ESM 203: Ocean Processes and Circulation

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ESM 203 Ocean Processes and Circulation

• Jeff Dozier Thomas DunneFall 2007

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What is sea water?

• Sea water is 96½ pure water
• Remaining 3½ is other materials
• Dissolved salts, gases and organic substances, as
  well as particles
• Physical properties are mainly related to the
  pure water, along with the dissolved salts
• Water mass characteristics
• Salinity, temperature, nutrients, oxygen
• Key property (for circulation) is sea water
  density
• Variability in vertical inhibit or enhance
  mixing
• Variability in horizontal drive currents

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Sea water density

• A function salinity, pressure, and temperature
• Ranges from about 1020 to 1040 kg m3
• Oceanographers use the sigma-T description

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Temperature

• Temperature generally decreases with depth in the
  ocean
• Changes in temperature generally regulate changes
  in density
• except where ice is formed (which increases
  salinity)
• Rule of thumb

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Temperature in the Equatorial Pacific
Equatorial Pacific - WOCE150W
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Temperature in the Southern Ocean
60S - WOCE150W
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Salinity

• Ocean waters are salty
• The salts (Cl, SO42, Na, K, etc) are in
  approximately constant proportions
• Residence time is huge
• To practically estimate salinity, we measure just
  one ion, Cl

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Salinity

• Salinity varies from 32 to 37 psu (practical
  salinity units, parts per thousand)
• Good water mass tracer
• Lower/higher values are unusual (river input,
  high evaporation, sea ice formation)
• Rule of thumb

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Pressure

• The weight of overlying sea water above a depth
  causes pressure (hydrostatic equation)
• Varies from 0 to gt500 bars (1 bar 105 N m2)
• Reference P0 at sea surface
• Rules of thumb

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Global sea surface temperature
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Global salinity
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Fluxes of water between surface and atmosphere
(slide from Planetary Hydrology lecture)
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Global salinity air-sea fluxes
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Convection the Conveyor Belt

• North Atlantic deep water (NADW) production
  drives the conveyor

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Role of sea ice

• Formation of seasonal sea ice drives formation of
  deep water
• Salinity of sea ice is 2 to 5 psu (compared to
  ocean 32 to 37 psu)
• Brine is left over when sea ice is formed
• Source of Arctic and Antarctic bottom water

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Seasonal sea ice, Antarctic
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Wind-driven circulation at the surface
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Major features of global wind field

• Ascending air leads to high precipitation (PgtE),
  and lower salinity
• Descending air leads to PltE, net evaporation, and
  salinity

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Coriolis effect
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Effect of wind stress

• Drag on surface layers causes mixing into lower
  layers

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Nansens Fram

• Nansen built the Fram to try to reach North Pole
  (1893-1896)
• Unique design to be locked in the ice and wait
• Once closer to Pole, Nansenone other dog team
  set out to try to reach
• (got to 8613N)
• http//www.arcticwebsite.com/ NansenFram.html

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Fram in the ice
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Ekman transport

• Nansen noticed that the movement of the
  ice-locked Fram was 20-40 to the right of the
  wind direction
• Nansen deduced (correctly) that the direction was
  caused by a balance of friction, wind stress, and
  Coriolis forces
• Ekman did the math
• Nansen later was instrumental in forming the
  League of Nations and in repatriating World War I
  refugees
• Awarded Nobel Peace Prize in 1922

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Ekman transport
Motion is to right of the wind direction (in
Northern Hemisphere)
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Ekman spiral

• Top layers direction results from balance of
  wind stress, friction, and Coriolis
• Layer 2s direction results from balance of
  stress from Layer 1, friction from Layer 3, and
  Coriolis
• Etc.

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Ekman transport of column down to Ekman depth
(40-200m)
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Divergence convergence

• Divergence leads to upwelling
• Convergence leads to downwelling

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Ekman pumping
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Ekman pumping and gyre circulation, I
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Ekman pumping and gyre circulation, II

• Convergence of Ekman transport piles the water up
• Geostrophy pushes it around the circle
• Comparatively little water is moved by Ekman
  transport (its the boundary layer)
• But it causes the gyre circulation, which moves a
  lot of water
• Downwelling in gyre interior lowers nutrient
  availability and algae biomass

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Gyre circulation, global scale

• Between Trade Winds Westerlies
• Convergence of Ekman transport
• Downwelling
• Subtropical gyres
• Between Westerlies and Polar Easterlies
• Divergence and upwelling
• Subarctic gyres

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Relationship between gyres and ocean
biogeochemistry

• Light drives photosynthesis
• CO2 is nearly always at saturation concentration
• Supply of nutrients is therefore critical
• Upwelling increases nutrients

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Ekman pumping and ocean biogeochemistry

• SeaWiFS mean chlorophyll concentration
• Low (blue) in center of subtropical gyres
• Driven by downwelling, which is caused by Ekman
  pumping

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Coastal upwelling on California coast

• California Current, April 1978
• Sea surface temperature from NOAA AVHRR
• Chlorophyll from Coastal Zone Color Scanner

SST
Chl
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Global currents, driven by Ekman transport
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El Niño-Southern Oscillation (ENSO)

• Normal Trade Winds pile warm water in western
  Pacific
• Water surface elevation about ½ m higher at
  Indonesia than Ecuador, surface temperature about
  8C warmer than off South America, where water
  (upwelling) is cold
• El Niño Trade Winds relax, less upwelling,
  therefore above-normal temperature off South
  America
• Fewer nutrients so fewer fish
• Often around Christmas, hence the name
• La Niña especially cold water off South America
• Southern Oscillation index air pressure anomaly
  between Tahiti and Darwin
• Negative (Tahiti Darwin) during El Niño

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http//www.elnino.noaa.gov/
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