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

Mechanisms of mean tropical precipitation changes Chou et al. 2009

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

The direct moisture effect (changes in moisture)

IPCC AR4

Column water vapor changes (IPCC AR4 ensemble)

The Hadley circulation

Precipitation changes (end of 21st century)

 widening of annual range

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

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

El Niño

( (850~200 ) MSU El Niño

SST

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

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



El Niño

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

The effect of deepened convection

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

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

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

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

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

  vertical advectionhorizontal advection thermodynamicdynamic



 :a weakening of tropical circulation

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

Effect of convection depth

 deepening of convection:~ %

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

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

Deeper convection more Eless E  Reduced upward motion; Less convergence of moisture flux  more evaporation

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

Changes in precipitation frequency and intensity

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

Precipitation frequency

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

Precipitation Intensity

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