Ocean Circulation

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Ocean Circulation
At 50N, youll find both polar bears and palm
trees.
Polar Bear Provincial Park Ontario, Canada
Scilly Isles Britain
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50N latitude
www.polarhusky.com/logisticsmaps-and-route/circump
olar
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Ocean Circulation

• Why the great difference in climate between
  interior Canada and the British Isles?
• Cold air flowing over Canadas cold interior
  loses heat, but is warmed when it reaches the
  Atlantic Ocean and the Gulf Stream
• The Gulf Stream is a current, a moving mass of
  water driven by wind and differences in water
  density

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Ocean Circulation

• There are 2 major types of currents
• Surface currents are wind-driven movements of
  water at or near the oceans surface (uppermost
  400 meters of ocean) involve 10 of the worlds
  ocean water
• Thermohaline currents are slow, deep currents
  that arise from density differences caused by
  variations in waters temperature and salinity

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Surface currents

• Surface currents move water horizontally, and are
  primarily driven by winds
• Waves on the sea surface transfer energy from the
  moving air to the water by friction the water
  flowing beneath the wind forms a surface current
• Only 2 of wind speed is transferred to ocean
  current (resulting current is 2 of wind speed)
• Water literally piles up in the direction of
  the wind and gravity pulls the water down slope

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Surface currents

• When water moves down slope, the Coriolis effect
  intervenes!
• Because of the Coriolis effect, surface currents
  in the Northern Hemisphere flow to the right of
  the wind direction, and in the Southern
  Hemisphere flow to the left of the wind direction
• Additionally, continents and submarine topography
  frequently block or deflect flow into a circular
  pattern, called a gyre

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The formation of gyres
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Clock-wise
Counter clock-wise
(Winds driven by uneven solar heating)
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The North Atlantic Gyre
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Surface currents

• The trade winds which blow from the southeast in
  the Southern Hemisphere, and from the northeast
  in the Northern Hemisphere set the current in
  motion between the tropics (equatorial currents)
• When equatorial currents reach the western
  boundary of the ocean basin, they must turn
  because they cannot cross land Coriolis deflects
  these currents away from the equator (western
  boundary currents)

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Western boundary currents Equatorial
currents
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The Gulf Stream is a western boundary current
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Western boundary currents

• Western boundary currents are fast, narrow, and
  deep surface currents that carry warm water from
  the equator to the poles
• Eastern boundary currents, on the other hand,
  flow back across the ocean basin carrying cool
  water from the poles to the equator (also
  deflected by continents and the Coriolis effect)

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Wind-driven surface currents
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Ekman Spiral and Transport

• The collision of air molecules (in wind) with
  water molecules at the sea surface generates the
  water current
• Once this surface film of water molecules in set
  in motion, they exert a frictional drag on the
  water molecules immediately beneath them, getting
  these to move as well
• If the wind blows persistently, motion is
  transferred downward into the water column

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Ekman Spiral and Transport

• As this wind-driven current deepens, its speed
  diminishes (decreases) because of the growing
  distance from the driving force (the wind)
• The currents flow direction also changes with
  depth, the result of Coriolis deflection!
• In the Northern Hemisphere, surface current flows
  to the right of the wind direction
• When this topmost layer sets the underlying layer
  of water in motion, this deeper layer also moves
  to the right of the direction of flow with each
  successively deeper layer deflected to the right
  of the layer immediately above it

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This spiraling flow pattern is called the Ekman
spiral
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Ekman Spiral and Transport

• Under the influence of a strong, persistent wind,
  the Ekman spiral may extend downward to a depth
  of 100-200 meters (330-660 feet)
• The net transport over this entire wind-driven
  spiral is 90 to the right of the wind direction
  in the Northern Hemisphere
• net transport represents the average of all
  directions and speeds of the Ekman spiral

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Ekman Transport
The immediate surface water moves in a direction
of 45 the overall transport of the water is 90
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Ekman Transport and Surface Currents

• The Ekman transport describes an ideal situation,
  but ideal conditions rarely exist in nature, so
  the actual movement of the surface currents
  deviate slightly from that expected from the
  Ekman spiral
• In shallow waters, for example, the Ekman
  transport can be very near the same direction as
  the wind

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Ekman Transport and Surface Currents

• Because Ekman transport deflects surface water
  90 to the right (in the Northern Hemisphere), as
  gyres rotate clockwise, a convergence of water
  occurs in the middle of the gyre
• Causes water to literally pile up in the center
  of the gyre
• Creates a hill of water that is up to 2 meters
  (6.6 feet) in height

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Dome of water formed by Ekman Transport
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Yes, Virginia, there really is a hill of water in
the middle of the North Atlantic
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Geostrophic Flow

• Gyres are in a constant balance between the
  pressure gradient formed by Ekman Transport and
  the Coriolis effect
• Coriolis wants to move water uphill against the
  concentration gradient and gravity wants to
  move the water downhill against the Coriolis
  deflection
• Resulting current flows parallel to the slope

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Geostrophic flow (currents)
Water flows downhill but is deflected to the
right by Coriolis effect!
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Water flows downhill under the influence of
gravity, but the Coriolis effect deflects it to
the right (in the Northern Hemisphere) All
currents in a gyre move as a result of
geostrophic flow!
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• Western boundary currents are very fast and deep
  because there is a westward intensification of
  water piling up due to the eastward rotation of
  the Earth Coriolis

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North America
Europe
Equator
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The Gulf Stream

• Remember, the Gulf Stream is a western boundary
  current
• Transports warm, tropical water northward
• Together with its eastward extension, the North
  Atlantic Current or Drift, keeps Ireland and the
  west coast of Great Britain warm, and parts of
  Norway ice- and snow-free year round

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• The Gulf Stream sometimes meanders, forming rings
  or eddies that trap cold or warm water in their
  centers
• Warm-water eddies bring coconuts and tropical
  fish to Long Island!

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Wind can cause vertical movement of water

• Wind-driven water is usually horizontal in
  nature, but can sometimes induce vertical
  movement in the surface water
• Upwelling is the vertical movement of cold, deep,
  nutrient-rich water to the surface
• Enhances productivity, which can support
  incredible numbers of large marine life
• Downwelling is the vertical movement of surface
  water to deeper parts of the ocean
• Decreases productivity, but transports necessary
  dissolved oxygen to organisms on the deep-sea
  floor

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Upwelling

• When surface waters move away (or diverge) from
  an area on the oceans surface, upwelling occurs
• Upwelling commonly occurs along the equator
  (equatorial upwelling) and along the west Coast
  of the United States (coastal upwelling)
• Creates areas of high productivity that are some
  of the most prolific fishing grounds in the world

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Equatorial Upwelling
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Equatorial Upwelling
As the SE trade winds pass over the equator, they
cause water in the northern hemisphere to veer to
the right, and water in the southern hemisphere
to veer to the left (Coriolis deflection). Surface
water diverges, resulting in equatorial upwelling
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Coastal Upwelling
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Coastal Upwelling (Global)
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Coastal Downwelling
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El Niño

• Under normal circumstances, upwelling brings
  cold, nutrient waters to the coasts of Peru,
  driving an important anchovy fishery

Pacific Warm Pool
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Normal conditions
Pacific Warm Pool
The Pacific Warm Pool contains some of the
warmest water on Earth very low in nutrients
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El Niño

• Every few years, a current of warm water occurs
  off the coast of Peru, reducing the commercial
  catch of anchovies
• Sea birds and seals that depend upon the
  anchovies for food suffered as well
• The warm current brought with it increased
  rainfall (good for Peru usually arid)
• Usually occurred around Christmas and was named
  El Niño (the child) today known as the El
  Niño Southern Oscillation (ENSO)

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El Niño

• During an El Niño, the high pressure zone along
  the coast of South America weakens, reducing the
  pressure gradient difference (high to low)
• Causes southeast trade winds to diminish (or in
  extreme cases, to blow in the opposite
  direction!)
• Without the trade winds, the warm water pool on
  the western side of the Pacific begins to flow
  towards South America

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El Niño

• The Pacific Warm Pool creates a band of warm
  water that stretches across the equatorial
  Pacific Ocean
• Begins moving in Sept and reaches the coast of
  Peru Dec or Jan temperature of seawater off
  Peru can increase lt10C (18F) during this time!
• Sea level can increase as much as 8 inches,
  simply due to thermal expansion of water along
  the coast!

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El Niño (underway)
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El Niño (established)
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El Niño
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El Niño

• As the warm water increases sea surface
  temperature, corals become bleached
• Near Peru, coastal upwelling brings warmer,
  nutrient-poor water to the surface, instead of
  cool, nutrient-rich water limiting productivity
• During a strong El Niño, the low pressure zone
  migrates (and remains) over South America, while
  high pressure moves towards Indonesia, bringing
  very dry conditions to Indonesia and Australia

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Normal conditions
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Normal conditions
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La Niña

• Occasionally, conditions opposite of El Niño
  occur these are known as ENSO cool phase, or La
  Niña
• La Niña is characterized by normal conditions
  intensified, resulting in stronger trade winds,
  causing more upwelling, and a band of cooler
  water stretching across the equatorial Pacific
• Usually occurs after an El Niño event enhanced
  productivity (good)

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El Niño and La Niña
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El Niño and La Niña

• El Niño conditions occur on average every 2 to 10
  years, but on a highly irregular basis
• Increased global warming may be promoting, and/or
  enhancing El Niño events increased sea surface
  temperatures can trigger more frequent and more
  severe events
• Strong El Niño events can alter global weather
  patterns, resulting in flooding, droughts, fires,
  tropical storms, and erosion

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Deep Ocean (Thermohaline) Circulation

• Subsurface, or deep ocean currents, arise from
  density differences between water masses produced
  by variations in temperature (thermal effect) and
  salinity (haline effect)
• For this reason, they are collectively referred
  to as thermohaline circulation
• As you saw in lab, when 2 water masses with
  different densities come into contact, the denser
  water mass slips beneath the less dense water mass

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Deep Ocean (Thermohaline) Circulation

• Deep water currents move large volumes of water
  and are much slower than surface currents
• It takes a deep water current an entire year to
  travel the same distance as a western intensified
  surface current can move in one hour!
• Although temperature and salinity both affect the
  density of seawater, temperature has a greater
  influence on density

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Deep Ocean (Thermohaline) Circulation

• Most water involved in deep-ocean currents
  originated at the surface in high latitudes
• There, the surface water cooled and its salinity
  increased as sea ice formed (both increasing its
  density)
• When this surface water becomes dense enough, it
  sinks, initiating deep ocean currents

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Deep Ocean (Thermohaline) Circulation

• Once the water sinks, it is removed from the
  physical processes that increased its density in
  the first place, so its temperature and salinity
  remain largely unchanged
• Thus, a temperature-salinity (T-S) diagram can be
  used to identify deep-water masses based on their
  characteristic temperature, salinity, and
  resulting density

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Sources of deep water

• Huge masses of deep water form beneath sea ice
  off Antarctica
• Here, rapid freezing produces very cold, high
  density water that sinks and becomes Antarctic
  bottom water, the densest water of the open ocean
• Large masses of deep water also forms in the
  North Atlantic during sea ice formation this
  dense mass is known as North Atlantic Deep Water

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Sources of deep water
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Conveyer-Belt Circulation

• For every liter that sinks from the surface into
  the deep ocean, a liter of deep water must return
  to the surface somewhere else
• Beginning in the North Atlantic, surface water
  carries heat to high latitudes via the Gulf
  Stream cooling in the North Atlantic during
  winter increases the density of this surface
  water until it sinks and flows southward towards
  the equator

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Conveyor-Belt Circulation
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Conveyor-Belt Circulation

• In 2005, researchers noticed that the flow of the
  northern Gulf Stream had decreased by 30 since
  1957
• Researchers also noted that the water of the
  North Atlantic was becoming fresher (less saline)
  as the Earth continues to warm (increasing
  precipitation and polar ice melting)

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A Global Warning?

• Freshening of the North Atlantic from global
  warming could slow or stop the sinking of cold,
  salty water, which drives the conveyor belt , and
  brings warm water via the Gulf Stream to the
  North Atlantic
• This warm water gives up heat to the atmosphere
  and moderates temperature in many parts of the
  globe

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The Great Pacific Garbage Patch, and other
interesting tidbits
http//www.youtube.com/watch?vuLrVCI4N67M