Ocean Movements

Chapter 25, Section 1 - Ocean Currents                                                                                                                                             

Current – A horizontal movement of water in a well-defined pattern (such as a river or stream).


Identified by…

Study of physical & chemical properties of ocean water.
Mapping the path of floating debris.

Two Categories…

Surface Current – A horizontal movement of ocean water caused by wind and occurring near the surface.
Deep Current – A stream-like movement of ocean water far below the surface.


Factors Affecting Surface Currents

  1. Air Currents
• Winds form due to convection & the Coriolis Effect.
• Uneven heating creates the Wind Belts.
• Tradewinds
• Westerlies
• Polar Easterlies
• Wind has kinetic energy à passes it to surface water.
• Water currents begin to follow the air currents.

  2. Earth’s Rotation
• Coriolis Effect – the curving of a moving object’s path due to the Earth’s rotation.
• Creates gyres.
• Huge circles of moving ocean water, above/below the equator.  (
Clockwise in North, Counterclockwise in South)

  3. Location of Continents
• Continents act as barriers, deflecting currents.
         • As continents drift, the currents change.

Major Surface Currents

  1. Equatorial
• North Equatorial (west)
• Equatorial Countercurrent (east)
• South Equatorial Current (west)
  2. Southern Hemisphere
• Antarctic Circumpolar (east)
• No continents to disrupt it.
• Indian Ocean
• Southern – driven by gyre.
• Northern – driven by monsoons.
  3. Northern Hemisphere
• Atlantic
• Gulf Stream à North Atlantic Gyre
• Forms Sargasso Sea
• Pacific
• Similar to Atlantic

Deep Currents

  • Cold, dense currents far below the surface.
  • Caused by difference in density.
• Cold water is denser à sinks.
• Freezing concentrates the water below à higher-salinity water sinks.
  • Antarctic Bottom Water
• Very cold (2oC), very salty.
• Moves N to 40oN, takes hundreds of years.
  • North Atlantic Deep Water
• Forms near Greenland.
  Flows south, under Gulf Stream, over Antarctic Bottom Water.

Global Ocean Conveyor

  • Transports heat around the globe.
  • Helps to regulate climate.

Turbidity Currents

  • Caused by underwater landslides.
  • Large amounts of sediment mix with water, making it heavy à sinks.


Chapter 25, Section 2 - Ocean Waves                                                                                                                                                

Wave

  • A periodic disturbance in a solid, liquid or gas as energy is transmitted through a medium.
  • Parts…
• Crest – highest point.
• Trough – lowest point.

• Wave Height –  vertical distance from crest to trough.
• Wavelength – distance between two consecutive crests (or troughs).
• Wave Period – The time it takes for two consecutive crests to pass a given point.

Wave Speed=  wavelength/(wave period)

Wave Energy

  • Friction between ocean water and wind creates ripples (small waves).
  • More wind à more energy transferred à larger waves.
  • Larger waves = more surface area à receive more energy from wind.
  • Larger waves grow larger (due to wind), smaller waves tend to die out.

Water Movement in Waves

  • Waves transport energy, not material.
  • Water particles move in circles in the wave.
• Diameter of circle = height of wave.
  • Wave motion decreases with depth.  (Waves receive their energy from wind.)
• Negligible water motion below a depth equal to half the wavelength.

Wave Size

  • Determined by…
• Wind speed.
• Wind duration.
• Fetch (distance wind blows across an area of sea to generate waves).
  • Strong, steady wind = large waves.
  • Strong, gusty wind = choppy water.
  • Waves collapse when height to wavelength ratio reaches 1:7.
  • Whitecaps form when wind blows the crests off.
  • Swell – one of a group of long, rolling waves of similar size.
• May travel for thousands of km’s.

Waves and the Coastline

Breakers

  • Waves reach a depth equal to half of their wavelength.
  • As they approach shore, the bottom is slowed.  The top maintains it speed.
  • Wavelength shortens, height increases, and the wave topples.
  • Breakers move sediments and erode the coastline.
• Steeper ocean floor = wave height increases rapidly, wave breaks with more force.
• Gentler slopes = wave rises slowly, rolls up onto shore.

Refraction

  • Waves bend directly toward shore as they approach shallow water.
• Part of wave in shallow water slows, part  in deeper water maintains speed.

Undertow

  • Irregular current formed as water from breakers flows back into deeper water by gravity.
  • Only tend to be strong along shores with deep drop-offs.

Rip Current

  • Form when water from large breakers returns to ocean through channels that cut through sandbars.
  • Can be strong enough to carry swimmers out.
  • Flow perpendicular to shore.
  • Can be identified by a gap in a line of breakers, or by turbid water.

Longshore Currents

  • Currents that flow parallel to the shore.
  • Formed when waves approach the shore at an angle.
  • Move large amounts of sediment down a coastline.
• Deposit it at a bay or inlet, creating a sandbar.

Tsunami

  • Giant seismic ocean waves.  (a.k.a. "Tidal Waves" - misleading because they are not caused by tides!)
  • Caused by…
• Earthquakes on ocean floor.
• Volcanic eruptions.
• Underwater landslides.
  • Long wavelengths – up to 500 km.
  • In deep water, wave height is less than 1m.
  • Wave period of about 1 hour.
  • Wave speed  up to 890 kph (jet airplane speed).
  • When it approaches the shore,…
• Speed decreases, height increases (30-40m).
• If trough arrives first, ocean seems to pull back.
• If crest arrives first, sudden, rapid rise in water level.


Chapter 25, Section 3 - Ocean Tides                                                                                                                                                

Tide

  • The periodic rise and fall of the water level in the oceans and other large bodies of water.
  • Formed as a result of the Moon’s gravitational pull.
             Caused by the differential gravitational force of the Moon on various parts of the Earth.

In 24 hours…

     …the Earth rotates once.

     …the Moon goes through 1/29 of its orbit.

So…

     …any spot on the Earth passes under the Moon

        every 24 hr, 50 min.

     …the Moon rises 50 min later each day.

     …tides occur 50 min later each day.


The Sun causes tides, as well, but it’s effect is only about half as strong as the Moon’s.

Spring Tides

  • Highest high, lowest low tides.
  • Occur when Sun & Moon align.
• At Full Moon & New Moon.

Neap Tides

  • Lowest high, highest low tides.
  • Occur when Sun & Moon are at right angles to each other.
• At First & Last Quarter Moon.

Tidal Breaking
  • Earth spins faster than the Moon orbits.
  • This means that the Earth turns beneath the tidal bulges.
  • This creates friction between the ocean and the ocean floor.
  • This drag slows the Earth’s spin, making the day longer.  (By 0.0023 sec per century.)
  • It also pulls the Moon, speeding up its orbit.  This allows the Moon to pull away from Earth’s gravity. (By 3 cm (1”) per year.)


Tidal Variations

  • Daily tidal patterns are influenced by continents and ocean basins…
• US Atlantic Coast
• 2 high tides
• 2 low tides
• Gulf of Mexico
• 1 high tide
• 1 low tide
• US Pacific Coast
• 1 very high tide
• 1 very low tide
• 1 lower high tide

• 1 higher low tide



Tidal Oscillations

  • Slow, rocking motion of ocean water that occurs as tidal bulges move around the ocean basins.
  • Can alter the effects of tidal bulges…
• Straight Coastlines & Open Ocean
• Not noticeable.
• Mediterranean Sea / Baltic Sea (enclosed seas)
• Reduce effects of tidal bulges.
• Small tidal range.
• Small Basins / Narrow Bays off Major Basins
• Amplify effects of tidal bulges.
• Very large tidal range.

• Ex – Bay of Fundy.


Tidal Currents

  • The movement of water toward and away from the coast as a result of the rise and fall of the tides.
• Flood Tide – Water flows toward the coast.
• Ebb Tide – Water flows away from the coast.
• Slack Water – Time between flood and ebb tides (no tidal currents).
  • Smaller in open ocean than near coast.
  • Strong in narrow bays along coastline.
  • Tidal Bore – A surge of ocean water that rushes upstream where a river meets the ocean.
• May appear as a large (up to 5 m) wave that rushes upstream (up to 20 kph).
• May travel many upriver for many km.