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 Stratopause 40 km Dry 20 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
Weather zone
Water
Vapor
Troposphere
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 Ozone – stratosphere
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 Latent 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 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

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

53. Airplane
Coriolis

54. Coriolis effect and atmospheric circulation
Coriolis effect influences wind direction
End up with 3 sets of cells – by 30 deg, flow has been deflected 90 deg
Air is deflected before getting all the way from equator to poles
Air only makes it about 1/3 of the way to the poles before it becomes dense enough to sink
Descending air turns back toward equator when it reaches the surface because it is again deflected to the right
Heats up when it gets back to equator and rises again.

58. Fig. 4-11

59. Fig. 4-7
Hadley Cell
Hadley Cell
Fig. 4-6

60. Fig. 4-18

61. Fig. 4-19 Pressure at sea-level (mbars)

62. Atmospheric circulation & precipitation
Air carries water vapor
In general, uplift of air masses induces precipitation
Conversely, descending air affects distribution of deserts
Descending arms of Hadley cells
Continental interiors
Leeward (downwind) of mountains
West coasts of major continents – due to upwelling and offshore currents

63. Deserts offset from convergence zones where there is high precipitation.
Deserts at divergence zones.

64. Features of the model At boundaries, air is moving vertically
Surface winds are weak and erratic
Equatorial region
Lots of rain as humid air rises and loses moisture (rain forests)
Doldrums
Intertropical convergence zone (ITCZ) – winds converge
30oN and S region
Sinking air is arid and evaporation >> precipitation (deserts and high salinity)
Horse latitudes

65. Features of the model
Air moves horizontally within the cells from areas of high pressure to areas of low pressure
Tropical areas – Hadley cells
Surface winds are strong and dependable
Trade winds or easterlies centered at ~15oN (northeast trade winds) and ~ 15oS (southeast trade winds)
Surface wind moves from horse latitudes to doldrums so come out of northeast in N hemisphere
Mid-latitude areas – Ferrel cells
Westerlies centered at ~ 45oN and ~45oS
Surface wind moves from horse latitudes to polar cells so comes out of southwest in the N hemisphere

67. The 6-celled model Not exactly correct either North - South variation
Land versus water distribution
Equator to pole flow of air different depending on amount of land at a particular longitude
ITCZ narrower and more consistent over land than ocean
Seasonal differences greater in N hemisphere (remember, more land)
The ocean’s thermostatic effect reduces irregularities due to surface conditions at different longitudes

68. Distributions of land masses
Differential heating and cooling
Land heats up and cools more rapidly

69. North - South variation (cont)
Offset at the equator
Geographical vs. meteorological equator
Meteorological equator is ~ 5oN of geographical equator (thermal equilibrium between hemispheres)
Meteorological equator and ITCZ generally coincide and change with seasons (moves N in northern summer)
Atmospheric and oceanic circulation is symmetrical around the meteorological equator NOT the geographical equator!
Seasonality

70. Seasonality important
Shifts in polar front and the ITCZ – meteorological equator

71. West-East variations
Air over chilled continents becomes cold and dense in the winter
Air sinks creating high pressure over continents
Air over relatively warmer waters rises (possibly with water vapor) creating low pressure zones over water
Air flows from high pressure to low pressure modifying air flow within cells
Reverse situation in summer
Effects pronounced in N hemisphere (mid-latitudes) where there is about the same amount of land & water

72. Winds over the Pacific
on two days in Sept
1996
Stronger winds in red-
orange
Notes:
Deviates from 6-cell
model
Strong westerlies hitting
Canada
Strong tradewinds
(easterlies) over Hawaii
Extratropical cyclone
east of New Zealand

73. Circulation of the Atmosphere
Most of the variation from the 6-cell model is due to
Geographical distribution of landmasses
Different response of land and ocean to solar heating
Chaotic flow
Over long term – 6-cell model is pretty good for describing average flow

74. Major surface wind and pressure systems of the world and their weather
These wind patterns move 2/3 of heat from tropics to poles.

75. Monsoons Pattern of wind circulation that changes with the season
Generally wet summers and dry winters
Linked to different heat capacities of land and water and to N-S movement of the ITCZ

76. Wet season In the spring, land heats (faster than water)
Warm air over land rises creating low pressure
Cool air flows from ocean to land
This humid air heats and rises (rains form)

77. Dry Season Land cools (faster than ocean)
Air cools and sinks over land creating high pressure
Dry surface wind moves seaward
Warms and rises over water (with or without evaporation and rain over water)

79. Monsoons
Most intense over Asia where you have a huge land mass in the N and a huge ocean to the S
Monsoon over India causes wet season (summer) from April – October (up to 10 meters – 425 inches of rain per year)
Smaller monsoon in N America (Gulf of Mexico and SE)

81. Dry season
Wet season
ITCZ
ITCZ

82. Sea and Land breezes
Daily changes in wind direction due to unequal heating and cooling of land versus water
Warm air during day on land rises and cool air from sea moves onshore (with or without water vapor)
Warmer air over water rises and cool air on land during the night sinks and moves offshore

84. Daytime Onshore Breeze
Nighttime Offshore Breeze

85. Fig. 4-17