Intensification of the tropical hydrological cycle?

1. Intensification of the tropical hydrological cycle? Richard P. Allan Environmental Systems Science Centre, University of Reading, UK Thanks to Brian Soden

2. Climate ImpactsHow the hydrological cycle responds to aradiative imbalance is crucial to society (e.g. water supply, agriculture, severe weather) Motivation

3. IPCC 2007 WGI

4. Hydrological cycle linked to radiative energy balance How will global water cycle respond to warming? Method: compare observations and reanalysis products with model simulations of the present day climate over the period 1979-2006 Introduction

5. Earth’s energy balance Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94

6. Earth’s energy balance SW heating +67 Wm-2 LW cooling -169 Wm-2 Precip: +78 Wm-2 Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94

7. Changing character of precipitation Convective rainfall draws in moisture from surroundings

8. Changing character of precipitation 1979-2002 • Moisture is observed & predicted to increase with warming ~7%K-1 (e.g. Soden et al. 2005, Science) • Thus convective rainfall also expected to increase at this rate (e.g. Trenberth et al. 2003 BAMS)

9. Precipitation also linked to clear-sky longwave radiative cooling of the atmosphere

10. Global precipitation (P) changes constrained by atmospheric net radiative cooling (Q) • Changes in Q expected to be ~3 Wm-2K-1 (e.g. Allen and Ingram, 2002) • If so, changes in P with warming ≈3%K-1 • …substantially lower than changes in moisture (~7%K-1)

11. Models also display a “muted” precipitation response (~1-3%K-1) 7 % K-1 ∆P (%) ∆T (K) Held and Soden (2006) J. Clim

12. “heavy rain”: ~7 % K-1 Mean: ~2 % K-1 ∆P (%) “light rain”: -? % K-1 ∆T (K) Held and Soden (2006) J. Clim

13. But recent results suggest that the “muted” precipitation response is not found in the observations Is the global water cycle/radiation budget not captured by models? Wentz et al. (2007) Science

14. Are model simulated changes in clear-sky radiative cooling robust? ↓OLR ? Cloud ? Aerosol  T, H2O, GHG  σT4   εAσT4  Ts

15. How does atmospheric net radiative cooling respond to warming? • Sensitivity test (Edwards/Slingo): • Assume no change in clouds, aerosol, ozone • Two warming scenarios (C1, C2) • Greenhouse gas increase (1980-99) • Shortwave absorption case

16. How does atmospheric net radiative cooling respond to warming? • Sensitivity test (Edwards/Slingo): • Assume no change in clouds, aerosol, ozone • Two warming scenarios (C1, C2) • Greenhouse gas increase (1980-99) • Shortwave absorption case

17. Increased moisture enhances atmospheric radiative cooling to surface ERA40 NCEP dSNLc/dCWV ~ 1 ─ 1.5 Wkg-1 SNLc = clear-sky surface net down longwave radiation CWV = column integrated water vapour Allan (2006) JGR 111, D22105

18. What about in observations, reanalyses and models? • Method: Compare monthly mean changes in temperature, water vapour and clear-sky longwave radiation, 1979-2006. • Estimate water vapour driven changes in surface radiation using satellite and climatological ocean data, and an empirical model (Prata 1996)

19. Tropical ocean variability SST Water vapour Clear net LW down at surface

20. Increase in clear-sky longwave radiative cooling to the surface ∆SNLc (Wm-2) CMIP3 CMIP3 volcanic NCEP ERA40 SSM/I-derived ~ +1 Wm-2 per decade

21. Tropical Oceans dCWV/dTs ~2 ─ 4 mm K-1 dSNLc/dTs ~3 ─ 5 Wm-2K-1 AMIP3 CMIP3 non-volcanic CMIP3 volcanic Reanalyses/ Observations

22. Tropical ocean variability LWc: TOA LWc: ATM Precip

23. Increase in atmospheric cooling over tropical ocean descent ~4 Wm-2K-1 CMIP3 volcanic Reanalyses/ Observations AMIP3 CMIP3 non-volcanic

24. RECAP… • Increased moisture (~7%/K) •  increased convective precipitation • Increased radiative cooling •  smaller mean rise in precipitation (~3%/K) • Implies reduced precipitation away from convective regimes (less light rainfall?) • Locally, mixed signal from the above

25. Method: Analyse separately precipitation over the ascending and descending branches of the tropical circulation • Use reanalyses to sub-sample observed data • Employ widely used precipitation datasets • Compare with atmosphere-only and fully coupled climate model simulations

26. Tropical Precipitation Response GPCP CMAP • Model precipitation response smaller than the satellite observations • see also Wentz et al. (2007) Science AMIP3 Allan and Soden, 2007, GRL

27. Tropical Subsidence regions dP/dt ~ -0.1 mm day-1 decade-1 OCEAN LAND AMIPSSM/I GPCP CMAP

28. Projected changes in Tropical Precipitation

29. Calculated trends • Models understimate mean precipitation response by factor of ~2-3 • Models severely underestimate precip response in ascending and descending branches of tropical circulation

30. Questions (1) Is the analysis flawed? (2) Are the observed changes physically plausible? Are the observations wrong? (3) Can decadal changes in cloud and aerosol explain the discrepancy? (4) Are the models missing fundamental physics?

31. (1) Is the analysis flawed? • Changes in the reanalyses cannot explain the bulk of the trends in precipitation

32. (2a) Are the observations wrong? • Many of the global precipitation datasets use satellite data • Changes in the observing system • Potential algorithm errors sensitive to temperature and/or water vapour amount • Evidence from land and ocean observations suggest models underestimate precipitation and evaporation response to warming • e.g. Wentz et al. 2007 Science; Zhang et al. 2007 Nature, Chou et al. 2007 GRL, …

33. Zhang et al. 2007 Nature

34. Surface Evaporation and wind speed • Surface evaporation depends primarily upon • Wind stress • Surface humidity gradient (expected to change due to Clausius Clapeyron equation ~7%/K) • Globally, evaporation must equal precipitation • Therefore a muted precipitation response requires a muted evaporation response (e.g. muted wind stress)

36. Changes in Evaporation over ocean • Observed increases in surface evaporation • larger than climate model simulations; caused by • increased surface humidity gradient (Clausius Clapeyron) • Little trend in wind stress changes over ocean (Yu and Weller, 2007; Wentz et al., 2007) but some evidence over land (Roderick et al. 2007 GRL) Yu and Weller (2007) BAMS

37. (2b) Are the changes plausible? • Increases in tropical precipitation and evaporation at ~7%/K plausible • But increases in ascent region precip ~ 30%K-1, much larger than Clausius Clapeyron • Implies intensification of hydrological cycle • At odds with observations and model simulations of a reduction in Walker circulation (Vecchi and Soden, 2007)

38. Vecchi and Soden (2006) Nature • Evidence for weakening of Walker circulation in models and observations

39. Vecchi and Soden (2006) Nature • Evidence for weakening of Walker circulation in models and observations

40. (3) What else could explain this apparent discrepancy? • Changes in land/ocean tropics/extra-tropical transport of heat and moisture? • Decadal variability in radiation balance and hydrological cycle? • Changes in clouds • Changes in aerosol

41. Climate models appear to underestimate variability in radiation budget Does this relate to clouds and/or aerosol? Wong et al. 2006 J. Climate, Wielicki et al. 2002, Science

42. Global dimming to Global Brightening Stanhill and Cohen EOS (below), Wild et al., Pinker et al. 2005 Science

43. See Mishchenko et al. (2007) Science Could decadal changes in aerosol have short-circuited the global water cycle through direct and indirect effect on cloud radiation?

44. Aerosol Hypothesis? • Could changes in aerosol directly and indirectly (through cloud) alter the radiation balance and precipitation? ↓ Aerosol

45. Aerosol Hypothesis? • Could changes in aerosol directly and indirectly (through cloud) alter the radiation balance and precipitation? ↓ Aerosol

46. Aerosol Hypothesis? • Could changes in aerosol directly and indirectly (through cloud) alter the radiation balance and precipitation? ↓ Aerosol  Rad cooling  Solar heating

47. Aerosol Hypothesis? • Could changes in aerosol directly and indirectly (through cloud) alter the radiation balance and precipitation? ↓ Aerosol  Rad cooling  Precip  Circulation?  Solar,  Evap

48. Summary • Global water and energy cycles coupled • Theoretical changes in clear-sky radiative cooling of atmosphere implies “muted” precipitation response • Models simulate muted response, observations show larger response • Possible artifacts of data? • Possible mechanisms (aerosol, cloud) • Implications for climate change prediction

49. References • Allen and Ingram (2002) Nature • Trenberth et al. (2003) BAMS • Wentz et al. (2007) Science • My meagre contributions: • Allan (2006) JGR • Allan and Soden (2007) GRL

50. Extra slides…