General Circulation of the Atmosphere

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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

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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

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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

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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

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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 lt water

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Summer Monsoon

• Air over land heats and rises drawing moist air
  in from tropical oceans

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Winter Monsoon

• Air over land cools and sinks drawing dry air in
  over the tropical oceans

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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

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Coriolis Effect

• Air moving from high to low pressure is deflected
  by Earths rotation
• Clockwise rotation in the northern hemisphere
• Counterclockwise rotation in the southern
  hemisphere

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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?

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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

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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

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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

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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

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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 lt45, but still to an angle

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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

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Gyre Circulation

• Wind driven and large scale
• Sea level in center 2 m higher than edge
• Eckman transport producing convergence
• Circulation extends to 600-1000 m
• Volume of water moved is 100 x transport of all
  Earths rivers
• Flow towards equator balanced by flow toward pole
  on westward margin
• In Atlantic, by Gulf Stream and North Atlantic
  Drift

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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

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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

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Equatorial Upwelling

• As surface water diverges, sea level falls,
  surface layer thins and cold water upwells

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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

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Coastal Upwelling

• Coastal divergence results in upwelling as cold
  water rises to replace surface water

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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

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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

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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

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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

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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 Earths
  climate

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Vertical Structure of Ocean

• Surface mixed layer
• Interacts with atmosphere
• Exchanges kinetic energy (wind, friction) and
  heat
• Typically well mixed (20-100 m)

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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

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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

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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
• 45N in Atlantic
• 50N in Pacific
• 10,000 km at 0.03-0.06 km h-1 250 y

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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

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Deep Atlantic Water Masses

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

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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

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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

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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

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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

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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

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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

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Sea Ice

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

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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

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Glacial Ice

• Mountain glaciers
• Equatorial high altitude or polar lower altitude
• Few km long, 100s m wide and 100s m thick

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Glacial Ice

• Continental ice sheets
• Large ice cube
• Existing ice sheets
• Antarctica and Greenland
• 3 of Earths surface or 11 of land surface
• 32 million km3 ( 70 m of sea level)