1. NATS 101 Lecture 20 Global Circulation

2. Supplemental References for Today’s Lecture
Aguado, E. and J. E. Burt, 2001: Understanding Weather & Climate, 2nd Ed. 505 pp. Prentice Hall. (ISBN )
Lutgens, F. K. and E. J. Tarbuck, 2001: The Atmosphere, An Introduction to the Atmosphere, 8th Ed. 484 pp. Prentice Hall. (ISBN )

3. Review Global Energy Balance (from long ago!)
Thermally Direct Circulations AGAIN!

4. Annual Energy Balance Heat transfer done by winds and ocean currents
Radiative Warming
Radiative Cooling
Radiative Cooling NH SH
Heat transfer done by winds and ocean currents
Differential heating drives winds and currents

5. 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

6. 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

7. Thermally Direct Circulation
DIV
CON H L
Warm air toward poles
Heat
Heat
Warm
Rising
Sinking
Cold L H
Cold air toward equator
CON
DIV
Heat
Heat
Equator
Poles

8. 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

9. 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 … ?
Equatorial region
Greatest heating
Polar region
Least heating
Polar region
Least heating

10. Waterworld Single Equator to Pole Cell
Cold air Poles
Hadley Cell
Divergence Aloft
Warm air Equator
Surface
Ahrens Fig. 7.14
Surface
Convergence Aloft

11. Consider a Rotating Waterworld
Triple Cell
Consider a Rotating Waterworld
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)
Hadley
Ferrel
Polar
Equator
Pole

12. Rotating Waterworld Prevailing Winds
Ahrens Fig. 7.16

13. Major Surface Pressure Zones
Ahrens Fig 7-17

14. 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.

15. 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

16. 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

17. Mid-Latitude Westerlies
Slopes of isobaric surfaces become steeper with altitude
Lutgens & Tarbuck Fig 8-6
Cold Warm

18. 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.

19. Ahrens Fig 7.17a

20. Ahrens Fig 7.17b

21. Pacific High, Bermuda High
Ahrens Fig 7-19
Subtropical Highs follow the sun!

22. Pacific High, Bermuda High
Ahrens Fig 7-20

23. Global Circulation - Precipitation
Ahrens Fig 13-3

24. 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

25. Next Lecture Topic- Atmosphere-Ocean Interactions El Nino and La Nina
Reading - Ahrens pg
Problems , 7.18