1. The Atmosphere - Driving Force for the
Ocean
- Composition, Density and Pressure
- Solar Heating and the Coriolis Effect
- Atmospheric Circulation
Reading:
4th Ed., Ch 8, Secs 2-9
5th Ed., Ch 8, Secs 2-10
Graphic: Cumulus clouds. R.F. Kresge, photographer. Courtesy of NOAA.
2. Atmospheric Composition
The atmosphere is a mixture of gases, consisting mostly of nitrogen and oxygen
About 4% of the volume of the lower atmosphere is water vapor
Graphic: Blue Planet Fig. 12.2, composition of th dry atmosphere. See Garrison,
4th Ed., Table 8.1, pg 186, or 5th Ed., Fig. 8.1, pg 178.
3. Atmospheric Pressure*
Atmospheric pressure = the weight of the air above
Atmospheric pressure decreases with height
At 5.5km altitude, atmospheric pressure is half the pressure at sea level
Graphic: Blue Planet, Fig. 12.6.
3. What Happens as Dry Air Rises?
As a balloon rises:
- it encounters lower atmospheric pressure and expands
- air molecules in the balloon get farther apart and collide less frequently so
temperature within the balloon drops
Graphic: A balloon rising through the atmosphere. Longer, heavier arrows
indicate more atmospheric pressure at lower altitudes.
4. Vertical Motion of Moist Air
As an air parcel rises:
- it expands
- it cools
- water vapor can condense from the cooler air forming clouds
Ascending air expands, cools and moistens
Descending air compresses, warms and dries out
Graphic: Garrison, 4th Ed. Fig. 8.1a, pg 187, 5th Ed. Fig. 8.2a, pg 179.
5. What Influences Global Atmospheric Circulation?
- Uneven solar heating
- Rotation of the Earth (the "Coriolis effect")
- Land/sea temperature contrasts
Graphic: Earth, as viewed from Apollo 17. Image courtesy of NASA.
6. Solar Radiation and Heat Transport
Tropics:
- gain more heat from the sun than they lose to space
- heat surplus
Poles:
- lose more heat to space than they gain from the sun
- heat deficit
Heat is transported from the tropics to the poles to even out the surplus and
deficits
Graphic: Garrison, 4th Ed. Fig. 8.4b, pg 189, 5th Ed. Fig. 8.5b, pg 181.
7. How is Heat Transported by the Atmosphere?
Convection currents transport heat in the atmosphere
Warm air rises
Cold air sinks
Convection currents result from this vertical motion and the horizontal motion
that closes the loop
Graphic: Garrison, 4th Ed., Fig. 8.6, pg 190, 5th Ed., Fig. 8.7, pg 182.
8. Atmospheric Circulation on a Non-Rotating Earth
Warm air rises in the tropics
Cool air sinks at the poles
Horizontal currents close the loop:
- flow to the north aloft
- flow to the south at the surface
Heat is transported from tropics to poles
Graphic: Garrison, 4th Ed., Fig. 8.7, pg 190, 5th Ed., Fig 8.8, pg 182.
9. But What About Earth Rotation?
The linear distance traveled by a point on the Earth's surface depends on
latitude
Points at the equator travel farther in one day than points at other latitudes
The difference in linear speed with latitudes produces the "Coriolis effect"
Graphic: Garrison, 4th Ed., Fig. 8.8, pg 189, 5th Ed., Fig. 8.9, pg 182.
10. What is the Coriolis Effect?
The Coriolis effect deflects motion:
- to the right in the northern hemisphere
- to the left in the southern hemisphere
The Coriolis effect influences all moving objects
It is very important for atmospheric and oceanic circulation
Graphic: Garrison, 4th Ed., Fig. 8.11, pg 192, 5th Ed., Fig. 8.12, pg 184.
(animation)
11. Earth Rotation and Atmospheric Circulation
Non-Rotating Earth Rotating Earth
Graphic: (left) Garrison, 4th Ed., Fig. 8.7, pg 190, 5th Ed., Fig. 8.8, pg 182,
(right) Garrison 4th Ed., Fig. 8.12, pg 193, 5th Ed., Fig. 8.13, pg 185.
12. Circulation Cells
Polar Cell - dry, cold air descends at poles
Ferrel Cell - governs circulation at midlatitudes
Hadley Cell - rising air in the tropics; sinking air at midlatitudes
Blue Planet, Fig. 13.12. See Garrison 4th Ed., Fig. 8.12, pg 193, 5th Ed., Fig.
8.13, pg 185.
13. Strong Surface Winds
Westerlies (30-60 N and S)
- Ferrel cell surface winds
- Influence midlatitude weather
Trade Winds
- Hadley cell surface winds
- Northeast trades (0-30 N) blow from NE to SW
- Southeast trades (0-30 S) blow from SE to NW
Blue Planet, Fig. 13.12. See Garrison 4th Ed., Fig. 8.12, pg 193, 5th Ed., Fig.
8.13, pg 185..
14. Weak Surface Winds
Doldrums - Calm areas near the equator where the northern and southern
hemisphere Hadley cells converge
Horse latitudes - Areas of very little wind between Hadley and Ferrel cells
Blue Planet, Fig. 13.12. See Garrison 4th Ed., Fig. 8.12, 5th Ed., Fig. 8.13, pg
185.
15. Deserts and Rainforests
Polar Regions: sinking air (cold and dry)
Midlatitudes: rising air (mild and wet)
Subtropics: sinking air (hot and dry)
Equatorial region:
- rising air at ITCZ
- intense thunderstorms
- very hot and wet
Graphic: False color view of ocean and land color, courtesy of the SeaWiFS
Project, NASA/Goddard Space Flight Center, and ORBIMAGE.
16. Summary - Atmospheric General Circulation
Circulation Cells
- Hadley, Polar, Ferrel
Strong Winds
- Westerlies, Trades
Weak Winds
- Doldrums, Horse Latitudes
Climate
-Deserts (descending air)
-Forests (ascending air)
- ITCZ
- Jet Stream and Polar Front
Blue Planet, Fig. 13.12. See Garrison 4th Ed,. Fig. 8.12, pg 193, 5th Ed., Fig.
8.13, pg 185.
17. Preview of Next Lecture
Sailing the Seas: Wind Driven Ocean Circulation
Reading:
4th Ed., Ch 9 Secs 2-5, 11-14
5th Ed., Ch 9 Secs 2-5, 11-14
Graphic: Gulf Stream sea surface temperatures (off the east coast of the US) as
measured by satellite. Courtesy of NASA.