Section 1 Ocean Currents
A current is a continuous flow of water along a broad path in the
ocean. It can be at the surface or it can be below the surface.
There are two main categories for ocean currents:
Surface Currents
Deep Currents
Factors That Affect Surface Currents
Surface Currents flow in the upper 1000 meters of the ocean.
They are driven by the wind. It is a transfer of energy from the air
to the water and then back again.
Global Wind Belts
Trade Winds: They are steady winds that always blow from North East in
Northern Hemisphere and South East in the Southern Hemisphere. In both
hemispheres they push currents westward across tropical latitudes of all three
major oceans. It is the historical path that the early trade ships sailed; they
used the prevailing winds to help push them along quicker in order to shorten
long trips.
Westerlies: They push ocean currents eastward toward higher latitudes. They
are warm water currents that flow on West side of the oceans.
Continental Barriers
Continental barriers deflect and divide the currents.
The Coriolis Effect
The Coriolis Effect forms gyres (huge circles of moving water)
Northern Hemisphere the currents flow Clockwise.
Southern Hemisphere the currents flow counter- clockwise.
This difference is due to the spinning (rotation) of the earth on its axis.
Equatorial Currents
Warm water currents re produced here.
This makes the Western side of ocean basins have WARM currents moving
away from the equator. The Eastern side of the ocean basin has COLD
currents moving toward the equator.
Currents in the Southern Hemisphere
The Antarctic Circumpolar Current (West Wind Drift)
Currents in the North Atlantic
Gulf Stream
The Gulf Stream is the swift moving, deep, warm Atlantic current that flows
along the Eastern Seaboard. It starts in the Caribbean Sea and then becomes
the North Atlantic Drift traveling all the way to Iceland and England. This keeps
some areas warmer than other locations at the same latitude. (Palm trees in
Ireland)
In the North Atlantic if also forms a gyre that is in the middle of the ocean,
called the Sargasso Sea. It is a warm calm center of the ocean.
Currents in the North Pacific
Kuroshio Current in North Pacific is similar to our Gulf current.
Major Surface Currents
Deep Currents
Deep currents are usually cold water currents. Cold Currents flow toward the
equator on the Eastern side of the ocean basins.
The three dense currents are Antarctic Bottom Water, North Atlantic Deep
Water, and the Antarctic Intermediate Water.
Density Currents flow under the surface due to density differences. They can
stay submerged for 500 – 2000 years and connects with global surface currents
to form circular paths. They are slow moving and the only source of oxygen for
deep-sea life because they retain what they absorb (salinity, oxygen,
temperature and density). Evaporation can cause them.
Antarctic Bottom Water
The densest water is found in polar-regions due to intense cooling and freezing.
When sea water freezes the salt is left behind, increasing salinity.
North Atlantic Deep Water
Canary Current
In the North Atlantic
Bengula Current
In the South Atlantic
Labrador Current
Found off the coast of Newfoundland. It can carry icebergs (remember the
Titanic). When it reaches the Gulf Stream (warm) the mixture produces the
thickest fog in the world.
Turbidity Currents
We learned in last chapter, caused by underwater sediment slides.
Counter-Currents
They are currents that flow in the opposite direction of the wind related currents.
They can flow at or beneath the surface. They occur because steady winds pile
water up on either side of an ocean basin
Section 2 Ocean Waves
A wave is the periodic disturbance in a solid, liquid or gas as
energy is transmitted through a medium.
Wave Height is the difference between the crest and trough
Crest is the high point of a wave
Trough is the low point of a wave
Wavelength is distance from one crest to the next crest
Period – is the time it takes a wave to pass a point
Speed =
Wavelength
Period
ex.
150 kilometers
12 minutes
= 12.5 km/minute
Wave Energy
Waves form from friction between the wind and the water. The longer the
winds blow, the larger the wave becomes.
Water Movement in a Wave
As a wave moves across the surface of the ocean, only the energy of the wave,
not the water, moves in the direction of the wave. The water molecules within
the wave move in a circular motion. During a single wave period, each water
particle moves in one complete circle. At the end of the wave period, a circling
water particle ends up almost exactly where it started.
Because waves receive their energy from wind pushing against the surface of
the ocean, the energy received decreases as the depth of the water increases.
Wave Size
There are three factors that determine the size of a wave: wind speed, length
of time the wind blows and the fetch.
Fetch is the distance that the wind blows across an area of water. The larger
the fetch, the larger the waves become.
White Caps
A white cap occurs when the wind blows the crest of a wave off.
Wave Swell is when a steady smooth wave comes at regular intervals up to 10
seconds apart. They are caused by winds and storms far away.
Waves and the Coastline
In shallow water, the wave bottom will touch the ocean floor and will cause it
to slow down and eventually break.
Breakers
Breakers begin to form as a wave approaches the shore line. The curl as they
slow down and crash on the beach.
Other actions that a wave takes are Swash and Backwash.
Swash is the motion of water going up the beach.
Backwash is the motion of water going back down the beach.
Refraction
Refraction is the process by which the ocean waves bend directly toward the
coastline as they approach the shallow water.
Undertows and Rip Currents
Undertows are seldom strong and will occur in areas along the shorelines that
have a steep drop-off.
Rip Currents flow perpendicular to the shore and are strong.
Long Shore Currents
A long shore current is a current that flows parallel to the shoreline. These
currents are very strong and can carry large amount of sand, often forming
sandbars.
Tsunamis
A tsunami is a wave formed from an underwater earthquake.
Tsunamis may travel at speeds of up to 890 km/h (as fast as a jet airplane).
A Tsunami as a Destructive Force
Near the shore, the height of a tsunami greatly increases as the tsunami's
speed decreases. As a tsunami approaches the shore, it may reach a height of
30 to 40 m. The arrival of a tsunami may be signaled by the sudden pulling
back of the water along the shore. This pulling back occurs when the trough of
the tsunami arrives before the crest. If the crest arrives first, a sudden, rapid
rise in the water level occurs.
Section 3 Tides
A tide is the periodic rise and fall of the water level in the oceans.
High Tide – moon pulls water to itself
Low Tide – water leaves the sides
Causes of Tides
In the 1600’s Sir Isaac Newton discovered that gravity pulled on the water
along the coastlines. The gravitational pull of the moon on the Earth’s water
caused the tides.
Behavior of Tides
Earth’s rotational period is 24 hours and the moon’s orbit around the earth is
approximately 29 days. Because the Earth and the moon orbit in the same
direction that the Earth rotates, all areas of the ocean pass under the moon
every 24 hours and 50 minutes. This makes the tides change every 6 hours
and 12.5 minutes. Each high tide gradually turns into a low tide, and each low
tide turns into a high tide. The time of the high and low tides change each day.
Spring Tides
Spring tides are when the sun is at a 0o or 180o to the moon (New and Full).
The pull is stronger which makes high tides higher and low tides lower.
Neap Tides
Neap tides are when the sun and the moon pull against each other at 90o
during the quarter moon phases the high tide is not very high and the low tide is
not very low. This occurs at the quarter and ¾ phases.
Tidal Variations
Atlantic Coast has two high tides and two low tides each day Gulf of Mexico has
one high and one low tide each day Pacific Coast has a mixed pattern. They
have Very High tides followed by very low tides and lower high tides followed
by higher low tides.
Tidal Oscillations
Tidal oscillations are the slow rocking motion of the ocean water that occurs as
the tidal bulges move around the ocean basin.
Tidal Currents
Tidal currents are the movement of water toward and away from the coast as a
result of the rise and fall of the tides.
Ebb Tide when the water moves toward the oceans
Flood Tide is when the water moves toward the coast
Slack Tide is when there are no currents