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The tropics in a changing climate
Chia ChouResearch Center for Environmental Changes
Academia Sinica
October 19, 2010
NCU
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Mechanisms of mean tropical precipitation changes
Chou et al. 2009
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• 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
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The direct moisture effect(changes in moisture)
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IPCC AR4
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IPCC AR4
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Column water vapor changes(IPCC AR4 ensemble)
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The Hadley circulation
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Precipitation changes (end of 21st century)
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widening of annual range
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EqvqqP pp
vertical moisture advection
horizontal moisture advection
P : precipitation
E : evaporation
ω: vertical velocity
q: Lq, moisture
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qp
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qp
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Hadley circulation
Precipitation change in a warmer climate
0 qLp
E, LW, H
Lq
0 qLp 0 qLp
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1997-98 El Niño
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( (850~200 ) MSU
aT ahP
El Niño
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SST
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Annual cycle of basic state
( for 8283 ,9192,9798) CMAP
'PSpatial asymmetry
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qp
E
qp
qv
EqvqqP pp
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h : moist static energy
Fnet : net flux into the atmosphere
ω: vertical velocity
q: Lq, moisture
T: CpT, temperature
')( netpp FqTvhh
Vertically integrated moist static energy budget
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)( Tqv
hp
'netF
hp
')( netpp FqTvhh
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El Niño
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Conclusion 1
• Asymmetry of mean tropical precipitation changes
widening of annual precipitation range• Mechanisms Global warming: thermodynamic component dominates ENSO: dynamic component dominates
qp
qp
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The effect of deepened convection
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Global water vapor budget (Held and Soden 2006):
MqP MqqMP
M: mass flux; q: PBL water vapor
thermodynamic dynamic
P: precipitation
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MqqMP
MM
PP
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)
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In global average, P = EP ≈ LW+SW (assuming H is small)
Vecchi and Soden (2007)
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0MM
increases at 7.5% per 1ºC T
increases at 1-3% per 1ºC T
?
NO
PP
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vqEP
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
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vqvq
EPEP )(
Vertically integrated water vapor budget
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vqq
vqq
EPEP pp
MM
PP
qvqEP p
qvqqEP pp
vertical advection horizontal advection
thermodynamic dynamic
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%5.7)(
EPEP
PP
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:a weakening of tropical circulation0
vqqp
vqq
EPEP p
~%5.7
vqq
vqq
EPEP pp
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vqq
vqq
EPEP pp
MM
PP
>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
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Effect of convection depth
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deepening of convection:~ 2.5-3.4%
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155 hPa
150 hPa
145 hPa
141 hPa
137 hPa
Convection top: 155 hPa ~ 137 hPa
(-1.2% ~ 3.3%)
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Chou and Chen 2010
Convection top: 155 hPa ~ 137 hPa
(-1.2% ~ 3.3%)
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Deeper convection
more E less E
Reduced upward motion; Less convergence of moisture flux
more evaporation
vqEP
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Conclusion 2
• Effect of convection depth: the deeper (shallower) convection, the weaker (stronger) the circulation
strength of tropical circulation: atmospheric stability; upper troposphere
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Changes in precipitation frequency and intensity
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Precipitation FrequencyScatterplot of model-simulated percentage change (%) for
globally averages
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Precipitation frequency
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Precipitation IntensityScatterplot of model-simulated percentage change (%) for
globally averages
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Precipitation Intensity
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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