Atmosphere Shallow Meridional Atmosphere Shallow Meridional Circulations: A New Mechanism Circulations: A New Mechanism

for ITCZ-Cold Tongue Interactionfor ITCZ-Cold Tongue Interaction

Chidong Zhang Chidong Zhang

RSMAS, University of MiamiRSMAS, University of Miami

Miami, FLMiami, FL

Equator ITCZ

1 km

10-12 km

1 - 5 km

Outline of the talk:1. Observations (Zhang et al. 2004, J. Climate)

2. Mechanism

3. Global reanalyses

4. Implications

Observations:

• Madden and Zipser (1970): Line Island Experiment soundings

• Tomas and Webster (1997), Trenberth et al (2000): divergent

wind component from global reanalyses

Trenberth et al (2000)

Atlantic (30˚W-10˚E)

S. Americas (60-40˚W)

N. Americas (110-80˚W)

Pacific (170-90˚W)

Australia (60-180˚E)

Africa (10-40˚E)

Januray July

FGGE dropsondes (May - June 1979)

EPIC2001 dropsondes (September 2001)

TAO soundings (1996 - 2001)

Wind profilers at Galapagos (2˚N, 157.4˚W)

and

Christmas Island

(2˚N, 157.4˚W) (1995 - 2001)

Wind profilers at Galapagos (2˚N, 157.4˚W)

and

Christmas Island

(2˚N, 157.4˚W) (1995 - 2001)

Mechanisms:• Wave propagation (Madden and Zipser 1970)• Vorticity advection (Tomas and Webster 1997)• Sea-breeze dynamics (Nolan and Zhang 2004)

warm

cool warm

cool

zb=1000m

T1=295Kp1=1010 hPa

T2=298Kp2=1008 hPa

=d=0.01Km-1

=d=0.01Km-1

=d=0.01Km-1

=0.006Km-1

p(z) = p0[(T0- z)/T0]g/RT

convergencedescent

Equator ITCZ

1 - 5 km

1 km

10-12 km

Effects of convection in the ITCZ:

ERA40 ( x 20)

RH

E. Pacific95 – 125˚W

Atlantic15 – 35˚W

hPa

hPa

RH

August

hPaERA40(a)

hPa

NCEP(b)

August

v at the equator

(a)

(b)

NCEP

ERA40

W. Africa(5˚E-15˚W)

April

ERA40Galapagos

Christmas Island ERA40

Implications:• Bimodal distribution in clouds in the ITCZ

Equator ITCZ

1 - 5 km

1 km

10-12 km

courtesy of Walter Peterson

courtesy of James Mather

Implications:• Bimodal distribution in clouds in the ITCZ• Momentum balance of the boundary layer

∂p/∂y/+v|U|Cd/h+(v-vT)we/h+∂v/∂t+u∂v/∂x+v∂v/∂y+ƒu =0

Momentum balance in the mixed layer (EPIC2001 dropsondes):

Implications:• Bimodal distribution in clouds in the ITCZ• Momentum balance of the boundary layer• Effect of ITCZ deep convection on surface wind

∂p/∂y/+v|U|Cd/h=0

Convective diabatic heating (Webster 1972; Gill 1980)

Convective ventilation (Lindzen and Nigam 1987)

“Entrainment Braking Mechanism”

∂p/∂y/+v|U|Cd/h+(v-vT)we/h=0

Implications:• Bimodal distribution in clouds in the ITCZ• Momentum balance of the boundary layer• Effect of ITCZ deep convection on surface wind• Meridional water vapor transport

TAO ship soundings

Implications to the Tropical Atlantic Variability:

• A new mechanism for the interaction among the ITCZ convection, surface trade wind, and the equatorial cold tongue

• A new mechanism for the West African Monsoon to affect the springtime development of the equatorial cold tongue

(Mitchell and Wallace 1992)

θ

50oC

20oC

θθe

90oC

60oC

θe

AEJ

θ

50oC

20oC

θθe

90oC

60oC

θe

AEJ

Parker et al (2003)

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