1. Winds and Global Circulation
Chapter 5
Winds and Global Circulation

2. Introduction
Reason for winds goes back to concepts of insolation and radiation
Differential (unequal) heating of Earth’s surface by latitude and at smaller scales leads to variations in pressure  gradient between locations  air motion
Like opening a bottle or can of beer
Earth’s rotation plays role in direction of flow

3. How Pressure Leads to Air Motion

4. So What is Pressure? Force per unit area
Dad says, “Have you checked the air in your tires lately?” – can measure because higher pressure in tire exerts force on gauge
In day-to-day life, why do we feel atmospheric pressure?
Air has mass and is being pulled downward by Earth’s gravity

5. Measurement of Pressure
Various units – in tire example, use pounds per square inch; also kg/cm2, pascals (Pa), millibars (mb), cm or mm of Mercury, inches of Mercury
Standard sea level pressure = 101,320 Pa = mb = ~ 76 cm Hg = 760 mm Hg = in Hg
Measure using barometer – mercury or aneroid

6. Mercury Barometer Very accurate! Hg in a glass tube
Tube placed in dish of Hg
Hg flows out of tube due to gravity  a vacuum at top
Hg stops flowing when air pressure pushing on Hg in dish = pull of gravity on Hg in tube
As pressure on Hg in dish increases, Hg in tube rises

7. Pressure Change with Altitude
What do you notice if you swim to the bottom of a pool?
Same concept with atmosphere – less of atmosphere above you as altitude increase, so less pressure
On weather maps, use a formula to adjust pressure readings to sea level, which enables pressure analyses (isobars)

8. Wind
Air motion or flow of air with respect to surface, usually only horizontal movements
Measured in meters per second or miles per hour (1 m/s approximately = 2 mph)
Speed determined using anemometer (most common is cup anemometer)
Direction determined with wind vane and always given as direction from which wind is coming (wind blowing from NW to SE = northwesterly wind); also use degrees

9. Measuring Wind Speed and Direction
Wind vane and anemometer Compass directions/
degrees

10. Forces Affecting Wind
Many features affect wind (buildings, hills, valleys, etc.); think of swirling motion in a football/baseball stadium
Three main forces determine wind speed and direction on larger scales:
1. Friction
2. Pressure gradient force
3. Coriolis effect

11. Friction “Drag” due to a surface
Causes wind speeds to decrease and has minor influence on wind direction
If looking at a profile of wind vs. height, would see a half-U shape, with slower speeds near the surface and increasing values higher in atmosphere
Negligible at high levels

12. Pressure Gradient Force
Due to difference in pressure (a gradient) between locations
PGF “pushes” air from areas of higher to lower pressure
A stronger gradient (greater difference in pressure) between locations produces stronger winds
Therefore, affects both direction and speed
If only PGF, then wind is perpendicular to isobars, but of course this is too simple....

13. Pressure Gradient Force

14. Pressure Gradient Force

15. Coriolis Effect
Deflection of motion of an object (including wind) from its path
So mainly affects direction
Due to rotation of Earth
In Northern Hemisphere, deflection is to the right of pressure gradient force, while in Southern Hemisphere, deflection is to the left of PGF
Most deflection at poles, least at equator

16. Coriolis Effect

17. Local Winds
Friction and PGF have greatest effect on small-scale winds; Coriolis negligible
Some local winds include:
Convective winds: heating causes pressure gradient; at low levels, wind flows toward warm region (convergence), and at high levels, wind flows away from warm region (divergence)
Mountain and valley breezes: during day, mountainsides heated causing air flow up valleys; opposite effect at night

18. Local Winds (cont’d)
Land and sea breezes: due to specific heat attributes (land heats and cools faster than water); during the day, land heats, and air flow from cooler water surface towards land (sea breeze); opposite at night, because water warmer than land surface (land breeze)

19. Cyclones and Anticyclones
Areas of low pressure
Air spirals inward and upward (convergence)
Due to Coriolis, air moves counterclockwise in NH and clockwise in SH
Associated with cloudy weather and precip
Anticyclones
Areas of high pressure
Air spirals outward and downward (divergence)
Due to Coriolis, air moves clockwise in NH and counterclockwise in SH
Associated with fair weather

20. Winds in Cyclones (L) and Anticyclones (H)

21. SUN How is heat transported from the Equator to the Poles? Earth Cold
High Pressure 0o
30oN
60oN
30oS
60oS
90oN
Warm
Low Pressure
SUN
Earth

22. L Warm air rises at the equator producing Low pressure and flows
towards the poles 0o
30oN
60oN
30oS
60oS
90oN L

23. H L H Cold air sinks at 30o N and S latitude Creating high pressure
30oS
60oS
90oN
Cold air sinks at 30o
N and S latitude
Creating high pressure
(Subtropical High
pressure) H L H

24. H L H Northeasterly and southeasterly surface winds flow from the
subtropical high pressure belts
(30o N and S) to the low
pressure belt (ITCZ) at
the equator (calm winds:
doldrums)
westerly surface winds
flow from the subtropical
high pressure belts towards
higher latitudes 0o
30oN
60oN
30oS
60oS
90oN H L H

25. IG4e_05_19
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26. L H L H L westerly surface winds are forced
to rise around 60o N and S latitude
when they encounter cold polar
easterly winds from the poles
resulting in Subpolar Low
pressure (SPL) belts 0o
30oN
60oN
30oS
60oS
90oN L H L H L

27. H L H L H L H cold air sinks at the poles producing polar high (PH)
pressure regions 0o
30oN
60oN
30oS
60oS
90oN L H L H L H

28. Three-cell Model

30. 2-D Glance at Surface Semi-permanent pressure cells around globe
Called semi-permanent, because location and intensity vary with season (Fig. 5.17, p. 164)
Seasonal shifts lead to
Movement of Intertropical Convergence Zone (ITCZ) – moves northward in July, southward in January – why?
Monsoon: shift in wind direction from offshore flow in winter (dry season) to onshore flow in summer (wet season)

31. Semi-permanent Pressure Cells

33. Winds Aloft
As you move farther from the Earth’s surface, friction has less impact, so can focus mainly on PGF and Coriolis
Geostrophic wind: wind that moves parallel to isobars and at a right angle to the PGF (Fig. 5.22a)
Flow is initially in direction of PGF, but in Northern Hemisphere, Coriolis deflects motion to the right
At some point, PGF = Coriolis (sum = 0), so speed and direction of air flow no longer changes
See Fig. 5.22b for illustration of final two points

34. Winds Aloft

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36. Other Characteristics of Winds Aloft
General pattern:
Weak easterly winds in tropics (Equatorial Easterlies)
High pressure at Tropics of Cancer and Capricorn, westerly winds to Arctic and Antarctic Circles (Westerlies)
Spiraling motions from Circles to poles

37. Specific Features of Winds Aloft
Rossby waves
Undulations or waves in Westerlies which move cold air toward equator and warm air toward poles
Primary mechanism for poleward heat transfer
Polar front
Sharp boundary between cold polar air and warm tropical air
Jet streams
Narrow zones (tube-like) of very fast wind speeds (center has highest speeds)
Occur along strong pressure gradients
Two affecting US – polar jet and subtropical jet

38. 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 transfer
Pools of cool air create areas of low pressure

39. Ocean Circulation
Why are we talking about the oceans during the same class at winds?
Atmospheric circulation drive the direction of surface ocean currents
Ocean currents also act as heat transfer mechanisms (help global energy balance)
General pattern is warm currents on eastern flank of continents and cold currents on western flank (Gulf Stream vs. California current)

40. Ocean Currents Upwelling is vertical movement in the oceans
Important because it brings nutrients to upper levels of ocean
Occurs along western edges of continents – one of strongest is along South America
What does this have to do with weather and climate?
Teleconnection – relationship between circulations in one region and weather/climate in another region; impacts vary across globe and even on same continent

41. Ocean Currents

43. El Nino El Niño-Warmer than normal waters in the Equatorial Pacific
La Niña-Cooler than normal waters in the Equatorial Pacific

44. Normal vs. El Nino and Associated Weather Effects