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NATS 101
Lecture 20
Global Circulation
Supplemental References for
Today’s Lecture
Aguado, E. and J. E. Burt, 2001: Understanding Weather & Climate, 2nd
Ed. 505 pp. Prentice Hall. (ISBN 0-13-027394-5)
Lutgens, F. K. and E. J. Tarbuck, 2001: The Atmosphere, An
Introduction to the Atmosphere, 8th Ed. 484 pp. Prentice Hall.
(ISBN 0-13-087957-6)
Review
• Global Energy Balance (from long ago!)
• Thermally Direct Circulations AGAIN!
Annual Energy Balance
Radiative
Cooling
NH
Radiative
Warming
Radiative
Cooling
SH
Heat transfer done by winds and ocean currents
Differential heating drives winds and currents
Global Energy Budget
• Averaged over entire earth, incoming solar
radiation is equal to outgoing IR
• Tropics absorb more solar radiation than
they emit IR to space
Surplus of radiant energy in tropics
• Poles absorb less solar radiation than they
emit IR to space
Deficit of radiant energy in poles
Global Circulation
• To balance the inequalities in the global
energy budget, energy must be
transported from the tropics to the poles.
40% of transport is done by oceans
60% of transport is done by atmosphere
Thermally Direct Circulation
DIV
Heat
H
CON
Warm air toward poles
Warm Rising
L
Sinking Cold
Cold air toward equator
CON
H
DIV
Heat
Equator
L
Heat
Poles
Heat
Global Circulation
• Winds throughout the world are
averaged over a long period of time
(over many winters)
Local wind patterns vanish
• Distinct patterns in the prevailing winds
emerge
Driven by the unequal heating of the
earth’s surface
Consider Waterworld
A Simple Model
• Earth uniformly covered
by water
Land-Sea heating
difference isn’t factor
• Sun is always directly
over the equator
No seasons
• Earth doesn’t rotate Use
average daily sun
No diurnal cycle and … ?
Polar region
Least heating
Equatorial region
Greatest heating
Polar region
Least heating
Waterworld
Single Equator to Pole Cell
Cold air
Sinking
@ Poles
Hadley
Cell
Divergence
Aloft
Warm air
Rising @
Equator
Convergence
@ Surface
Ahrens
Fig. 7.14
Divergence
@ Surface
Convergence
Aloft
Consider a Rotating Waterworld
Triple Cell
Hadley
Ferrel
Polar
Pole
Equator
• Equator-to-Pole temperature difference and
rotation of Earth produce 3 circulation cells
Hadley Cell (Strong Thermally Direct)
Ferrel Cell (Indirect: Forced by Hadley & Polar)
Polar Cell (Weak Thermally Direct)
Rotating Waterworld
Prevailing Winds
Ahrens Fig. 7.16
Major Surface Pressure Zones
Ahrens Fig 7-17
ITCZ
• Inter-Tropical Convergence Zone
• Near equator Northeast Trades (N.H.)
Converge with Southeast Trades (S.H.)
along this zone.
• Is not evident as a continuous band
around the globe on a day-to-day basis.
Jet Streams
•Swiftly flowing air currents, generally near tropopause.
•Subtropical Jet Stream: On polar side of Hadley Cell;
westerly wind
•Polar Front Jet Stream: On Equator side of Polar Front;
westerly wind
Ahrens Fig 7-22
Why Jet Streams in Mid-Latitudes?
Strong Thermal Contrast
Temperature gradient produces increasing PGF
with altitude & thus increasing wind speeds.
Aguado & Burt Fig 8-6
Mid-Latitude Westerlies
Slopes of isobaric
surfaces become
steeper with
altitude
Cold
Warm
Lutgens & Tarbuck Fig 8-6
Real World Circulation
• Land-Ocean heating difference, along
with the difference between tropics and
poles, and rotation of earth.
• Sun not always directly over the Equator
(cause of the seasons).
• Expect high pressure over cold land in
the winter.
• Expect low pressure over warm land in
the summer.
Ahrens Fig 7.17a
Ahrens Fig 7.17b
Pacific High, Bermuda High
Ahrens Fig 7-19
Subtropical Highs follow the sun!
Pacific High, Bermuda High
Ahrens Fig 7-20
Global Circulation - Precipitation
Ahrens Fig 13-3
Summary
• Global Circulation
Differential Heating Between Tropics and Poles
Three Cells
Mid-Latitude Westerlies
Patterns shift slightly with seasons
• Precipitation
Major Deserts occur under Sub-Tropical High
Mid-latitude storms occur along Polar Front
Next Lecture
• Topic- Atmosphere-Ocean Interactions
El Nino and La Nina
• Reading - Ahrens pg 189-197
• Problems - 7.17, 7.18