRO_MK3 JJA Prec. Anom.

10. UKMO_HadCM3 JJA Prec. Anom.

11. MIROC_3.2 JJA Prec. Anom.

12. MRI_CGCM2 JJA Prec. Anom.

13. NCAR_PCM1 JJA Prec. Anom.

14. MPI_ECHAM5 JJA Prec. Anom.

15. GFDL_CM2.1 • Prec change (2070-99)-(1951-80); Clim wind 850-700hPa

16. MPI_ECHAM5 • Prec change (2070-99)-(1951-80); Clim wind 850-700hPa

17. MRI_CGCM2 • Prec change (2070-99)-(1951-80); Clim wind 850-700hPa

18. HadCM3 • Prec change (2070-99)-(1951-80); Clim wind 850-700hPa

19. NCAR_CCSM3 • Prec change (2070-99)-(1951-80); Clim wind 850-700hPa

20. MIROC_3.2_Med.Res. • Prec change (2070-99)-(1951-80); Clim wind 850-700hPa

21. Trend of the 10-model ensemble median Precipitation change: measures at the local level > 99% significance (1979-2099) Neelin, Munnich, Su, Meyerson and Holloway , 2006, PNAS

22. Inter-model precipitation agreement Number of models (out of 10) with > 99% significant* dry/wet trend (1979-2099) and exceeding 20% of the median clim./century *[Spearman-rho test] Neelin, Munnich, Su, Meyerson and Holloway, 2006, PNAS

23. Hypothesis for analysis method: • models have similar processes for precip increases and decreases but the geographic location is sensitive …to differences in model clim. of wind, precip; to variations in the moistening process (shallow convection, moisture closure,…)

24. Hypothesis for analysis method: • models have similar processes for precip increases and decreases but the geographic location is sensitive • Check agreement on amplitude measure: • Spatial projection of precip change for each model on that model’s own characteristic pattern of change

25. Projection of JJA (30yr running mean) precip pattern onto normalized positive & negative late-century pattern for each model Neelin, Munnich, Su, Meyerson and Holloway , 2006, PNAS

26. ENSO precip. anoms: obs. vs. atm. models • Warm-cold composite for Xie-Arkin obs, ECMWF-AMIP2, NCEP-AMIP2, QTCM Observed & 3 models forced by observed sea surface temperature (AMIP2=Atm. Model intercomparison project) See also Sperber and Palmer 1996, Giannini et al 2001; Saravanan & Chang, 2000; Joseph & Nigam 2006 (El Niño avg 1982-83, 87-88, 92-93, 95-96 – La Niña avg 1984-85, 89-90, 96-97)

27. ENSO teleconnections to regional precip. anomalies Su & Neelin, 2002; Chiang & Sobel 2002

28. The “upped-ante” mechanism1 Margin of convective zone Neelin, Chou & Su, 2003 GRL

29. The Rich-get-richer mechanismFormerly M¢ (anomalous Gross Moist Stability) mechanism1 Descent region: incr. descent Þ less precip. Center of convergence zone: incr. moisture Þ lower gross moist stability Þ incr. convergence, precip Chou & Neelin, 2004; Held and Soden 2006

30. Temperature T and Moisture q equations

31. Moisture & moist static energy (MSE) budgets Moisture budget for perturbations P' = – <q' Ñ· `v > – <`v ·Ñq' >– <`q Ñ·v' >– < v '·Ñ`q>+ E' … R.get.R Upped-ante Convgc Fb • Use MSE budget to obtain Ñ·v'(Chou & Neelin 2004) • Neglect Ñ·v' ,v '(Held and Soden 2006; plausible for spatial avgs ifÑ·v'at smaller scales thanÑ·`v ) Yields precip anoms as T'Þ q' ÞÑq' , M'

32. Budget diagnostics for mechanisms Upped-ante Rich-get-richer Rad cooling, (v·ÑT)' ocean transp, … GMS multiplier effect SST disequilibrium • Moist Static Energy transport by divergent flow » MÑ·v • Gross Moist Stability M=Ms-Mq, (Mq inc. with moisture) MSE budget for perturbations T' + ocean mixed layer / land MÑ·v' = –M'Ñ·v – (v·Ñq)'– c¶tT '+ F net'+ (v·ÑT)'… Yields precip anoms as T'Þ q' ÞÑq' , M'; v' , q' Þ E' etc. P' » (1+ )–(v·Ñq)' + Ñ·v(–M') – c¶tT '+… top s Mq [ ] s M

33. QTCM 2xCO2 Expt. suppressing change in moisture advection (testing the upped-ante mechanism) Suppression Expt 2xCO2 Precip. change (mm/day) Control 2xCO2 Precip. change Neelin, Chou & Su, 2003 GRL

34. ECHAM4 + ocean mixed layer 2xCO2 equilib. Precip. anom. rel. to control Moisture anom. (1000-900 hPa) Moisture anom. (900-700 hPa) ---Clim. Precip. (6 mm/day contour) Chou, Neelin, Tu & Chen (2006, J. Clim.)

35. ECHAM4/OPYC3 2070-2099 IS92a (GHG only) Precip. anom. rel. to control Moisture anom. (1000-900 hPa) Moisture anom. (900-700 hPa) ---Clim. Precip. (6 mm/day contour) Chou et al. (2006, J. Clim.)

36. ECHAM4 DJF Contributions to the moisture/MSE budget Assoc. with upped ante Assoc. with rich-get-richer (M') mechanism Convergence feedback on both Chou et al, 2006, J. Clim.

37. 4panel Observed Fnet climatology July Net flux into atmosphere Solar, IR, sensible, latent (Net surface flux=0 over land) Shaded over/under +/- 30 W/m2 Precipitation Low-level wind Chou and Neelin 2003, J. Clim.

38. Ventilation by relatively low moist static energy air from oceanic/nonconvective regions: helps set poleward extent of monsoons Chou et al 2001 QJRMS; Chou and Neelin 2003 GRL; 2005 JClim

39. The role of ventilation in mid-holocene N. Africa • Precip. change rel. to control • Expt: 6 ka bp orbital parameters & grassland albedo specified through all N Africa • Control: present day orbital and albedo forcing • What stops precip from advancing northward? Ventilation. Su & Neelin 2005, JGR

40. The role of ventilation in mid-holocene N. Africa N. African zonal avg: • moisture (dashed) • critical moisture for convection (increases with Temperature) • 1. Control; • 2. 6ka orbital PMIP expt; • 3. 6ka and grassland albedo over N. Africa • Despite low albedo, ventilation by inflow keeps moisture from rising to convective threshold in north Su & Neelin 2005, JGR

41. The role of ventilation in mid-holocene N. Africa • Expts with ventilation (inflow moisture advection + moisture diffusion) reduced yield greater poleward movement of precip Su & Neelin 2005, JGR

42. The role of ventilation in mid-holocene N. Africa • Precip. change rel. to present • 1. 6 ka bp orbital parameters & interactive vegetation • 2. Same but reduced ventilation • Ventilation leading control on poleward extent of 200 mm/y; veg. feedback enhances prec. Hales, Neelin & Zeng 2006, JClim

43. Can one observe a critical moisture for convection? • Tropical Rainfall Measuring Mission Microwave Imager (TMI) data • Wentz & Spencer (1998) algorithm • Average precip P(w) in each 0.3 mm w bin (typically 104 to 107 counts per bin in 5 yrs) • 0.25 degree resolution • No explicit time averaging Western Pacific Eastern Pacific Peters & Neelin, 2006, Nature Physics

44. How well do the curves collapse when rescaled? Western Pacific Eastern Pacific • Rescale water vapor and P by critical value & amplitude from power law fit above critical Peters & Neelin, 2006

45. Dependence on Tropospheric temperature • Averages conditioned on vert. avg. temp. T, as well as w (T 200-1000mb from ERA40 reanalysis) • Power law fits above critical: wc changes, same  • [note more data points at 270, 271] ^

46. Convective margin prototype Land Ocean (1) (2) Steady-state, 1D temperature and moisture equations (in moist static energy form) for a semi-infinite land region lying to the west of an ocean region Lintner and Neelin, 2007, GRL

47. Perturbations to xc Between the old and new margins, precipitation shuts down, so the largest droughts occur here. Stochastic wind smooths margins, climate pertn to T or inflow q shifts

48. Nonconvecting region solution For inflow moisture q0 at coast (x=0) for constant inflow uq, q rises to convective threshold qc(T), giving convective margin position (distance from coast) xc: with inflow distance scale -1 = -uqMs(MqpFT)-1.

49. Summary=Outline • Illustration of model precipitation sensitivity • Global warming/El Niño • Sahel bafflement • Some principles: • Widespread warming/ local precipitation balances • Moist static energy budget • A few mechanisms: upped-ante, rich-get-richer, ventilation • Role of ventilation: Mid-Holocene example • Prototype for convective margins • Convective threshold change versus inflow air

50. Summary: mechanisms • tropospheric warming increases moisture gradient between convective and non-convective regions • the"upped-ante mechanism": • negative precipitation anomaly regions along margins of convection zones with wind inflow from dry zones • the“rich-get-richer mechanism" (a.k.a. M' mechanism): • Positive/negative precipitation changes in regions of with high/low climatological precipitation • [+ocean heat transport anomaly in equatorial Pacific]