1. General Circulation of the Atmosphere
Tropical heating drives Hadley cell circulation
Warm wet air rises along the equator
Transfers water vapor from tropical oceans to higher latitudes
Transfers heat from low to high latitudes

2. The Hadley Cell
Along equator, strong solar heating causes air to expand upward and diverge to poles
Creates a zone of low pressure at the equator called
Equatorial low
Intertropical Convergence Zone (ITCZ)
The upward motions that dominate the region favor formation of heavy rainfall
ITCZ is rainiest latitude zone on Earth
Rains 200 days a year – aka. doldrums

3. General Circulation Air parcels rise creating low pressure
Heat and expand
Become humid
Transfer heat (sensible & latent) to poles
Transfer of moisture towards poles
In mid latitudes
Dry air sinks creating high pressure
Air flows away from high pressure

4. General Circulation Hadley cell circulation creates trade winds
Dry trade winds move from subtropics to tropics and pick up moisture
Trade winds from both hemispheres converge in the ITCZ
Trade winds warm and rise
Contribute to low pressure and high rainfall in the ITCZ

6. Monsoons – Circulation in ITCZ
ITCZ shifts with seasons
Circulation driven by solar heating
Circulation affected by seasonal heat transfer between tropical ocean and land
Heat capacity and thermal inertia of land < water

7. Summer Monsoon
Air over land heats and rises drawing moist air in from tropical oceans

8. Winter Monsoon
Air over land cools and sinks drawing dry air in over the tropical oceans

10. Circulation at Mid & High Latitudes
Sinking air from Hadley circulation creates high pressure in subtropics
Circulation modified
Coriolis effect
Monsoons
Flow of cold air from high latitudes

11. Coriolis Effect
Air moving from high to low pressure is deflected by Earth’s rotation
Clockwise rotation in the northern hemisphere
Counterclockwise rotation in the southern hemisphere

13. Ocean Circulation?
Circulation in the troposphere is caused by atmospheric pressure gradients
Result from vertical or horizontal temperature differences
Temperature variations caused by latitudinal differences in solar heating
Ocean surfaces are heated by incoming surface radiation
Do the oceans circulate for the same reason as the atmosphere?

14. No!
90% of solar radiation that penetrates oceans absorbed in upper 100 m
Warm water at surface is less dense than the colder water below
Water column is inherently stable
Very little vertical mixing
Water has a high heat capacity
Lots of heat required for a small change in temperature
Lateral temperature and salinity differences are small over large areas

15. Ocean Circulation Ultimately driven by solar energy
Distribution of solar energy drives global winds
Latitudinal wind belts produce ocean currents
Determine circulation patterns in upper ocean
Distribution of surface ocean temperatures strongly influence density structure
Density structure of oceans drives deep ocean circulation
Negative feedback
Surface temperature gradients drive circulation
Net effect is to move warm water to poles and cold water towards tropics

16. Heat Transfer in Oceans
Heating occurs in upper ocean
Vertical mixing is minimal
Average mixed layer depth ~100 m
Heat transfer from equator to pole by ocean currents
Oceans redistribute about half as much heat at the atmosphere

17. Surface Currents Surface circulation driven by winds
As a result of friction, winds drag ocean surface
Water movement confined to upper ~100 m
Although well-developed currents ~1-2 km
Examples, Gulf Stream, Kuroshiro Current
Coriolis effect influences ocean currents
Water deflected to right in N. hemisphere
Water deflected to left in S. hemisphere

19. Eckman Spiral Eckman theory predicts
1) surface currents will flow at 45° to the surface wind path
2) flow will be reversed at ~100 m below the surface
3) flow at depth will be considerably reduced in speed
Few observations of true Eckman Spiral
Surface flow <45°, but still to an angle

20. Eckman Transport
Observations confirm net transport of surface water is at a right angle to wind direction
Net movement of water referred to as Eckman Transport

23. Gyre Circulation Wind driven and large scale
Sea level in center 2 m higher than edge
Eckman transport producing convergence
Circulation extends to m
Volume of water moved is 100 x transport of all Earth’s rivers
Flow towards equator balanced by flow toward pole on westward margin
In Atlantic, by Gulf Stream and North Atlantic Drift

24. Downwelling In areas of convergence
Surface water piles up in center of gyre
Sea level in the center of gyre increases
Surface layer of water thickens
Accumulation of water causes it to sink
Process known as downwelling

25. Equatorial Divergence
Areas of the ocean where divergence of surface currents occurs
Equatorial divergence (e.g., Atlantic)
In N. hemisphere, NW trades result in westward flowing N. equatorial current
Eckman transport moves water to N
In S. hemisphere, SW trades result in westward flowing S. equatorial current
Eckman transport moves water to S
Divergence occurs along the equator

26. Equatorial Upwelling
As surface water diverges, sea level falls, surface layer thins and cold water “upwells”

27. Eckman Transport Along Coasts
Winds along a coast may result in Eckman transport that moves water towards or away from the coast
Divergence from easterly winds and southward moving currents
SW coast of N. America
W coast of N. Africa
Divergence from northward moving currents
West coasts of S. America and S. Africa

28. Coastal Upwelling
Coastal divergence results in upwelling as cold water rises to replace surface water

29. Geostrophic Currents
Eckman transport from wind-driven currents piles water up in gyre center
Gravity pulls water down slope
Slope is opposite to Coriolis effect
Net effect is flow 90° to slope
Result is a geostrophic current
Geostrophic currents push water in the same direction as the wind-driven flow

30. Boundary Currents Gyre circulation pushes water to the west
Flow of water around gyres is asymmetric
In the western part of gyre water is confined to a narrow fast-moving flow
Western boundary current
In the eastern part of gyre flow is diffuse, spread out and slow
Eastern boundary current
Eastern currents tend to be divergent
Eckman transport away from continent

31. Gulf Stream Western boundary current in Atlantic
Narrow, fast-moving from Cuba to Cape Hatteras
Decreases speed across N. Atlantic
Flow broadens and slows becoming N. Atlantic Drift
Movement to the south along the Canary Current is very slow, shallow and broad

32. Deep Ocean Circulation
Driven by differences in density
Density of seawater is a function of
Water temperature
Salinity
Quantity of dissolved salts
Chlorine
Sodium
Magnesium
Calcium
Potassium

33. Thermohaline Circulation
Deep ocean circulation depends on temperature (thermo) & salinity (hals)
Controls seawater density
Density increases as:
Salinity increases
Temperature decreases
Horizontal density changes small
Vertical changes not quite as small
Water column is stable
Densest water on bottom
Flow of water in deep ocean is slow
However, still important in shaping Earth’s climate

34. Vertical Structure of Ocean
Surface mixed layer
Interacts with atmosphere
Exchanges kinetic energy (wind, friction) and heat
Typically well mixed ( m)

35. Vertical Structure of Ocean
Pychnocline (~1 km)
Zone of transition between surface and deep water
Characterized by rapid increase in density
Some regions density change due to salinity changes – halocline
Most regions density change due to temperature change – thermocline
Steep density gradient stabilizes layer

36. Bottom Water Formation
Deep-ocean circulation begins with production of dense (cold and/or salty) water at high latitudes
Ice formation in Polar oceans excludes salt
Combination of cold water and high salinity produces very dense water
Dense water sinks and flows down the slopes of the basin towards equator

37. Antarctic Bottom Water (AABW)
Weddell Sea major site of AABW formation
AABW circles Antarctica and flow northward as deepest layer in Atlantic, Pacific and Indian Ocean basins
AABW flow extensive
45°N in Atlantic
50°N in Pacific
10,000 km at km h-1; 250 y

38. North Atlantic Deep Water (NADW)
Coastal Greenland (Labrador Sea) site of NADW formation
NADW comprises about 50% of the deep water to worlds oceans
NADW in the Labrador Sea sinks directly into the western Atlantic
NADW forms in Norwegian Basins
Sinks and is dammed behind sills
Between Greenland and Iceland and Iceland and the British Isles
NADW periodically spills over sills into the North Atlantic

39. Deep Atlantic Water Masses
Deep Atlantic water comes from high latitude N. Atlantic, Southern Ocean and at shallower depth, the Mediterranean Sea

40. AABW and NADW Interact
NADW flowing south in the Atlantic joins the Antarctic Circumpolar Current
NADW and AABW combine
Spin around Antarctica
Eventually branch off into the Pacific, Indian and Atlantic ocean basins

41. Ocean Circulation Surface water at high latitudes forms deep water
Deep water sinks and flows at depth throughout the major ocean basins
Deep water upwells to replace the surface water that sinks in polar regions
Surface waters must flow to high latitudes to replace water sinking in polar regions
Idealized circulation – Thermohaline Conveyer Belt

42. Thermohaline Conveyor Belt
NADW sinks, flows south to ACC and branches into Indian and Pacific Basins
Upwelling brings cold water to surface where it eventually returns to N. Atlantic

43. Ocean Circulation and Climate
Warm surface waters move from equator to poles transferring heat pole-ward and into the deep oceans
Oceans vast reservoir of heat
Water heats and cools slowly
Pools of water warmer than normal heat the atmosphere
Pools of water colder than normal cool the atmosphere
Timescale of months to years
Time needed for heating/cooling of water

44. Ocean Circulation and Climate
On long timescales, average ocean temperature affects climate
Most water is in deep ocean
Average temperature of ocean is a function of
Process of bottom-water formation
Transport of water around ocean basins
Deep water recycle times is ~1000 y
Thermohaline circulation moderates climate over time periods of ~ 1000 y

45. Ice on Earth Important component of climate system
Ice properties are different from water, air and land
Two important factors affecting climate
High albedo
Latent heat stored in ice

46. Sea Ice Salt rejection during sea ice formation
Important for bottom water formation
Sea ice stops atmosphere from interacting with surface mixed layer

47. Sea Ice Distribution Most sea ice in Southern Ocean
Enormous amount form and melt each season
Average thickness ~1 m
Landmasses in Arctic prevent sea ice movement
Arctic sea ice persists for 4-5 years
Reach thickness of 4 m in central Arctic and 1 m on margins

48. Glacial Ice Mountain glaciers
Equatorial high altitude or polar lower altitude
Few km long, 100’s m wide and 100’s m thick

49. Glacial Ice Continental ice sheets Large ice cube Existing ice sheets
Antarctica and Greenland
~3% of Earth’s surface or 11% of land surface
32 million km3 (= 70 m of sea level)