Chapter 15 Global Circulation: Big Picture: Idealized View

Ferrel cell

Ferrel cell

Polar cell

Fig 1. Earth’s radiation budget

What do ‘surplus’ and ‘deficit’ refer to?What wavelength ranges are for shortwave and longwave? What are their sources?What is shortwave in?Why are shortwave absorbed not symmetrical with latitude?At what latitude does most heat and mass transfer occur?

Fig 2. 1735, Hadley’s circulation cell idea developed.

Ideas: Latent heat release drives convection. Trade winds are Easterlies due to the rotationof the Earth, and he realized it 100 years before Coriolis wrote the theory for motion in a uniformly rotating coordinate system. Main problems: No westerlies anywhere to conserveangular momentum, and the Equator to pole temperature difference is too large.

Fig. 3. Rossby theory of circulation, 1930s

William Ferrel’s cell idea in 1856: Rossby clarified and explained more completely. Prevailingwesterlies at mid latitudes are a wavy river of air, Rossby waves, where the Coriolis force is therestoring force, and strong horizontal gradients of pressure and temperature (baroclinic instabilities – density depends on pressure and temperature). This conceptual model capturesmany features of the Earth’s circulation.

clouds

clouds

polar frontWarm air rides up the colder air

doldrums

deserts

Jet Streams (pink) between warm (orange) and cold (blue) regions

Jet streams (shown in pink) are well-known examples of thermal wind. They arise from the horizontal temperature gradient from the warm tropics to cold polar regions.

From Wikipedia

Geostrophic Wind: due to pressure gradient along x direction

Barotropic atmosphere(density depends only on pressure)

Baroclinic atmosphere(density depends on pressure and temperature) size of temperaturesymbol represents value .

The vertical variation of geostrophic wind in a barotropic atmosphere (a) and in a baroclinic atmosphere (b). The blue portion of the surface denotes a cold region while the orange portion denotes a warm region. Temperature difference is restricted to the boundary in (a) and extends through the region in (b). The dotted lines enclose isobaric surfaces which remain at constant slope with increasing height in (a) and increase in slope with height in (b). This causes thermal wind to occur only in a baroclinic atmosphere. (from Wikipedia)

T(z)T(z)

Thermal wind

y yx

y y

Fig 4a Longitudinally averaged winds (zonal mean) for 20 years on the equinox March 21st. Westerlies are solid lines. Note jet stream

maxima at 200 hPA.

Thermal Wind Estimation (will do derivation later)

Fig 4b Zonal mean averagecirculation

Dec Jan Feb

June July August

Note the location of the ITCZ

What latitude is cloudiest?

What latitude has clear conditions?

Fig 4c Zonal mean temperature for 20 years at equinox on March 21st.

Note the strong latitudinal gradient.

Fig 7 Lorenz theory of Atmosphere Energy Cycle

Energy density (units 105 Jm-2) Energy flows (Wm-2)

Key points: Terminology and mechanisms at play. What are APE, KE, and eddies?

Barotropic and Baroclinic AtmosphereBarotropic Baroclinic

Baroclinic fluid is ‘full of life as it converts thermal energy into kinetic energy’

From Marshall and Plumb