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

qq

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

qq

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

qq

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

qq

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