Transcript Ocean Currents

Ocean Currents
G.Burgess
2009
Major Ocean Currents
1. Antarctic circumpolar current
2. California current
3. Equatorial current
4. Gulf Stream
5. North Atlantic Drift
6. Portugal Current
7. Labrador Current
8. Canary Current
9. Humbolt Current
10. North and South Equatorial Counter Current
11. North Pacific Drift
12. West Wind Drift
13. Florida Current
14. Antilles Current
Factors that affect motion of ocean water
1. Sun and Wind
2. Gravity and Density: changed by
heat and dissolved materials
3. Ekman Current Spiral and Coriolis
Effect
4. Geographic location and structure
of ocean basin
Sun and Winds
• Sun: direct light heats the equatorial regions most and
polar regions least.
• Warm air rises, cooler air rushes in to replace the warm
air. (causes air currents; wind)
• Cause surface circulation
• Friction between moving air and water surface will cause
the top layer and successive layers to move and
circulate
• Water direction is dependant on Coriolis effect, velocity,
and Ekman Spiral
Coriolis Effect
• Discovered by Gaspard Gustave de Coriolis(french)1844
• As the Earth rotates, it causes the air above to deflect to the right
in the Northern Hemisphere and to the left in the Southern
hemisphere
Ekman Current Spiral
• The ocean is made up of layers
of water
• When steady winds blow, the
top layer moves, the successive
layers below move as a result of
friction with the top layer.
• The layers below move at a
slower speed than the layer
above
• The Coriolis effect causes this
motion to also move to the right
in the northern hemisphere and
to the left in the southern
hemisphere.
http://oceanworld.tamu.edu/students/currents/currents3.htm,
accessed Sept.27, 2009.
•water is deflected 45° from the
direction of the wind.
•Deeper = more deflection
•Net movement of current is 90°
from the wind direction.
Gravity and Density
• Form deep circulation currents
• Gravity pulls dense water toward ocean floor
• Density of ocean water depends on concentrations of
salt, dissolved materials, and water temperature
• More salt and dissolved material = greater density
• Colder water = more dense
• Warm water = less dense
• Less dense water flows over more dense water
– Heavy cold polar water moves toward equator while
light warm equatorial water moves to the poles
Location and Floor structure
• Coast lines and ocean floor protrusions present obstacles for
currents to go around
• Northern hemisphere currents are deflected around the right side of
topographic highs and left side of depressions (deflections are
reversed in the southern hemisphere)
• the coastlines of the continents act as boundaries and cause surface
currents to form closed circular patterns called GYRES
• There are 5 gyres;
– 1) North Atlantic 2) South Atlantic
– 3) North Pacific 4) South Pacific
– 5) Indian Ocean Gyre
Surface circulation
• Wind is caused by the atmosphere being heated by the
sun
• As air heats up it rises, cooler air rushes in to replace the
warm air
• This motion causes the surface water to move.
• As the surface water moves, it applies frictional stress on
the layers of water below and move them (Ekman Spiral)
Deep water circulation
•
•
•
•
Caused by variations in water temperature and salinity
Heavy, cold polar water moves to the equator
Light, warm equatorial water moves to the pole
(Alexander von Humbolt, 1814)
Local Circulation
• Occurs in small scale coastal regions
• Affected by wind, waves and ocean floor topography
• Coastal features (bay, inlet, island, rivers, etc.) also
affect the flow of these surface currents
• Along depths of <100m, surface currents can be affected
by local changes in temperature and salinity causing
water currents to upwell and sink
• Upwelling brings cold, nutrient rich water to the surface
providing food for local food webs
• Sinking moves O2 to lower depths providing bottom living
organisms
Measuring Currents
• Oceanographers measure ocean currents for many reasons;
predicting climate change, travel of hurricanes, changes to daily
weather, migration of ocean species, cleanup of spilled materials,
etc.
Types of Measurements taken:
• Direct current measurement:
– Getting data directly from water motion
– velocity, direction, geographic location, depth
– Velocity: m/s in surface water to km/yr in deep circulation
– Usually converted and describes as Knots/hr
– Direction: compass bearing- degrees
– Location: longitude-latitude coordinates and depth
Types of direct measurements
• Eulerian methods-measure a current as it passes a fixed
point
– Ekman meter- 1905-records both direction and
velocity
• Lagranian methods: tracing and monitoring movement of
floats or tagged objects
– Drift bottles: bottles dropped in water, usually with
card in side, found by chance. Other bottles may have
dyes, radioactive material, radio-buoy.
– Droques: parachute type drifting buoy,
Types of indirect measurements
• Satellite
• trash
Geostrophic Motion
• Fluid motion may be unaccelerated and frictionless
• Gyres are large scale systems of rotating ocean currents
caused by winds of Westerlies and Trade winds.
• Gyres tend to pile-up water low-density water near their
centers forming gently sloped hills.
• Water moves down slope with gravity and is deflected by
the Coriolis effect at right angles
• Currents are deflected around the right side of
topographic highs and left side of topographic lows
Tides
• caused by the rotation of the Earth and pull of the Earth,
Moon and Sun.
• Tidal currents are caused by the horizontal motion
resulting from the vertical motion of the tide
• Diurnal Tides: occur as a single high tide and single low
tide during a 24hr period. (moon is furthest away from
the equator)
• Semi-diurnal Tides: 2 high tides and 2 low tides occur
each day (moon is over to the equator)
• Mixed tides occur as the moon is travelling toward the
poles (heights of tides change)