Ocean Currents

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Ocean Currents

• Ocean water circulates in currents caused by wind
  and by density differences
• Currents are the flow of water between areas of
  different surface levels or different densities

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Ocean Currents

• Ocean water moves constantly sideways, up, down
  because it is
• Blown by wind
• Moved by differences in density below the surface
  zone
• Heated unevenly
• Evaporated unevenly

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Ocean Currents

• Ocean water moves constantly sideways, up, down
  because it is
• Affected by Earths rotation (Coriolis)
• Pulled by sun moon

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CURRENTS
Currents are important because they moderate
climates, mix nutrients and oxygen, and transport
larvae and nekton.
They are caused by the
1. Spin of the Earth on its axis and the Coriolis
effect
2. Ekman Transport results in movement at a 90o
angle to the wind
3. Warm air from equator flows toward poles,
drives air circulation, makes wind
4. Continents deflect east-west water movement,
but not air
NASA
NOAA
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Coriolis Effect

• Causes currents to move Clockwise in the Northern
  Hemisphere Counterclockwise in the Southern
  Hemisphere
• Causes westward moving currents to be intensified
  (move much quicker)

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EKMAN TRANSPORT
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Surface Currents

• Affect the uppermost 10 of world ocean
• Some rapid riverlike, some slow and diffuse
• Largest organized into gyres

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The circular patterns of surface water currents
are called gyres. The characteristics of a
current depend upon where the water is coming
from in the gyre.
In the Northern hemisphere the gyre is (right)
clockwise, and in the southern hemisphere the
currents are (left) counter clockwise.
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Coriolis Effect

• Water near ocean surface moves to the right of
  wind direction in Northern Hemisphere
• Water near ocean surface moves to the left of
  wind direction in Southern Hemisphere

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Eastern boundary currents are located on the
eastern side of the OCEAN - not the continent.
East Coast of United States
Gulf Stream in red
Western boundary currents are located on the
western side of oceans.
Florida
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Comparison of Boundary Currents

• Western

• Eastern

• currents bring warm water from the equator

1. currents bring cold water from the poles
2. high salinity low oxygen content
2. low salinity high oxygen content
3. nutrient poor, infrequent upwelling
3. nutrient rich due to upwelling
4. lower biomass
4. high biomass
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Surface Currents
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Effect of Surface Currents on Climate

• Surface currents distribute heat worldwide
• Warm water flows to higher latitudes
• transfers heat to the air and cools
• Moves back to low latitudes
• Absorbs heat
• Repeats

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Upwelling and Downwelling

• Describe the vertical movement of water

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Vertical Movement

• Upwelling upward movement of water
• Often due to divergence of surface currents
  (Equatorial Upwelling)
• Can be caused by wind blowing parallel to shore
  or offshore (Coastal Upwelling)
• Brings nutrients

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Vertical Movement

• Downwelling downward movement of water caused
  by
• Convergence of surface currents
• Increase in surface water density
• Wind blowing parallel to shore or onshore
  (Coastal Downwelling)
• Brings dissolved gases and nutrients to deeper
  ocean

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EL NIÑO
Normally, trade winds blow warm surface water
toward the western Pacific.
When the Trade Winds falter, warm water piles up
in the western Pacific about 1 ½ feet higher
than the average ocean surface due to wind and
Ekman transport.
This causes the thermocline to rise near the
surface in the eastern Pacific.
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Cold, nutrient-rich water wells up from the
thermocline to the surface waters off the coast
of Peru - good for fishing.
This image shows normal sea surface temperature
in the Equatorial Pacific Ocean in January 1997.
Notice the warm water is in a pool in the western
Pacific Ocean.
NOAA
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El Niño describes abnormal oceanic and
atmospheric conditions.
First noticed off the coast of Peru around
Christmas time, the condition was called El Niño
referring to the Christ child.
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El Niños weaker trade winds blow warm water
eastward, across the equatorial region of the
Pacific.
NOAA
This depresses the thermocline across the
Pacific, and stops upwelling.
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Because warm water evaporates faster, the air
fills with more water vapor and it produces more
rain.
El Niño occurs primarily in the tropical Pacific
Ocean but it affects global weather.
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La Niña
NOAA
When too much westerly wind blows surface water
near the Americas, it has an effect on the
weather that is often opposite to El Niño, called
La Niña.
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Comparison of El Niño and La Niña

• El Niño

• La Niña

warmer ocean temperatures
colder ocean temperatures
occurs every 3-7 years
frequency unknown
wetter than normal summers
drier than normal summers
weak trade winds
strong trade winds
decreases hurricanes forming in N. Atlantic
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1997 was the warmest year of the century and the
strongest El Niño in 40 years. Oddly, five of
the centurys warmest years have been in the last
decade.
NASA
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Thermohaline currents are found on the ocean
floor. Thermo refers to temperature and
haline refers to salinity.
NASA
Water slows down below the pycnocline (area
defining water masses of different densities) to
1-2 meters/day.
Cold water sinks at the poles and crawls toward
the rising warmer water at the equator.
NASA
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Density Structure of the Ocean

• Seawater weighs 2-3 more than pure water
• Cold, salty water is more dense than warm, less
  salty water
• Much of the ocean can be divided into three
  density zones
• surface, pycnocline deep

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Density Zones

• Surface or Mixed Zone
• Temperature, salinity density constant due to
  currents and waves
• In contact with atmosphere
• Exposed to sunlight
• Least dense water
• 2 all of ocean water
• 150 meters deep

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Density Zones

• Pycnocline aka Thermocline aka Halocline
• Density increases with depth, temperature
  decreases, salinity increases
• Isolates surface water from denser layer below
• 18 of all ocean water

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Density Zones

• Deep Zone
• Below pycnocline
• Depth below 1000 m
• Little change with depth
• 80 of all ocean water

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Density stratificaiton in the ocean
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Temperature, Salinity Stratification

• Surface zone well mixed, little temperature
  change with depth
• Pycnocline temperatures drop rapidly, density
  increases, temperature decreases
• Deep zone, little temperature change with depth
  a cold, stable zone

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• Below pycnocline/thermocline water very cold 1-3
  deg C
• Average temperature of ocean 3.9 deg C

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Density Stratification Water Movement

• Water mass body of water with same temperature
  and salinity
• Even deepest water originates at surface
• Evaporation makes dense salty water
• Dense water mass trapped at depth- ocean is
  stratified

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Density Stratification Water Movement

• Dense water masses form
• near poles (as water freezes)
• in enclosed areas (where water evaporates)

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Density Stratification Water Movement

• Water masses below pycnocline do not mix no
  energy too cold

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Density Stratification Vertical Water Movement

• Vertical movement possible when surface-water and
  deep-water density similar
• Water in tropics very stable (no exchange)
• Water in northern polar ocean very unstable (due
  to salinity differences)

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Thermohaline Circulation

• 90 of ocean water below surface zone gravity
  driven
• Dense water sinks, less dense water rises
• Density is a function of temperature and salinity
  movement of water due to density differences
  called thermohaline circulation
• Can take hundreds of years to circulate

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• Water masses form at the surface
• Densest and deepest form where water becomes very
  cold and salty (at the poles)

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Circulation of the Atlantic Ocean
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The Global Heat Connection

• Because they transfer large quantities of heat,
  ocean currents affect world weather and climate
• The transport of tropical water to the polar
  regions is part of the global conveyor belt for
  heat transfer

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Ocean Flow

• The slow, steady flow of water
• distributes dissolved gases and solids
• mixes nutrients
• transports juvenile marine organisms among ocean
  basins

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