ATS150Introduction to Climate ChangeSpring 2010

Study Guide for Exam #2

We’ll have the second exam in ATS 150 on Friday March 26, from 1 PM to 1:50, in the regular classroom. The exam will count 25% of your grade for the course. You will need to bring a pencil or pen to the exam, but you won’t need a “blue book.” The format of the exam will be pretty much the same as the first exam.

I’m mainly trying to see that you understand concepts presented in the lectures. I don’t want you to memorize a bunch of stuff from the notes or the textbook. The exam will be “open notes,” meaning you can use printed copies of the lecture notes, or books, or stuff you’ve written down during the exam. You may not consult one another, or look stuff up on the internet. The exam will cover stuff we’ve talked about in class. The textbook may or may not be helpful in studying (I hope so), but I will not ask you anything on the exam that is in the book but has not been covered in class. Also, there will be no calculations or math problems on the exam.

The idea of this study guide is not to tell you precisely what questions will be on the exam. Rather, I’m trying to tell you what topics from the class will be covered. Basically, the exam covers all the lecture materials since the previous exam: “Energy Budget of the Earth,” “Pressure, Wind, and Weather,” “Circulation of the Oceans,” and “Climate Feedback Processes.” There won’t be any questions about climates of the past, which we’ll begin in earnest next week.

Here’s a list of stuff to study:

  1. Energy Budget of the Earth
  2. Flows of energy between surface and atmosphere
  3. 100 units of sunshine at top of atmosphere
  4. 51 units absorbed at surface
  5. 96 units of thermal (IR) absorbed at surface
  6. 117 units of thermal IR emitted by surface
  7. 7 units of rising hot air, 23 units of evaporated water at sfc
  8. Geographic distribution of energy in and energy out
  9. Absorbed solar energy depends mostly on latitude/season
  10. Outgoing longwave radiation (OLR) much more “lumpy”
  11. Thermal emission (OLR) mostly from hot dry places
  12. Thermal emission (OLR) much weaker from cloudy places
  13. Net heating more than 100 W/m2 over tropical oceans
  14. Net cooling more than 150 W/m2 over polar regions
  15. Effects of evaporating water and rising hot air
  16. Evaporating water takes a lot of energy (5 times as much to evaporate 1 kg of water as to warm it from 0 to 100 C)
  17. Almost 4 times as much atmospheric heating by condensation of evaporated water as from rising hot air
  18. Water evaporated from sunny subtropical oceans is carried into deep tropics where it rains out (heats air)
  19. Pressure, Wind, and Weather
  20. Wind is “pushed around” by 5 forces (3 real, two imaginary)
  21. Real forces: Gravity, pressure differences (gradient), friction
  22. Imaginary forces: Coriolis and centripetal
  23. Heating lifts the air against gravity
  24. Lifted air pushes against adjacent air (pressure gradients)
  25. Combination of lifting and pushing produced by geographic variations in heating and cooling causes planetary-scale circulations of the atmosphere (wind) and oceans (currents)
  26. These circulations act to balance Earth’s energy budget, moving heat from hot places (tropical surface) to cold places (upper air and poles)
  27. Pressure-gradient force
  28. Caused by different amounts of heating/cooling in different places
  29. Air moves (wind blows) from high pressure to low pressure
  30. Coriolis force
  31. Not a real force, but the apparent deflection of winds and ocean currents that results from Earth’s spin underneath us
  32. Always pushes 90 to the right of motion in the Northern Hemisphere (to the left in Southern Hemisphere)
  33. Strength of deflection is proportional to speed of motion
  34. Geography of air circulation and patterns of winds
  35. “Hadley Cell:” rising hot air caused by tropical convergence and rain forces air to blow toward Equator near surface, away from Equator aloft (also causes rainforests!)
  36. “Trade Winds:” Inflowing surface air toward Equatorial convergence is deflected to right in Northern Hemisphere (NE Trades) and to left in Southern Hemisphere (SE Trades)
  37. “Subtropical subsidence:” sinking branch of Hadley Cell near 30 latitude in both hemisphere associated with deserts
  38. “Midlatitude westerlies:” warmed air flowing poleward is deflected eastward (“westerly wind”) in both hemispheres
  39. Polar vortex, jet streams, and winter storms
  40. Polar air extremely cold in polar night (because outgoing thermal radiation is unopposed by solar heating)
  41. Thermal contraction of air in polar winter causes very strong pressure gradient forces to try to “fill it in”
  42. “Jet Streams:” Coriolis force deflects air “falling” into polar night to spin very fast in direction of Earth’s spin (toward east, westerly jet stream winds)
  43. Polar vortex/jet stream blocks direct poleward heat flow
  44. Waves in jet stream (winter storms, warm/cold fronts) are the main mechanism for mixing polar & subtropical air masses
  45. Circulation of the Oceans:
  46. Subtropical Gyres, western and eastern boundary currents
  47. “Ekman Transport:” Water is pushed by Coriolis force to right of wind in Northern Hemisphere (left in SH)
  48. Ekman flow moves water toward tropics in midlats (because of westerlies) and poleward out of tropics (because of easterly Trade Winds, causing it to “pile up” in subtropics
  49. Gyres:” Coriolis force causes elevated water in subtropical oceans to rotate clockwise in NH, counterclockwise in SH. The subtropical gyres carry huge amounts of heat poleward!
  50. “Western Boundary Currents:” (part of gyres; e.g. Gulf Stream) Fast-flowing currents on western sides of oceans (east coasts) that carry warm water poleward
  51. “Eastern Boundary Currents:” (part of gyres; e.g. California Current) slow-flowing cold return flow on eastern sides of oceans (west coasts) that carry cold water toward tropics
  52. “Coastal upwelling:” equatorward flow along eastern boundaries (west coasts) causes Ekman flow offshore, so very cold water is forced to surface from depth. Causes cold deserts, low clouds and fog (e.g., Baja, Namibia, Peru)
  53. Equatorial Oceans and El Nino
  54. No Coriolis force at Equator, so water Trade Winds push warm surface water westward across equatorial oceans
  55. “Equatorial upwelling:” caused by diverging surface water as NE Trades push water NW and SE Trades push water SW
  56. Warm water “piles up” and becomes very deep. “Warm pool” (Sea-Surface Temp, SST > 30 C) the size of Siberia in Western Pacific and Indian Ocean, hundreds of meters deep! (source of energy for Indian Monsoon and torrential rains)
  57. Cold water forced to rise in Eastern Pacific because warm water pushed away, very productive fishery on desert coast
  58. “El Nino:” Occurs when Trade Winds relax, warm water sloshes eastward, caps EQ upwelling in eastern Pacific. Monsoon rains often fail, torrential rains in east (esp Peru). Changes in rainfall heating reduce EW pressure gradient, reinforce weakening of Trade Winds. Weather ensues!
  59. Thermohaline (heat-salt) Circulation and Conveyor Belt
  60. Hot dry NE Trade Winds blowing off of Sahara evaporate lots of fresh water from subtropical Atlantic, leaving salt behind
  61. Salty North Atlantic water gets very cold near Greenland
  62. Cold salty N. Atlantic water very dense, sinks like a rock! Forms “North Atlantic Deep Water” which slowly fills ocean bottoms
  63. NADW flows south along bottom of Atlantic all the way to Antarctic, then around & round and back up through Pacific and Indian Oceans. Surface water slowly returns in Atlantic.
  64. Thermohaline “conveyor belt” takes about 1000 years to cycle
  65. This is the only way the deep ocean ever “sees” the atmosphere
  66. Moves a HUGE amount of heat poleward, releases 1/3 as much heat to North Atlantic region as received from sunlight there!
  1. Climate Feedback Processes
  2. Climate Forcing, Response, and Sensitivity
  3. Define climate sensitivity as strength of response (e.g., change in surface temperature) for a given change in forcing (e.g., change in heating by solar radiation)
  4. Without feedback, climate sensitivity is about 0.25 K per 1 Wm-2
  5. Feedback happens when the response changes the forcing
  6. Changes in radiation change the surface temperature
  7. Surface temperature changes the radiation
  8. Changes can either reinforce/amplify the forcing (positive feedback) or counteract/damp the forcing (negative feedback)
  9. Positive feedback can amplify either warming or cooling (e.g., can make cooling stronger as in Ice Ages)
  10. Negative feedback can counteract either warming or cooling (e.g., can resist temperature changes over time)
  11. Kinds of negative climate feedback (stabilizes climate)
  12. Longwave radiation feedback: warm surface radiates more energy, so warming is reduced (or vice versa).
  13. Low clouds reflect more sunlight. Warming can evaporate more water from oceans, making more low clouds to shade surface and reflect more sunlight to space (resists warming). Low clouds also emit OLR to space, but at almost same temp as surface so extra reflected solar radiation “wins out” over extra OLR.
  14. Kinds of positive climate feedback (amplifies climate changes)
  15. Water vapor feedback (very strong!). Warming evaporates more water from oceans, water vapor greenhouse effect adds more longwave radiation to surface, extra warming evaporates more water (enhances warming or cooling)
  16. Ice-albedo feedback. Cooling causes snow and ice cover to expand, increases albedo, enhances cooling. Warming cuases snow and ice cover to contract, reduces albedo, enhances warming. Very important in allowing ice ages.
  17. High-cloud feedback. High clouds reflect sunlight like low clouds, but high clouds are very cold so OLR to space is much less than surface. Reduced OLR “wins out” over reduced solar. If warming produces extra high clouds, warming will be enhanced.

1