1. Winds and the Global Circulation System
Objectives:
Describe the measurement and variation of atmospheric pressure
Explain the processes that impact upon wind direction and weather systems
Describe the pressure and circulation patterns of the atmosphere
Evaluate the role of upper atmospheric circulation
Describe the circulation patterns of the oceans

2. Atmospheric Pressure
As the atmosphere is held down by gravity, it exerts a force upon every surface (pressure = force per unit area)
At sea level the force is the weight of 1 kg of air that lies above each square centimeter of the surface
(around 15 lbs per inch)

3. -Atmospheric pressure decreases rapidly with altitude near the surface
-Therefore a small change in elevation will often produce a significant change in air pressure

4. Differences in air pressure = a pressure gradient
The pressure gradient forces acts at right angles to the isobars (90 degrees)
820
830
840
850
860
820
830
840
850
860
870
880
890
strong pressure gradient
weak pressure gradient

5. The planet Earth also rotates
in the northern hemisphere air appears to be deflected to the right
in the southern hemisphere, deflected to the left
this deflective force = Coriolis force
because the wind is deflected it now flows parallel to the isobars = geostrophic wind

6. Imagine a turntable
when not turning, a ball traces straight line
when moving, ball traces a curved line

7. low pressure high pressure gradient force winds pressure geostrophic
992
996
1000
1004
1008
1012
1016
1020
high pressure

8. Friction forces
As wind flows over the surface friction reduces the speed
Friction also changes the direction of the geostrophic wind
The pressure gradient force over powers the Coriolis effect
As a result wind flow across the isobars

9. In Northern hemisphere – cyclones spiral counter clockwise
Anticyclones spiral clockwise
In Southern hemisphere – opposite
Cyclones spiral clockwise
Anticyclones spiral counter clockwise

10. Land and Sea breezes
During the day, air over land heats up and the sea is relatively cool (sea breeze)
land = low pressure and sea = high pressure

11. At night air over land cools and the sea is relatively warm (land breeze)
land = high pressure and sea = low pressure

12. H H L L L High pressure (anticyclone) air descends
Side View
From above H H L L L
air descends
surrounding air is relatively low

13. L L H H H Low pressure (depressions, cyclone) air ascends
Side View
From above L L H H H
air ascends
surrounding air is relatively high

14. General Circulation of the Earth’s Atmosphere
Deflection is least at the equator and greatest at the poles

15. In Northern hemisphere – cyclones spiral counter clockwise
Anticyclones spiral clockwise
In Southern hemisphere – opposite
Cyclones spiral clockwise
Anticyclones spiral counter clockwise

16. EARTH SUN But heat is transported from the Equator to the Poles - how?
Cold
High Pressure
Warm
Low Pressure
EARTH
SUN

17. L Warm air rises and flows polewards
PRESSURE L
So, around the equator = (ITCZ) Inter-Tropical Convergence Zone
= low pressure because solar heating

18. Doldrums
Samuel Taylor Coleridge
“The Rime of the Ancient Mariner”
“Down dropt the breeze, the sails dropt down, 'Twas sad as sad could be”
“Day after day, day after day, We stuck, nor breath nor motion ; As idle as a painted ship Upon a painted ocean.”
Intense heating causes air to rise, leading to little horizontal motion at times
Very calm conditions

19. Cooled air sinks at 30 degrees N&SH
HIGH
PRESSURE
Subtropical High

20. Trade winds are predictable
North East in Northern hemisphere
South East in Southern hemisphere
At subtropical high conditions are calm
known as the “horse latitudes”

21. Cooled air sinks at 30 degrees N&S
Air flows northwards towards the Poles
Some air flows back towards the Equator
HIGH
PRESSURE

22. Warm air meets cold air and rises
Warmer air rises L
Cold air sinks
Subpolar low pressure because the warmer air of the mid-latitudes rises as it meets cold polar air

23. H At the Poles the air is cold and dense
this produces an area of high pressure H

24. From the subtropical high to the subpolar low = “westerlies”
Includes variable low and high pressure systems
At the Polar Front - storms form
From the Polar High to the subpolar low are the Polar easterlies - variable

25. Hadley
Cells

26. Monsoons
-January high pressure over the land produces dry winds.
-Air flows towards the ITCZ.

27. Monsoons
July - position of the ITCZ moves North
Low pressure over the land causes winds to flow off the ocean.
This brings heavy rainfall.

30. Permanent area of high pressure over Antarctica
Seasonal pressure changes over the Arctic

31. Upper Atmosphere At 5-7 kms above the surface
Influenced only by pressure gradient force and Coriolis force
Geostrophic winds that flow parallel to isobars

32. 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

33. Jet Streams Narrow bands of high velocity
Form along the polar front and above the Hadley cell in the subtropics

34. Circulation patterns (CURRENTS) produced by:
Winds
Density differences in sea water
Coriolis force
Shape of ocean basins
Astronomical factors (TIDES)

35. Ocean Temperatures Surface warms – especially in the summer
Layer of rapid temperature drop - thermocline

36. Ocean Currents wind Driven mostly by wind blowing over the surface
However, currents move slowly
Lag behind wind speed so often called drifts
wind

37. Ocean currents
Large continuously moving loops (gyres)
Produced by winds, Coriolis force and land masses

38. Gyres Large circular currents
Subtropical gyre corresponds to the subtropical high pressure

39. N. and S. Equatorial currents corresponds to the trade winds
Equatorial countercurrent corresponds to the ITCZ

40. West winds around Antarctica create circumpolar gyre
Portions branch off toward the equator

41. Deep-sea currents
Driven by differences in temperature and salinity
Much slower than surface currents
Thermohaline circulation
Depends on conditions in the North Atlantic

42. El Nino/Southern Oscillation (ENSO)
Every year warming occurs off the coast of Peru (~2˚/C)
Suppresses upwelling
But every 4 or 5 years it is much more pronounced = El Nino
Because there is no upwelling fish die
May have far wider effects

43. In normal year, low pressure dominates in Malaysia and northern Australia.
In an El Niño year, low pressure moves east to the central part of the western Pacific