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.
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.
…the Moon goes through 1/29 of its orbit.
…any spot on the Earth
passes under the Moon
…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.
• Highest high, lowest low
tides.
• Occur when Sun & Moon
align.
• At Full Moon &
New Moon.
• 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.
