1. Lecture 8: Atmospheric Circulation Introductive Physical Oceanography
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2. Reading: Outline: Stewart chapter 4.1 to 4.3, 4.6
Mean large-scale wind and pressure structure
Walker and Hadley circulations
Westerly winds (Ferrel cell)
Surface wind stress
Mean precipitation-evaporation fields
Temperature structure, potential temperature

3. Solar Insolation (Kump, Kasting and Crane text)

4. Seasonality Obliquity (tilt of axis) results in seasons
Perihelion - closest to sun
Aphelion - farthest from sun

5. Radiation Balance at the Top of Atmosphere

6. The Layered Atmosphere
From NASA Science site

7. The Layered Atmosphere

8. Adiabatic Lapse Rate and Potential Temperature
-(dT/dz)dry parcel = g/cp = d
Where subscript d = dry.
In dry atmosphere, adiabatic lapse rate  = 9.8 C/km
In moist atmosphere, adiabatic lapse rate  = 6-7 C/km
Potential temperature
Reference level is the ground, taken as 1000 mbar.

9. Troposphere (对流层)
Troposphere is vertically well-mixed through convection (heating from below). Temperature decreases with height only because pressure decreases with height. (on average).
Sometimes we have an “inversion” when there is colder air on the ground - very stable.

10. Stratosphere (平流层)
Stratosphere is warmest at top: due to absorption of the sun’s UV radiation. (The ozone maximum is in the middle of the stratosphere.)

11. Sea-Land Breeze Circulation

12. Monsoon
Summer Winter

13. Convergence and Divergence
ITCZ = Intertropical Convergence Zone
Actually not exactly on equator: seasonal variations in insolation, so ITCZ sits at maximum heating location. Offset in the mean because of asymmetric distribution of land between hemispheres. ITCZ is well developed north of equator.

14. ITCZ Seasonal Change

15. Observed ITCZ
Pacific Ocean
Atlantic Ocean 9/04

16. Wind System
Surface winds in Hadley cell: deflect to right in NH, deflect to left in SH 
TRADE WINDS in both hemispheres

17. Ocean Surface Wind

18. Northern Hemisphere Sea Level Pressure (SLP)

19. Southern Hemisphere Sea Level Pressure

20. Wind Stress
Wind speed is measured above the sea surface. In practice measurements are at different heights. For consistency, adjust all measurements to a 10 meter height.
Actual stress on the ocean:
 = cDu2
where u is the wind speed at 10 meters,  is the air density 1.3 kg/m3, and cD is the (dimensionless) drag coefficient, which is determined empirically.

21. as wind speed increases: cD= (0.61 + 0.063u) x 10-3
Wind Stress
From Stewart online text
Typical value for drag coefficient:
Low wind speed: cD= 1.1 x 10-3
as wind speed increases: cD= ( u) x 10-3

22. Surface Winds and Surface Pressure

23. Hydrologic Cycle (Water Cycle)

24. Vapor Pressure
Warm air holds a lot more water vapor than cold air. In equilibrium, warmer air will free more water molecules from liquid water, and raise the amount of vapor. This eventually falls out as precipitation.

25. Water Cycle

26. Mean Cloud Cover

27. Precipitation-Evaporation
Evaporative heat loss

28. Precipitation-Evaporation
Net evaporation-precipitation (red evaporates, blue freshens)

29. Surface Salinity

30. Net Heat Exchange

31. Ocean Surface Temperature

32. Questions (Due in 2-week)
At about what latitude in the northern hemisphere does the switch from trades to westerlies occur?
What direction is the prevailing wind at the equator?
If the wind speed at 10 meters height is 10 m/sec, what is the wind stress on the ocean? (use drag coefficients from lecture or from Stewart electronic notes)
How many main low pressure and high pressure centers are in the world? What are their names? Please identify their latitude and longitude.

33. Learning about the Ocean, the Climate and the Nature