1. Lecture 14 4 February 2005 Atmospheric and Oceanic Circulations (continued) Chapter 6

2. Figure Credit: “Earth’s Climate” by W. Ruddiman

4. Credit: www.physicalgeography.net Wind simply put, wind is the horizontal flow of air in response to differences in air pressure these pressure differences are usually due to uneven solar heating at the surface wind flows because of pressure gradient ‘heat rises’

5. Wind winds are designated as direction from not direction to (oceanographers do it the opposite) wind compass so, a westerly wind would be coming from what angular direction?

6. Four forces that determine winds 1. Gravity - pulls gas molecules close to Earth density & pressure decrease with height 2. Pressure gradient force - the difference in air pressure between areas 3. Coriolis force - deflects wind from a straight line to the right or left depending on hemisphere 4. Friction force - the drag on air flow from the Earth’s surface

7. Pressure vs. Pressure Gradient The value of pressure itself is NOT important The CHANGE in pressure over DISTANCE is Change over distance is a GRADIENT The GRADIENT in pressure gives winds & ocean currents their “push”

8. Pressure Gradient Force (PGF) isobar - a line of equal pressure (analogous to isotherm) gradient is 16 mb (note the closer isobars) the PGF acts at right (90º) angles to the isobars

9. Pressure Gradient Force note the 1008 mb isobar

10. Wind speed = Const * Pressure Gradient Here, a 4x increase in PGF corresponds to a 4x increase in wind speed Credit: www.physicalgeography.net

11. Pressure Gradient Force and Isobars if there were no other forces acting on wind, it would flow in straight lines (perpendicular to isobars) from high to low pressure zones

12. Coriolis Force (just the facts) Rotation of Earth acts to deflect any motion from a straight line Deflection is to right (NH) to the left (SH) Coriolis “force” act on a right angle to the motion Coriolis Force is NOT a real “force” but is caused by viewing motion on a rotating planet

13. Coriolis Force Show the merry-go-round video

14. the amount of rotation about a vertical axis (’spinning’) is maximum at the poles and minimum at the equator Figure Credit: “Earth’s Climate” by W. Ruddiman

15. Earth’s Rotation every point on earth rotates around a central axis at 15 degrees/hour Latitude Speed of rotation (mph) 0˚0˚ 1041 30 ˚ 902 50 ˚ 670 60 ˚ 521 90 ˚ 0

16. Coriolis Force an object with an initial east-west velocity will maintain that velocity, even as it passes over surfaces with different velocities as a result, it appears to be deflected over that surface (right in NH, left in SH)

17. Coriolis Force and Deflection of Flight Path

19. Coriolis Force and Flight Paths II. Airplane animation

20. Figure Credit: “Earth’s Climate” by W. Ruddiman The Coriolis Force affects air flow in response to pressure gradients in the atmosphere

21. geostrophic winds - PGF and Coriolis forces are opposite and balanced Credit: www.physicalgeography.net in the northern hemisphere (upper troposphere), the CF deflects the wind to the right until wind flows parallel to isobars ~7km

22. Geostrophic Winds Balance between Pressure Gradient & Coriolis Forces Flow along isobars not across Works for upper atmosphere winds & ocean currents

23. 500 mb Pressure Map

24. PGF, CF & isobars in upper troposphere isobars

25. Friction Force surface friction reduces wind speed and reduces the Coriolis force (remember CF increases with wind speed) because of this, it causes winds to move across isobars at an angle the friction force operates only in the bottom 0.5-1 km of the atmosphere, and it acts opposite to the direction of motion

26. Figure Credit: “Earth’s Climate” by W. Ruddiman

27. PGF + Coriolis + Friction Forces isobars

29. The inter-tropical convergence zone (ITCZ) solar heating in the tropics expands air and decreases its density - leading to increased buoyancy How would this change the average molecular weight of air? average molecular weight of air is ~29 g/mol average density of air is 1.3 kg/m^3 what happens to air density if you add water vapor? It also gets more humid (adding water vapor)

30. Convection on your Stove

31. Convection on Earth

32. as this air rises, it cools and water condenses out, leading to intense precipitation Credit: http://ess.geology.ufl.edu/ess/Notes/AtmosphericC irculation/convect.jpeg

33. Credit: http://www.geog.ucsb.edu/~jeff/wallpaper/itcz_goes11_lrg.jpg A satellite (GOES) view of the ITCZ over the eastern Pacific

34. the position of the ITCZ tracks the sun (it is found in the summer hemisphere) - the location of the ITCZ determines the rainy season in many tropical countries, especially those in Africa the horizontal winds within the ITCZ are calm - the doldrums

35. The C in ITCZ the intense uplift of air creates horizontal pressure gradients at the surface Credit: NASA JPL as a result, winds converge towards the equator from both hemispheres what about the complete cycle - where does the uplifted air go?

37. Equator-to-pole cross section of circulation

38. Hadley cell circulation this circulation refers to the complete circulation of rising air in the tropics, descending air over 30 °N and °S, and trade winds converging at the equator the descending branch of the Hadley circulation brings hot, dry air to the surface - leading to high pressure areas and suppressed precipitation

39. Subtropical high-pressure cells these cells occur where the tropical air descends in either hemisphere

40. Figure Credit: “Earth’s Climate” by W. Ruddiman Monsoon Circulation

41. Figure Credit: “Earth’s Climate” by W. Ruddiman Monsoon Circulation

42. Figure Credit: physicalgeography.net Monsoon Circulation

43. Asian monsoon intense, dry winds flow from the Asian interior in response to the gradient between the continental high pressure and the equatorial (ITCZ) low pressure

44. Asian monsoon in summer, the subsolar point and the ITCZ shift northward, reversing the pressure gradient - as the winds flow over the Indian ocean they gain moisture

45. Daytime land-sea breeze results from differential heating of land and sea - not from radiation differences - but from the different specific heats of land and water

46. Nighttime land-sea breeze at night, the land cools more rapidly than the sea and thus overlying air becomes more dense and has a higher pressure