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MOTION IN THE OCEAN

Waves and Tides

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Waves

A disturbance which moves through or over the surface of a fluid

Mostly caused by:

Winds

Earthquakes

Gravitational Pull

Volcanos

Form of great energy

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When a wave passes through the ocean, individual water molecules move up and down but they do not move forward or backward.

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http://www.military.com/video/logistics-and-supplies/naval-equipment/warship-takes-on-massive-waves/1910420040001/

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http://oceanexplorer.noaa.gov/edu/learning/player/lesson09.html

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Wave Characteristics

Parts of a Wave

Crest = high point

Trough = low point

Height = vertical distance from crest to trough

Wavelength = Horizontal distance between crest to crest or trough to trough

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Size of Wind Generated Waves

Depends on 3 things:

Wind Speed
Wind Duration (length of time wind blows)
“Fetch” Extent of open water across which the wind can blow

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Water Motion in Waves

Water travels in vertical circular orbits

Wave moves, particles don’t!

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Importance of Waves

Shaping Coastlines

Erode cliffs
Grind rock into sand

Ecology

Returns O₂to water
Stir up food for filter feeders

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Types of Waves�

CHOP – Short period (back bays)

SWELL – Long period (boat rolls; seasickness)

SWASH – water up beach BACKWASH – back down

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Caused by undersea quake or volcano

Wavelength = ~150 mi. Wave height = 6” – 1’

Can NOT perceive in boat Speed > 500 mph

Slows down to ~25 mph at shore; water builds up to ~65+ ft

TSUNAMI “TIDAL WAVE”

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Tsunami Waves

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Creation of a Tsunami

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Before the 2004 Indian Ocean tsunami reached Sri Lanka, many animals, like this elephant, moved to high ground and survived

The man on the left is about to

be killed by the tsunami that struck

Hilo, Hawaii in 1946.

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Hundreds of thousands of people died in the December 26, 2004 Indian Ocean tsunami. But as described in these news reports, others saw warning signs, and escaped with their lives.

Thousands died on the beaches of Thailand in the December 2004 tsunami. But a 10-year-old British girl saved more than 100 people because she had just studied the killer waves in school. When the sea suddenly began to boil, then pulled away from the resort she was visiting, leaving fish and boats stranded high and dry, Tilly Smith recognized that a tsunami was approaching. Fortunately, her frantic warnings were heeded, and the beach was evacuated just moments before the huge waves crashed ashore.

Kalutara Beach Detail�Imagery collected December 26, 2004. Description: Receding waters and beach damage from tsunami

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"Primitive" Tribe Survives Tsunami�

Off the coast of India, the remote Andaman Islands are home to the aboriginal Onge (pronounced OHN-ghee) a tribe that follows an ancient hunter-gatherer lifestyle. On December 26, 2004, when the creek that ran through their settlement suddenly went dry and the sea pulled away from the shore, the Onge took immediate action. Believing that evil spirits were at work, the tribe scattered pig and turtle skulls around their settlement and threw stones into the sea, then quickly gathered their possessions and ran inland.

Moments later, a tsunami slammed into the island of Andaman. All 96 Onge survived the tsunami because of their awareness of natural processes. Many other people in the region were less attuned to nature, and paid with their lives. When the seabed was exposed by the approaching tsunami, tourists visiting the coast of Thailand wandered curiously toward the receding ocean, and Indonesian fisherman ran to pick up fish left high and dry, oblivious to the danger.

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TABLE TALK

Apparently, some animals are alerted to the approach of a large tsunami in ways that humans can't detect. Speculate about what warning signs animals might pick up that people cannot.
Describe several warning signs that often precede tsunami.
Why do you think so many people ignore these indicators?
If you detect any tsunami warning signs, what should you do?

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http://www.onr.navy.mil/focus/ocean/motion/waves1.htm

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Early thoughts about what �caused the tides.

Some believed the earth was a living animal and the tides a result of its breathing.
Others, who thought the ocean’s waters were the earth’s blood, decided the tides were its pulse, caused by the beating of its great heart.

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Early thoughts about what �caused the tides.

Early followers of the religious prophet Muhammad believed that the rise and fall of the waters were caused when the “Angel of the Sea” placed his foot in and out of the ocean.
Primitive natives thought the tides were a sign of a sea god’s anger and made human sacrifices to please him.

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Sir Isaac Newton to the Rescue!

Newton’s Universal Gravitation theory helped to explain how the gravity of the moon (and the sun) were related to tides.

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Tides

The rhythmic rise and fall of the ocean’s water

High tide = rising, incoming tide, flow

Low tide = receding, outgoing tide, ebb

Slack tide = vertical movement stops

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http://oceanexplorer.noaa.gov/edu/learning/player/lesson10.html

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What Causes Tides?

1. Gravitational pull of sun & moon on Earth

Moon closer, therefore > effect

Like magnet, pulls water away from surface = TIDAL BULGE

http://oceanservice.noaa.gov/education/kits/tides/media/supp_tide04.html

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Tides are generated by:

the gravitational pull of the moon and sun

- moon has 2x greater gravitational pull than the

sun

- sun is 10 million x more massive than the

moon and is 390 times farther away

TIDES: Tides are the slow, periodic vertical rise and fall of the sea surface. They are usually described as being either diurnal or semi-diurnal. Diurnal tides have one high water and one low water in each lunar day (about 24.8 hours), while semi-diurnal tides have two high and two low waters in the same time period. While these tidal changes are easier to observe where land and water meet, they exist everywhere -- even in the middle of the ocean. Tidal ranges along the shoreline vary by location. For example, the tides in Canada's Bay of Fundy, an Atlantic Ocean inlet west of Nova Scotia, rise and fall as much as 50 feet, while the tidal range in Lake Superior is measured in inches.

High and low tides are the result of the attractive forces (gravitational pull) of the moon and sun on a rotating Earth.

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Tides are very long, slow waves

They have a wave period of 12 hours 25 min

Tidal day is 24 hours 50 min

NJ has 2 high and 2 low tides daily

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2. Centrifugal Forces

Produced by motions of Earth, sun, & moon

Bulge on opposite side because centr. force > pull of moon

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Centripetal force

Gravity

Newton’s law states that all particles of mass have a gravitational attraction for all other particles, and that the gravitational force is proportional to the sum of the two masses and inversely proportional to the square of the distance between their centers of mass.
The gravitational attraction between the moon and the Earth is very small compared to gravity felt by an object at the Earth’s surface due to the Earth itself. Although the sun has a much greater mass than the moon it is much further from the Earth and the gravitational attraction between the sun and the Earth is smaller than that between the moon and the Earth.

Orbital Motions and Centripetal Force

In any two body system, such as the Earth and moon or the Earth and sun, one body does not orbit around the other. Rather the two bodies orbit around a common balance point that is closer to the larger body. For the Earth and moon, this balance point is beneath the Earth’s surface but not at the Earth’s center, Similarly the common point of rotation between the Earth and sun is inside, but not at the center of, the sun
Any body in orbit must be held in that orbit by a centripetal force that can be supplied by the gravitational attraction.
Centripetal force varies with distance from the center of rotation. All points within each rotating body follow the same diameter circle of rotation, and centripetal force is the same at all points on and within each of two orbiting bodies.
The gravitational force varies with the square of the distance and is slightly higher on the side of a body facing the other orbiting body and slightly lower on the opposite side.

The Balance between Centripetal Force and Gravitational Force

The small imbalance between centripetal force and gravity at different points on the Earth is responsible for tides.
The Earth’s own gravity is millions of times larger than the gravitational attraction of the moon or sun at the Earth’s surface and, therefore, the imbalance between centripetal force and gravity can be compensated by an immeasurably small change in an object’s weight if the force imbalance is directed in the same direction as Earth’s gravity (vertically toward Earth’s center).

Distribution of Tide-Generating Forces

The moon’s gravitational attraction and the required centripetal force are exactly balanced at all points along a ring around the Earth drawn almost exactly midway between the point closest to and farthest away from the moon
At the points directly toward the moon and directly on the opposite side of the Earth from the moon, Earth’s gravity and the imbalance between the moon’s gravitational attraction and the required centripetal force is completely compensated by an immeasurably small change in an object’s weight.
At all other points on the Earth there is a component of the imbalance between the moon’s gravitational attraction and the required centripetal force that acts parallel to the Earth’s surface and so cannot be compensated. This component is the tidal force.
The tidal force acts toward the moon on the moon’s side of the Earth and away from the moon on the opposite side.
The tidal force is zero at the points directly toward and directly away from the moon, increases away from these points, and then decreases to zero again at points along the ring around the Earth drawn almost exactly midway between the point closest to and the point farthest away from the moon, where the moon’s gravitational attraction and required centripetal force are balanced.

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Types of Tides

2x’s/month

Spring Tide

- Moon and sun are in direct line with one another

- Results in unusually high tidal range

Tidal Range = vertical distance between high & low tides

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Neap Tide

sun and moon are at right angles

Pulls cancel each other out – causes a weak pull

unusually low tidal range

2 x’s / month

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Spring vs. Neap Tides

http://oceanservice.noaa.gov/education/kits/tides/media/supp_tide06a.html

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The monthly tidal cycle�(29½ days)

About every 7 days, Earth alternates between:

Spring tide

Alignment of Earth-Moon-Sun system (syzygy)
Lunar and solar bulges constructively interfere
Large tidal range

Neap tide

Earth-Moon-Sun system at right angles (quadrature)
Lunar and solar bulges destructively interfere
Small tidal range

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Distance bet. Moon & Earth

Perigee Tides

Moon closest to earth, very high tides (causes flooding)

Apogee Tides

Moon farthest away from earth, very low tides

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Types of Tides Continued

Diurnal Tides

1 high & 1 low / day
Parts of Gulf of Mexico and Asia

Semi-Diurnal Tides

2 high & 2 low / day
Atlantic coasts of North America and Europe

Mixed

2 high & 2 low / day (height varies)
Pacific coast

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http://oceanservice.noaa.gov/education/tutorial_tides/media/supp_tide01.html

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Importance of Tides

Expose & submerge orgs

Circulate water in bays & estuaries

Trigger spawning (grunion, horseshoe crab)

Circulates food, wastes, etc

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Impacts of Tides

Predicting tides has always been important to people who look to the sea for their livelihood. Commercial and recreational fishermen use their knowledge of the tides and tidal currents to help them improve their catches. Depending on the species and water depth in a particular area, fish may concentrate during ebb or flood tidal currents. In some areas, strong tidal currents concentrate bait and smaller fish, attracting larger fish. In addition, knowledge of the tides has also been of interest to recreational beachgoers and surfers.�

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Currents

What are currents?

- “Rivers” of circulating water

Causes

- Wind

- Rotating Earth

- Density Changes

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Ocean Circulation

Currents are also a product of the wind.
Current can be in surface layers of deeper water layers
Both winds and the currents they drive are affected by the Coriolis Effect.
Because the earth spins continuously, anything that passes over the earth is deflected.

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Ocean Circulation

In the northern hemisphere, winds and currents are deflected to the right.
In the southern hemisphere, winds and currents are deflected to the left.
This deflection termed the Coriolis Effect. Since the Earth is spinning, it causes this defection of winds and currents.
This deflection often causes currents to travel in circular patterns called gyres.

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Coriolis Effect

- N. Hemis – clockwise; Right

- S. Hemis – counterclockwise; Left

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https://www.youtube.com/watch?v=i2mec3vgeaI

Coriolis Effect

https://www.youtube.com/watch?v=Pb69HENUZs8

http://video.mit.edu/watch/the-coriolis-effect-4407/

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Ocean Circulation

Despite repeated mixing by winds and currents at the ocean’s surface, the ocean is still stratified into three layers:

Surface layer – from surface to about 200 meters; this layer stays well mixed most of the year.
Intermediate layer - from 200 – 1500 meters; major temperature change (thermocline) is located here. Less mixing occurs here.
Bottom layer – below 1500 meters; low mixing and normally uniformly cold.

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Deep Ocean Currents�DRIVE THE OCEAN CONVEYOR BELT

Separated from surface currents by boundary called a “Thermohaline”

Flow beneath surface; cross equator

Move North to South

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In some locations, large volumes of water may sink or rise.

Water sinks due to changes in temperature and salinity – this is known as an area of down-welling.

Down-welling brings gases from the surface to deeper layers.

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Importance Of Deep Currents

Upwelling

Brings deep water to surf.
Circulates nutrients up
Moves plankton & larvae

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Surface Ocean Currents

Broad, slow drifts; never cross equator

Wind generated; circular gyres

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Gulf Stream

- Brings warm water from equator north along east coast of N. A.

- N. Atlantic

Sometimes form eddies – circulating water that pinches off from the current

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MIGRATION

NAVIGATION

WEATHER

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Localized Surface Currents

Longshore Current.

Flows parallel to shore; move sediment

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RIP CURRENT

- Caused by converging longshore currents

- Very dangerous ; Red Flag

- DO NOT fight rip current; swim parallel to shore to get out of channel

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http://www.deepseanews.com/2012/06/must-watch-video-on-rip-currents/

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http://www.hamilton.k12.nj.us/webpages/aschwing/resources.cfm?subpage=358703