Atmospheric Circulation

1. Atmospheric Circulation Moving things around on present day Earth

2. Short-term cycles Long term (organic) Long term - rock (inorganic/tectonic)

3. Global cycles • Biogeochemical cycles are the major way that elements are moved on Earth’s surface • Driven by solar input (primary production) • Elements cycle between reservoirs that operate on different time scales • Cycles have positive and negative feedbacks and subject to perturbations • Interaction with physical processes through tectonic/rock cycles • Oceans and atmosphere are important conduits transporting matter and energy

4. Oceans & Atmosphere • Shorter timescales of exchange • Exchange time in atmosphere – hours to decades • Mediates rapid cycling between oceans and continents • Exchange time in oceans • Surface and deep water – years • Deep circulation – 100’s to 1000’s of years

6. Atmosphere & Ocean • Gases and water freely exchange at the ocean-atmosphere interface • Movement of air (and water) by wind help minimize worldwide temperature extremes. • Weather is influenced by the movement of water in air (state of the atmosphere at a specific time and place) • Climate is the long-term average of the weather in an area

7. In general • Atmosphere exchanges material with biota and oceans rapidly • Cycles that include an atmospheric component tend to have more rapid recycling (N and C) • Cycles without an atmospheric component can be slower (immobile) because tied to geological cycles (P)

8. Atmosphere • Major conduit for transport between oceans and land • Major role in controlling climate (heat transport) • Composition evolved as a result of evolution of life • Changing due to human activities • Well-mixed so harbinger of global change • Structure – layered • Held on earth’s surface by gravity

9. Mt. Everest (8,850 m)

10. Atmosphere structure 0oC Mesosphere 30 mi Pressure decreases with altitude – 1 atmosphere of pressure at Earth’s surface at sea level. Stratopause 40 km Dry 20 mi Ozone Temperature Stratosphere 20 km 10 mi -55oC Tropopause Water Vapor Troposphere Weather zone 20oC 80% of atmospheric mass is in the troposphere

11. Structure of the Atmosphere • Troposphere is densest and is where our weather occurs • Substances in the stratosphere persist for long periods because there are few removal processes • In troposphere, temperature decreases with altitude • In stratosphere, temperature increases with altitude due to interactions with particles and radiation from the sun • The ozone layer is within the stratosphere • Ozone absorbs UV at top of stratosphere

12. Troposphere • Well-mixed • Limited exchange with overlying stratosphere • Heated by long-wave radiation (heat) re-radiated from Earth’s surface • Temperature decreases with altitude in troposphere • Heating from below results in convection, remember? • Rising warm air creates thermal instability

13. Composition of the atmosphere • 78% nitrogen and 21% oxygen • Other elements make up < 1% • Air is never completely dry and water can be up to 4% of its volume. • Residence time of water vapor in the atmosphere is ~10 days.

15. also H2S, H2, (CH3)S

16. Atmosphere • N2 – fairly inert; long residence time (20 my) • O2 – accumulated over time; complex controls; shorter residence time (~10,000 years) • CO2 – trace constituent; complex controls; short residence time (~3 years) • Affected by processes with cycles at various timescales (from rock to seasonal) • Long-term variations • Greenhouse

17. Atmosphere • Trace constituents – reduced gases • Microbially produced at present and removed in rain/oxidation • Greenhouse gases • Ozone – stratosphere • Problematic in troposphere • Water vapor • Varies tremendously • Important in distributing heat • Greenhouse gas

18. Properties of the atmosphere • Air has mass (and density) • Molecular movement associated with heat causes the same mass of warm air to occupy more space than cool air. So, warm air is less dense. • Humid air is less dense than dry air at the same temperature because molecules of water vapor (H2O) weigh less than N2 and O2 molecules displaced.

19. Density structure of troposphere • Influenced by temperature and water content • Water vapor is less dense than dry air so causes density of air to decrease and air to rise • Warming air makes it less dense so it rises • Condensation of water vapor releases heat which warms the air • Warm air can hold more water vapor than cold air

20. Air density affected by pressure • Air lifted to altitude experiences less pressure so expands and cools • Air compressed as it descends from altitude warms

21. Air movement • Water vapor rises, expands and cools • Condenses into clouds or precipitation (cooler air can’t hold as much water) • Atmosphere can lose water by precipitation • As air loses water vapor it becomes more dense and air will then fall, compress and heat

23. Atmospheric circulation • Powered by sunlight – uneven solar heating • About 51% of incoming energy is absorbed by Earth’s land and water • Energy absorption varies depending on the angle of approach, the sea state and the presence of ice or other covering (e.g., foam)

24. Heat budget • Energy imbalance – more energy comes in at the equator than at the poles • 51% of the short-wave radiation (light) striking land is converted to longer-wave radiation (heat) and transferred into the atmosphere by conduction, radiation and evaporation. • Eventually, atmosphere, land and ocean radiate heat back to space as long-wave radiation (heat) • Input and outflow of heat comprise the earth’s heat budget • We assume thermal equilibrium (Earth is not getting warmer or cooler) or the overall heat budget of the earth is balanced

26. Atmospheric circulation • Uneven solar heating of earth • Atm and oceans move heat poleward • Air moves from high pressure to low pressure • Poleward movement of warm air (less dense) • Equatorward movement of cold air (more dense)

27. Movement of heat • Sensible heat • Transported by a body that has higher temperature than its surroundings (conduction and/or convection) • Latent heat • Phase changes of water • Evaporation takes up heat and condensation releases heat

28. Uneven solar heating • Heat budget for particular latitudes is NOT balanced • Sunlight reaching polar latitudes is spread over a greater area (less radiation per unit area) • At poles, light goes through more atmosphere so approaches surface at a low angle favoring reflection • Tropical latitudes get greater radiation per unit area and light passes through less atmosphere so they get more solar energy than polar areas

29. Solar radiation • Radiation hits the earth in parallel rays • Incident angle varies with latitude • Energy is spread out over more area • Less heat per area • Passes through more atmosphere • Which absorbs radiation • Poles are cooler because they receive lower intensity solar radiation do to angle of incident radiation. N S

30. Solar radiation • Second reason the poles are cooler is the tilt of the earth on its axis • Variation in daylength • Even when poles have long daylength, the incident angle is long. • Third reason is that poles are farther from the sun 23.5o N S

32. Fig. 4-1

34. Fig. 4-2

36. Seasons & solar heating • Mid-latitudes – N Hemisphere receives 3x the amount of solar energy per day in June than in December • Due to the 23.5o tilt of Earth’s rotational axis • N Hemisphere tilts toward the sun in June and away in December • Tilt causes seasons

38. Figs. 4-15 and 4-16

39. Circulation • Atmospheric and oceanic circulation are governed by the redistribution of this energy • Water moves heat between tropics to poles • Ocean currents and water vapor move heat. • Higher latent heat of vaporization means vapor transfers more heat per unit mass than liquid water.

40. Atmospheric circulation • Warm air rises and cool air sinks • Warm air expands and rises • Expansion causes cooling and contraction causing increasing density and sinking • Air will rise where its warmer and sink where its cooler

41. Convection

42. Logically on the earth, one can imagine this

43. Fig. 13.11

44. Fig. 4-25

45. Air movement • Air is warmed at equator so rises • As it rises, it dumps its moisture because its expanded and cooled • Air moves south to replace air that’s risen • Creates zone of low pressure (sinking air creates high pressure and rising air creates low pressure.

46. Fig. 4-3

48. Atmospheric circulation • But, this is NOT what happens • Atmospheric circulation is governed not only by uneven solar heating but, • The Earth’s rotation • Eastward (CCW) rotation of the Earth on its axis deflects moving air or water (or any object with mass). • CORIOLIS effect (1835)

49. Coriolis Effect • Rotation of the Earth CCW • Relative speeds of sphere at different latitudes • Caused by an observer’s moving frame of reference on a spinning Earth • Curve is slightly to the right of initial path in the northern hemisphere • Curve is slightly to the left of initial path in the southern hemisphere

50. Relative speeds of objects at different radii moving at the same angular speed