Ocean Currents

G.Burgess

2009

Major Ocean Currents

1. Antarctic circumpolar current2. California current3. Equatorial current4. Gulf Stream5. North Atlantic Drift6. Portugal Current7. Labrador Current8. Canary Current9. Humbolt Current10. North and South Equatorial Counter Current11. North Pacific Drift12. West Wind Drift13. Florida Current14. 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 Atlantic2) 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)