the tropics in a changing climate chia chou research center for environmental changes academia...
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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 frequencyTRANSCRIPT
The tropics in a changing climate
Chia ChouResearch 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
IPCC AR4
Column water vapor changes(IPCC AR4 ensemble)
The Hadley circulation
Precipitation changes (end of 21st century)
widening of annual range
EqvqqP pp
vertical moisture advection
horizontal moisture advection
P : precipitation
E : evaporation
ω: vertical velocity
q: Lq, moisture
qp
qp
Hadley circulation
Precipitation change in a warmer climate
0 qLp
E, LW, H
Lq
0 qLp 0 qLp
1997-98 El Niño
( (850~200 ) MSU
aT ahP
El Niño
SST
Annual cycle of basic state
( for 8283 ,9192,9798) CMAP
'PSpatial asymmetry
qp
E
qp
qv
EqvqqP pp
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
)( Tqv
hp
'netF
hp
')( netpp FqTvhh
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
qp
qp
The effect of deepened convection
Global water vapor budget (Held and Soden 2006):
MqP MqqMP
M: mass flux; q: PBL water vapor
thermodynamic dynamic
P: precipitation
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)
In global average, P = EP ≈ LW+SW (assuming H is small)
Vecchi and Soden (2007)
0MM
increases at 7.5% per 1ºC T
increases at 1-3% per 1ºC T
?
NO
PP
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
vqvq
EPEP )(
Vertically integrated water vapor budget
vqq
vqq
EPEP pp
MM
PP
qvqEP p
qvqqEP pp
vertical advection horizontal advection
thermodynamic dynamic
%5.7)(
EPEP
PP
:a weakening of tropical circulation0
vqqp
vqq
EPEP p
~%5.7
vqq
vqq
EPEP pp
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
Effect of convection depth
deepening of convection:~ 2.5-3.4%
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 E less E
Reduced upward motion; Less convergence of moisture flux
more evaporation
vqEP
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 FrequencyScatterplot of model-simulated percentage change (%) for
globally averages
Precipitation frequency
Precipitation IntensityScatterplot 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