1. Chapter 6 Atmospheric and Oceanic Circulations

2. Wind is a vector variable
Temperature is a scalar variable.

3. Atmospheric and Oceanic Circulations
Wind Essentials  
Driving Forces within the Atmosphere  
Atmospheric Patterns of Motion  
Oceanic Currents  

4. Wind Essentials Air Pressure and Its Measurement
Mercury barometer
Aneroid barometer
Wind: Description and Measurement  
Wind
Anemometer
Wind vane
Global Winds  

5. Barometers
Figure 6.2

6. Air Pressure Readings
mb= kpa=14.66lb/in2
Figure 6.3

7. Atmospheric Pressure and Elevation
Which point (p1 vs. p2) has higher air pressure? Why?
How are pressure change with elevation?
Uniformly decrease with elevation
decreases faster with higher close to see level than at high elevations. p2 p1 7

8. Wind Vane and Anemometer
Wind: horizontal movement of air across Earth surface.
Vector: Speed measured by Anemometer
Direction measured by wind vane
wind direction is defined as the
direction from which it originates.
Standard measurement of wind
is 10 m above ground.
Old weather forecast refer wind speed in scales, a commonly used one is Beaufort Wind Scale.
Figure 6.4

9. Driving Forces within the Atmosphere
Pressure Gradient Force  
Coriolis Force  
Friction Force  
Gravity

10. Pressure Gradient Force
Pressure difference is primarily caused by uneven heating of Earth surface
Without pressure gradient force, the air will not move, then there will be no Coriolis force, no friction force.
Figure 6.7

11. Coriolis Force
It deflects anything that flies or flows across Earth surface: wind, airplane, ocean currents etc.
Coriolis Force only changes the direction of movement, not the speed. It is always perpendicular to the direction of movement, to the right hand side on Northern Hemisphere.
F=2vΩsin(φ), where v=wind speed, Ω=angular velocity of earth rotation 7.29x105 radians per second, φ=latitude
The stronger the wind, the stronger Coriolis force.
Figure 6.9

12. Pressure + Coriolis + Friction
Pressure Gradient Force only
Pressure Gradient+Coriolis+Friction Forces
Pressure Gradient +Coriolis Forces
Friction force: Always in the opposite direction of wind.
Strength: depending on wind speed, surface condition (topography, vegetation, …)
Figure 6.8

13. Scales of Atmospheric Movement
The movement of atmosphere around the globe is a composite of multiple scale motion, like a meandering river contains larege eddies composed o f smaller eddies containing still smaller eddies.
Macroscale: Large Planetary wide movement of atmosphere, e.g. trade winds, monsoon, hurricanes, which can blow for weeks or longer.
Mesoscale: lasts for several minutes or hours, usually less than 100 km across, e.g. thunderstorms, tornadoes  
Microscale: smallest scale of air motion, lasts for seconds at most for minutes, e.g. wind gust, dust devils  

14. The Westerlies tropopause Earth sun Pressure gradiant N. Pole Equator
500mb
600mb
Pressure gradiant
700mb
800mb
900mb
1000mb
Equator
N. Pole 14

15. Single Cell Model (Hadley 1735)
sun
Figure 6.12 15

16. Three-Cell Model (1920s) Ferrel Cell Polar Cell Hadley Cell
Figure 6.12

17. General Atmospheric Circulation and pressure zones
Figure 6.12

18. General Atmospheric Circulation
Figure 6.12 18

19. Primary High-Pressure and Low-Pressure Areas
Equatorial low-pressure trough: thermal
Polar high-pressure cells: thermal
Subtropical high-pressure cells: dynamic
Subpolar low-pressure cells: dynamic

20. Equatorial Low-Pressure Trough
Intertropical convergence zone (ITCZ)
Clouds and rain
Trade winds: The trade winds were named during the era of sail ships that carried trade across the seas.

21. Global Barometric Pressure
Icelandic Low
Siberian High
Aleutian Low
Azores High
Hawaiian High
The subtropical high pressure zone broke into three high pressure centers: Hawaiian, Azores, Siberian Highs
The subpolar low pressure zone broke into two low pressure centers: Aleutian and Icelandic Lows
Figure 6.10

22. High Pressure Center
East side drier and more stable, feature cooler ocean currents than west side. Earth major deserts extend to the west coast of each continent. 22

23. Global Barometric Pressure
Pacific high
Bermuda high
The high pressure centers are pushed northern. As a result, the subpolar low pressure centers are weakened significantly.
Figure 6.10

24. Global Patterns of Pressure
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

25. Wind Portrait of the Pacific Ocean
Westerlies
Pacific High
Trade wind
Wind pattern derived from
a radar scatterometer aboard
Seasat on a day in September.
Note: compare wind pattern
and the visible earth below:
Figure 6.6

26. June–July ITCZ
Figure 6.11

27. Monsoonal Winds Larger than average northward migration of ITCZ.
Regional wind systems seasonally changes direction and intensity associated with changes temperature and precipitation.
Winter: cold dry wind blow off the continents
Summer: warm moist-laden wind blow from sea toward land
Figure 6.20

28. Upper Atmospheric Circulation
Jet stream: a fast flowing narrow air currents in the upper atmosphere
Rossby waves

29. Jet Streams
An concentrated band of wind occurring in the westerly flow aloft.
Flat in vertical direction
Speed up to 190 mph
Stronger in winter
It is caused by the large pressure gradient caused by the large horizontal temperature difference over short distance.
Influence surface weather systems.
30-70oN
20-50oN
Figure 6.17

30. Rossby Waves
Note: in the upper atmosphere, artic area has low pressure, thus the air circles counterclockwise parallel to the pressure gradient (Why?). But this circle is not perfect. Instead, it follows a wavy path.
Discovered by Carl G. Rossby in It refers to the waving undulations of geostrophic winds of the arctic front.
Figure 6.16

31. Rossby Waves Smooth westward flow of upper air westerlies
Develop at the polar front, and form convoluted waves eventually pinch off
Primary mechanism for poleward heat transfere
Pools of cool air create areas of low pressure
Figure 6.16 31

32. Local Winds
Land-sea breezes
Mountain-valley breezes
Katabatic winds

33. Land-Sea Breezes
Figure 6.18

34. Mountain-Valley Breezes
Katabatic Wind: A regional scale gravity driven wind, usually needs a high plateau to cool the air, and become dense and flow downslope.
Figure 6.19

35. Oceanic Currents Function: Mixing sea water
Surface warm water with deep cold water
CO2 absorption
Climate
Biogeochemical processes: phytoplankton growth
Driving force: the frictional drag of winds
Thus we have an Atmosphere-Sea are coupled system. Once the current starts to move, the Coriolis force will kick in. Then there is “friction” between upper and lower water, the shear stress.   

36. Major Ocean Surface Currents
Surface ocean currents are driven by air circulation around subtropical high pressure cells.
Figure 6.21

37. Equatorial Currents/Western Intesificaitn
Corresponding to trade winds on both sides of the Equator, these winds drives the surface current westward along the equator, called equatorial currents.
The equatorial currents push water piles up against the eastern shores of the continent. This is called western intensification. The piled up water will go either up north or down south . The Gulf Stream is one caused by western intensification.

38. Upwelling/Downwelling Currents
Upwelling Currents: When surface water is swept away from a coast, an upwelling current occurs. This cool water generally is nutrient rich, e.g. Pacific Coast of North and South America
Downwelling Currents: Accumulation of surface water (e.g. western end of equatorial current) can gravitates downward to generate a downwelling current.
Figure 6.22 38