PGF. Coriolis Force. Geostrophic winds. Friction. Surface winds, Northern Hemisphere vs. Southern Hemisphere. Convergent and Divergent Air. Hadley cells. Global circulation model, breakdown of each zone. Jet streams, Rossby Waves. Monsoons. Seasonal wind changes (global). Multi-year atmospheric oscillations: ENSO, NAO. Ocean currents, Ekman spiral/transport, upwelling/downwelling.

1. Atmospheric and Oceanic
Circulation
Chapter 4--Part 2

2. 3 Forces Affecting Air in Motion
•Pressure Gradient Force
•Coriolis Force
•Friction

3. Force #1:
The Pressure Gradient Force
• The pressure gradient force is the force exerted by
a gas (in this case, air) at higher pressure trying to
move to an area of lower pressure
• The PGF pulls air out of the high and into the low at
a 90º angle relative to the isobars
• The greater the “slope”, or gradient, between one
pressure region and the next, the faster the air will
move

4. Where the Isobars are Close Together,
Winds are Faster & Stronger
HEY… Hold ON.
What’s UP with the curving motion?

5. Force #2:
The Coriolis Force
• A force which causes fluids in motion over
great distances and objects moving at high
speed to be deflected:
to the right in the Northern Hemisphere
to the left in the Southern Hemisphere
(Note: Air acts like a fluid in many ways.)

6. PGF + Coriolis Force =
“curving” wind

7. Coriolis Force—doing the math
• The Coriolis force is a force existing in a
rotating coordinate system with constant
angular velocity to a reference frame. It acts
on a body moving in the rotating frame to
deflect its motion sideways.

8. Formulae (partial - for the mathematically curious)
• In non-vector terms: at a given rate of rotation of the observer, the
magnitude of the Coriolis acceleration of the object is proportional to the
velocity of the object and also to the sine of the angle between the
direction of movement of the object and the axis of rotation.
• The vector formula for the magnitude and direction the Coriolis
acceleration is
where (here and below) is the velocity of the particle in the rotating
system, and is the angular velocity vector (which has magnitude equal
to the rotation rate and is directed along the axis of rotation) of the
rotating system. The equation may be multiplied by the mass of the
relevant object to produce the Coriolis force:
• The × symbols represent cross products. (The cross product does not
commute: changing the order of the vectors changes the sign of the
product.)
• The Coriolis effect is the behavior added by the Coriolis acceleration.
The formula implies that the Coriolis acceleration is perpendicular both
to the direction of the velocity of the moving mass and to the rotation
axis.

9. • A force which causes fluids in motion over
great distances and objects moving at high
speed to be deflected:
to the right in the Northern Hemisphere
to the left in the Southern Hemisphere
Remember:

10. Coriolis Force: In The Toilet
• Given the definition of coriolis force, is it
valid to assume that the water in your toilet,
sink, or bathtub will be deflected to the right
while draining?

11. • A force which causes fluids in motion over
great distances and objects moving at high
speed to be deflected
to the right in the Northern Hemisphere
to the left in the Southern Hemisphere

12. • A force which causes fluids in motion over
great distances and objects moving at high
speed to be deflected
to the right in the Northern Hemisphere
to the left in the Southern Hemisphere

13. Geostrophic Winds
• When the Coriolis Force and Pressure
Gradient Force balance one another, winds
spin around a high or low pressure cell,
parallel to the isobars
• These winds occur in the upper atmosphere,
where there is no friction
• They are known as geostrophic winds

14. Geostrophic winds

15. Force #3:
Friction

16. Putting it together:
3 Forces Affecting Air in Motion

17. Surface winds:
Make a simple drawing
• Be able to draw it in your sleep...

18. Northern Hemisphere and
Southern Hemisphere Winds

19. Convergent and Divergent Air

20. Hadley Cells

21. A Simplified Global Circulation
Model

22. The ITCZ

23. Subtropical Highs

24. Some are so prominent, they even
have their own special names

25. Between the ITCZ and the SHPs
are the Trade Winds

26. The Hadley Cell at Work

28. The Westerlies

29. Subpolar Lows

30. Polar Easterlies

31. Polar Highs

32. A Simplified Global Circulation
Model

33. The Jet Stream(s)

34. Rossby Waves:
Undulations in the Jet Stream

35. World Regions with Monsoon Patterns

36. Monsoons in India and Asia

37. Minor Monsoons: Australia and W. Africa

38. Seasonal Movement of the ITCZ

39. Seasonal Pressure Changes Cause
Seasonal Wind Changes

40. ITCZ shifts more dramatically over
land than it does over water

41. 41
Multi-year Atmospheric Oscillations
• ENSO--El Niño-Southern Oscillation
–Ocean-Atmosphere connection
• (we will discuss this phenomenon in Chapter 7)
• NAO--North Atlantic Oscillation
–Affects Europe, eastern US, Greenland/Canada
region; no defined pattern
• AO--Arctic Oscillation
–Associated with NAO
• PDO--Pacific Decadal Oscillation
41

42. El Niño/Southern OscillationEl Niño/Southern Oscillation
(ENSO)(ENSO)

43. 43
NAO--Positive Phase
• Stronger Azores
high and deeper
Icelandic low
• Stronger winter
storms, more of
them to the north
• Mild, wet eastern
U.S.; warm, wet in
N. Europe
• Cold, dry Med., west
Greenland, NE
Canada

44. 44
NAO--Negative Phase
• Weaker Azores high,
Icelandic low
• Reduced PGF =
weaker storms and
less of them
• Cold snaps in
eastern U.S. bring
more snow; cold, dry
in N. Europe
• Wetter Med.;
Greenland, NE
Canada milder

45. 4545
Ocean CurrentsOcean Currents
• Forces driving ocean currentsForces driving ocean currents
–Frictional drag of windFrictional drag of wind
–Coriolis forceCoriolis force
–Temperature, density, and salinity differencesTemperature, density, and salinity differences
–Location of continents and shape of the sea floorLocation of continents and shape of the sea floor
–TidesTides
4545

46. Warm and Cold Surface CurrentsWarm and Cold Surface Currents
• Direction and temperatureDirection and temperature

47. Ekman Spiral
(Northern Hemisphere)
wind
Force of
wind on
surface
Coriolis
force
Current

48. Ekman Transport

49. 49
Upwelling Currents
• Where the net movement of water is away
from the coast, cold, dense water rises up
from the bottom of the ocean to replace the
water that has moved away.
49

50. 50
Downwelling Currents
• Where the net movement of water is toward
the coast, warmer surface water piles up
and pushes down toward the bottom of the
ocean, displacing colder water, below.
50

51. 51
Open-ocean Upwelling
• Near the equator,
upwelling occurs
where surface winds
cause ocean water to
diverge. As surface
waters move apart,
cold bottom water rises
up to replace what’s
been pushed away.

52. Currents: Thermohaline CirculationCurrents: Thermohaline Circulation