Atmosphere Shallow Meridional Circulations: A New Mechanism for ITCZ-Cold Tongue Interaction Chidong Zhang RSMAS, University of Miami Miami, FL

Equator ITCZ 1 km km 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) JanurayJuly

FGGE dropsondes (May - June 1979)

EPIC2001 dropsondes (September 2001)

TAO soundings ( )

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

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

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

warm coolwarm cool z b =1000m T 1 =295K p 1 =1010 hPa T 2 =298K p 2 =1008 hPa  =  d =0.01Km -1  =0.006Km -1 p(z) = p 0 [(T 0 -  z)/T 0 ] g/RT convergence descent

Equator ITCZ km 1 km km Effects of convection in the ITCZ:

ERA40 (  x 20) RH E. Pacific 95 – 125˚W Atlantic 15 – 35˚W hPa RH August

hPa ERA40 (a) hPa NCEP(b) August v at the equator

(a) (b) NCEP ERA40 W. Africa (5˚E-15˚W) April

ERA40 Galapagos Christmas Island ERA40

Implications: Bimodal distribution in clouds in the ITCZ Equator ITCZ km 1 km 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|C d /h+(v-v T )w e /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|C d /h=0 Convective diabatic heating (Webster 1972; Gill 1980) Convective ventilation (Lindzen and Nigam 1987)

“Entrainment Braking Mechanism” ∂p/∂y/  +v|U|C d /h+(v-v T )w e /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)

Parker et al (2003)

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