Life on an Ocean Planet

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• Choose to view chapter section with a click on
  the section heading.
• Surface Currents
• Deep Currents
• Studying Ocean Currents

Chapter Topic Menu
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• Understanding what causes currents and where they
  flow is fundamental to all marine sciences. It
  helps explain how heat, sediments, nutrients, and
  organisms move within the seas.
• Causes of Currents
• Three major factors drive ocean currents.
• 1. Wind.
• If the wind blows long enough in one
  direction,it will cause a water current to
  develop.
• The current continues to flow until internal
  friction,or friction with the sea floor,
  dissipates its energy.
• 2. Changes in sea level.
• Sea level is the average level of the seas
  surface at its meanheight between high and low
  tide.
• The oceans surface is never flat, ocean
  circulation cause slopes to develop. The steeper
  the mound of water, the larger and faster the
  current. The force that drives this current is
  the pressure gradient force.
• 3. Variations in water density.
• Differences in water density also cause
  horizontal differences in water pressure. When
  the density of seawater in one area is greater
  than another, the horizontal pressure gradient
  between the two areas initiates a current that
  flows below the surface.

Surface Currents
Chapter 9 Pages 9-3 9-4
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Gyres

• The combination of westerlies, trade winds, and
  the Coriolis effect results in a circular flow in
  each ocean basin. This flow is called a gyre.
• There are five major gyres one in each major
  ocean basin
• 1. North Atlantic Gyre
• 2. South Atlantic Gyre
• 3. North Pacific Gyre
• 4. South Pacific Gyre
• 5. Indian Ocean Gyre
• The flow of currents in all parts of theocean is
  a balance of various factors,including the
  pressure gradient force,friction, and the
  Coriolis effect.

Surface Currents
Chapter 9 Pages 9-5 9-6
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Ekman Transport

• The Ekman transport is an interesting
  phenomenondiscovered in the 1890s by Fridtjof
  Nansen.
• The wind and the Coriolis effect influences
  waterwell below the surface because water tends
  to flowin what can be imagined as layers.
• Due to friction, the upper water currents push
  the deep water below it. This deep layer pushes
  the next layer below it. The process continues in
  layers downward. Each water layer flows to the
  right of the layer above causing a spiral motion.
• This spiraling effect of water layers pushing
  slightly to the right from the one above (to the
  left in the Southern Hemisphere) is called the
  Ekman spiral.
• There is a net motion imparted to the water
  column down to friction depth. This motion is
  called the Ekman transport.
• The net effect, averaging of all the speeds and
  directions of the Ekman spiral, is to move water
  90 to the right of the wind in the Northern
  Hemisphere, or to the left in the Southern
  Hemisphere.

Surface Currents
Chapter 9 Pages 9-6 to 9-8
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Western and Eastern Boundary Currents

• Satellite images show that the oceans are really
  hilly, not calm or flat.
• These images show that water piles up where
  currents meet. Where currents diverge, valleys
  form.
• There is a dynamic balance between the clockwise
  deflection of the Coriolis effect (attempting to
  move water to the right) and the pressure
  gradient created by gravity (attempting to move
  the water to the left).
• The balance keeps the gyreflowing around the
  outside ofthe ocean basin.
• Geostrophic currents arecreated by the Earths
  rotation.
• This current results from thebalance between
  the pressuregradient force and theCoriolis
  effect.

Surface Currents
Chapter 9 Pages 9-8 to 9-10
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Western and Eastern Boundary Currents (continued)

• Western boundary currents are found onthe east
  coasts of the continents and are stronger and
  faster than eastern boundary currents due to
  western intensification. Western boundary
  currents flow through smaller areas than eastern
  boundary currents.
• Trade winds blow along the equator pushingwater
  westward, causing it to pile up on the western
  edge of ocean basins before it turns to the
  poles. The Earths rotation tends to shift the
  higher surface level in the center of the gyre
  westward. The higher surface level is now west of
  center and forces the current to squeeze
  through a narrower area.
• Total water volume balances out.Western boundary
  currents handle thesame volume, but through
  smaller areas,so water must move more rapidly.

Surface Currents
Chapter 9 Pages 9-10 to 9-16
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Countercurrents

• Countercurrents and undercurrents are water flows
  that differ from the major ocean currents.
• Countercurrents are associated with equatorial
  currents it runs opposite of its adjacent
  current.
• It is hypothesized they develop in equatorial
  regions because of the doldrums. Without wind
  pushing water westward, water driven in from the
  east enters the basin more quickly than it exits.
  This causes a countercurrent to develop.
• Undercurrents flow beneath theadjacent current
  and are foundbeneath most major currents.
• They can significantlyaffect land masses and
  land temperatures.

Surface Currents
Chapter 9 Pages 9-16 9-17
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Upwelling and Downwelling

• Upwelling is an upward vertical current that
  brings deep water to the surface. Downwelling is
  a downward vertical current that pushes surface
  water to the bottom.
• Coastal upwellings occur when the wind blows
  offshore or parallel to shore. In the Northern
  Hemisphere this wind blowing southward will cause
  an upwelling only on a west coast.
• The same wind on the east coast in the Northern
  Hemisphere sends surface water toward shore
  causing a downwelling.
• These currents have strongbiological effects
• Upwelling tends to bring deepwater nutrients up
  into shallow water.
• Upwellings also relate to significant weather
  patterns.
• Downwellings are important in carrying and
  cycling nutrients to the deep ocean ecosystems
  and sediments.

Surface Currents
Chapter 9 Pages 9-17 to 9-20
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Heat Transport and Climate

• Currents play a critical role by transporting
  heat from warm areas to cool areas and affects
  climate by moderating temperatures. Without
  currents moving heat, the worlds climates would
  be more extreme.
• El Niño Southern Oscillation (ENSO)
• El Niño tremendously affects worldweather
  patterns.
• This brings low pressure and highrainfall in the
  Western Pacific.
• The opposite happens in the EasternPacific with
  high pressure andless rainfall.

Surface Currents
Chapter 9 Pages 9-20 to 9-22
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El Niño (continued)

• For reasons still not clear, every 3 to 8 years a
  rearrangement of the high- and low-pressure
  systems occur.
• High pressure builds in the Western Pacific and
  low pressure in the Eastern Pacific. Trade winds
  weaken or reverse and blow eastward the
  southern oscillation.
• This causes warm water of the west to migrate
  east to the coast of South America. The loss of
  upwelling deprives the water of nutrients. A
  normally productive region declines with the
  collapse of local fisheries and marine
  ecosystems.
• Over the eastern Pacific, humid air rises causing
  precipitation in normally arid regions. Flooding,
  tornados, drought and other weather events can
  lead to loss of life and property damage.

Surface Currents
Chapter 9 Pages 9-22 to 9-24
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Thermohaline Circulation and Water Masses

• Thermohaline circulation is water motion caused
  by differing water densities.
• In the deep-ocean layers, water density
  variation, not wind, is the primary causeof
  current.
• Circulation drives most of the vertical motion of
  seawater and the oceans overall circulation.
• Thermohaline circulation works because
  waterdensity increases due to cooling,
  increasedsalinity or both.
• When water becomes dense, it sinks, causing a
  downward flow.
• This means water in some other place must rise
  to replace it, causing an upward flow.
• Density differences drive the slow circulation
  of deep water.

Deep Currents
Chapter 9 Pages 9-26 9-27
Five distinct water masses result from density
stratification.
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How Deep Water Forms

• The intermediate, deep, and bottom watermasses
  form primarily, but not entirely, athigh
  latitudes (around 70 North and South).
• The densest ocean waters, Antarctic Bottom
  Waters form in the Antarctic in winter, sink to
  the bottom and spread along the ocean floor to
  about 40 north latitude.
• In the Arctic the North Atlantic Deep Waters
  form, but often get trapped there by the
  topography of the ocean basin.
• In the Northern Hemisphere along the east coast
  of the Siberian Kamchatka Peninsula the Pacific
  Deep Waters form. Not as dense as bottom water
  they make up the deep layers.
• Mediterranean Deep Waters form due to evaporation
  rather than cooling, with a salinity of 38.
  Flowing out of the Mediterranean they form the
  intermediate water layer resting above the bottom
  layer and deep layer.

Deep Currents
Chapter 9 Pages 9-28 to 9-30
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Deep-Water Flow Patterns

• The enormous water quantities sinking at the
  poles and in the Mediterranean create the
  thermohaline circulation pattern.
• Dense water descends into low areas and bottom
  water upwell to compensate.
• The rising warm water enters wind-driven currents
  and is carried to the poles. There it cools,
  becomes more dense, and sinks again, repeating
  the process.
• The Ocean Conveyor Belt
• The interconnected flow of currents that
  redistribute heat is called the ocean conveyor
  belt or the Earths air conditioner.
• The ocean conveyor belt is important because it
  moderates the worlds climate. This marriage of
  surface and deep water circulation carries heat
  away from the tropics and, in turn, keeps the
  tropics from getting too hot.
• Some scientists hypothesize that some Ice Ages
  may have resulted from a disruption of the
  conveyor belt.

Deep Currents
Chapter 9 Pages 9-30 to 9-33
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Two Distinct Approaches

• There are two main approaches to study currents
• 1. Lagrangian method, also called the float
  method.
• Studying the current by tracking a drifting
  object. This involves floating something in the
  current that records the information as it
  drifts.
• 2. Eulerian method, also called the flow method.
• Studying the current by staying in one place and
  measuring changes to the velocity of the water as
  it flows past. This method uses fixed instruments
  that meter/sample the current as it passes.
• Instrumentation and Methods
• There are five examples of instruments or
  methods that scientists apply for studying
  currents.
• For Lagrangian study methods researchers use
• 1. A drogue. The advantage over a simple
  surfacefloat is that the holey sock ensures
  that the current andnot the wind determine where
  it drifts.

Studying Ocean Currents
Chapter 9 Page 9-34
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Instrumentation and Methods (continued)

• 2. The Argo float drifts at depth before
  periodically rising to the surface to transmit to
  a satellite a temperature and salinity profile of
  the water it rose through.
• For Eulerian study methods researchers use
• 3. Various types of flow meters. These
  devicesuse impellors and vanes to measure and
  recordcurrent speed and direction. The
  information gatheredis either transmitted
  immediately or stored forretrieval later.
• 4. A more sophisticated device is the Doppler
  AcousticCurrent Meter. This instrument
  determines currentdirection and speed.
• 5. Oceanographers can now use satellites to help
  them.Although they are primarily used for
  studying the surface,these instruments use laser
  and photography to study currents.

Studying Ocean Currents
Chapter 9 Pages 9-35 to 38