12.003Atmosphere, Ocean and Climate Dynamics

Lecture XIIHadley Circulation

Thursday, October 6, 2011

Lecture XII Outline

1. Horizontal motions in the atmosphere2. Angular momentum conservation and the Hadley Circulation3. Midlatidute eddies4. The atmospheric energy transport

Thursday, October 6, 2011

Radiative-convective equilibrium

High p

Low p

High p

Low p

Thursday, October 6, 2011

Hadley Model George Hadley (1685-1768), British meteorologist, speculated that pressure gradients drive a pole-to-eq1taor overturning cell (the Hadley cell)

➡ heating in tropics drives ascending motions➡ pressure gradients at tropopause drive northward flow➡ cooling at poles drive descending motions

Thursday, October 6, 2011

Hadley Model Hadley realized that air must conserve angular momentum

➡ as air move poleward it gets closer to rotation axis➡ air must spin faster along➡ westerly winds

Thursday, October 6, 2011

Observations: Meridional Overturning Circulation

Thursday, October 6, 2011

Observations: Zona-Average Zonal Winds

Thursday, October 6, 2011

Conservation of angular momentumM0 = mΩa2 = mΩa2 cos2 θ +uacosθ (1)

dTdz

=− 1+L/RT q∗

1+Lβ/cp q∗Γd ≈−

1+31 q∗

1+167 q∗Γd (2)

dTdz

�

Tropopause≈−1+31×1×10

−3

1+167 1×10−3 Γd ≈ 0.9Γd ≈ 9K/Km (3)

dTdz

�

Tropics≈−1+31×17×10

−3

1+167 17×10−3 Γd ≈ 0.9Γd ≈ 4K/Km (4)

1

Thursday, October 6, 2011

Conservation of angular momentumM0 = mΩa2 = mΩa2 cos2 θ +uacosθ (1)

dTdz

=− 1+L/RT q∗

1+Lβ/cp q∗Γd ≈−

1+31 q∗

1+167 q∗Γd (2)

dTdz

�

Tropopause≈−1+31×1×10

−3

1+167 1×10−3 Γd ≈ 0.9Γd ≈ 9K/Km (3)

dTdz

�

Tropics≈−1+31×17×10

−3

1+167 17×10−3 Γd ≈ 0.9Γd ≈ 4K/Km (4)

1

Thursday, October 6, 2011

Breaking up of Hadley’s cell

Thursday, October 6, 2011

Breaking up of Hadley’s cellSpinup of an atmospheric circulation model (simulation starts from radiative-convective equilibrium solution)

Thursday, October 6, 2011

Descending branch of Hadley’s cell

Thursday, October 6, 2011

Hadley cell and moisture transportrt

Schematic of circulation Annual-mean zonal-mean relative humidity (%)

Thursday, October 6, 2011

Atmospheric energy transport

Thursday, October 6, 2011

Atmospheric circulation

Non-rotating planet Rotating planet

Thursday, October 6, 2011

Energy balance modelsEnergy budget for an atmospheric latitude band, i.e.• zonally averaged band• vertically averaged band

Thursday, October 6, 2011

Surface temperature and heat transport

Thursday, October 6, 2011

Energy transport by the Hadley cellHeat transport

Thursday, October 6, 2011

Energy transport by the Hadley cellHeat plus potential energy (dry static energy) transport

Thursday, October 6, 2011

Energy transport by the Hadley cellHeat plus potential energy plus latent heat

(moist static energy) transport

Thursday, October 6, 2011

Energy transport at midlatitudesEnergy transport is by geostrophic eddies

Thursday, October 6, 2011

Total atmospheric energy transport

Thursday, October 6, 2011

Total atmospheric energy transport• Hadley cell transport energy in the tropics➡ weak meridional temperature gradients below 30o

• Eddies transport energy at midlatitudes➡ strong temperature gradients beyond 30o

Thursday, October 6, 2011

Energy balance of the Earth

Thursday, October 6, 2011

Surface temperature and heat transport

Thursday, October 6, 2011