The tropics in a changing climate

1. The tropics in a changing climate Chia Chou Research Center for Environmental Changes Academia Sinica October 19, 2010 NCU

2. Mechanisms of mean tropical precipitation changes Chou et al. 2009

3. Based on these mechanisms, to further examine changes in the tropics: — the direct moisture effect (thermodynamic component) — the effect of deepened convection — changes in precipitation intensity and frequency

4. The direct moisture effect (changes in moisture)

5. IPCC AR4

6. IPCC AR4

7. Column water vapor changes(IPCC AR4 ensemble)

8. The Hadley circulation

9. Precipitation changes (end of 21st century)

11. widening of annual range

12. horizontal moisture advection vertical moisture advection P : precipitation E : evaporation ω: vertical velocity q: Lq, moisture

15. Hadley circulation Precipitation change in a warmer climate Lq    E, LW, H

16. 1997-98 El Niño

18. ( (850~200 ) MSU El Niño

19. SST

20. Spatial asymmetry ( for 8283 ,9192,9798) CMAP Annual cycle of basic state

22. Vertically integrated moist static energy budget h : moist static energy Fnet : net flux into the atmosphere ω: vertical velocity q: Lq, moisture T: CpT, temperature

23. 

24. El Niño

25. Conclusion 1 • Asymmetry of mean tropical precipitation changes  widening of annual precipitation range • Mechanisms  Global warming: thermodynamic component dominates  ENSO: dynamic component dominates

26. The effect of deepened convection

27. Global water vapor budget (Held and Soden 2006):  thermodynamic dynamic P: precipitation M: mass flux; q: PBL water vapor

28. 1-3% in P per 1ºC T (model simulations)   <0  slowing of tropical circulation dynamic component   7.5% in q per 1ºC T (Clausius-Clapeyron) thermodynamic component Held and Soden (2006); Vecchi and Soden (2007)

29. In global average, P = E P ≈ LW+SW (assuming H is small) Vecchi and Soden (2007)

30. increases at 1-3% per 1ºC T increases at 7.5% per 1ºC T ?  NO 

31. Vertically integrated water vapor budget convergence of moisture flux P: precipitation; E: evaporation q: water vapor (moisture); v: horizontal velocity ω: vertical velocity; ‹ ›: vertical integration

32. Vertically integrated water vapor budget

33. vertical advection horizontal advection  thermodynamic dynamic 

34. 

35.  :a weakening of tropical circulation

36.    >0 or <0 No constraint  7.5% in q per 1ºC T 7.5% in q per 1ºC T   <0  1-3% in P per 1ºC T (controlled by energy budget)

37. Effect of convection depth

38.  deepening of convection:~ 2.5-3.4%

39. 155 hPa 150 hPa 145 hPa 141 hPa 137 hPa Convection top: 155 hPa ~ 137 hPa (-1.2% ~ 3.3%)

40. Convection top: 155 hPa ~ 137 hPa (-1.2% ~ 3.3%) Chou and Chen 2010

41. Deeper convection more E less E Reduced upward motion; Less convergence of moisture flux more evaporation

42. Conclusion 2 • Effect of convection depth: the deeper (shallower) convection, the weaker (stronger) the circulation strength of tropical circulation: atmospheric stability; upper troposphere

43. Changes in precipitation frequency and intensity

44. Precipitation Frequency • Scatterplot of model-simulated percentage change (%) for globally averages

45. Precipitation frequency

46. Precipitation Intensity • Scatterplot of model-simulated percentage change (%) for globally averages

47. Precipitation Intensity

49. Conclusion 3 • Frequency is enhanced for median and heavy precipitation, while reduced for light precipitation • Intensity is enhanced for heavy precipitation, but inconsistent for median and light precipitation