Transcript Waves and Tides

Waves and Tides
What are waves and tides?
Waves/ Characteristics of Waves
• Waves are disturbances in the ocean that transmit
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energy from one place to another.
The most familiar types of waves
– – the ones that cause boats to bob up and down on the
open sea,
– end as breakers on the beach
– Tides are also a type of wave
Waves can interfere and cancel out one another
Waves shape and erode beaches and shorelines
Waves in the oceans can travel thousands of miles before
reaching land.
Wind waves range in size from small ripples to huge waves
over 30 m high
FORMATION OF WAVES
• The great majority of large breakers one
observes on a beach result from distant winds.
• Five factors influence the formation of wind
waves:
1. Wind speed
2. Distance of open water that the wind has blown
over (called the fetch)
3. Width of area affected by fetch
4. Time duration the wind has blown over a given
area
5. Water depth
PARTS OF A WAVE
• A group of waves consist of several crests
separated by troughs.
• The height of the wave is called the
amplitude.
• The distance between the waves is known
as the wavelength.
• The time between successive wave crests
is the period.
WAVE FORMATION:
*draw this picture in your notes!
Parts of a wave
• Crest: highest part
• Trough: lowest part
• Wavelength: distance between
successive crests or troughs
• Wave height: vertical height from
crest to trough
Classification of Waves
• Waves are classified into types based on their periods.
• They range from ripples up to tsunamis and tides.
• In between these extremes are chop and swell, the most
familiar types of surface waves.
• Ocean waves behave like light rays, they are reflected or
refracted by obstacles they encounter, such as islands. When
different wave groups meet, they interfere, adding to, or
canceling out each other.
• *As waves pass over the surface, the particles of water do not
move forward with the waves. Instead, they gyrate in little
circles or loops. Underwater, the particles move in even smaller
loops. At a depth below about half the distance between crests,
they are still.
WAVES….
• The greater each of the variables, the
larger the waves.
• Waves are characterized by wind
speed, wind duration, and the fetch; also
by:
– Wave height (from trough to crest)
– Wave length (from crest to crest)
– Wave period (time interval between arrival
of consecutive crests at a stationary point)
– Wave propagation direction (any of the ways
in which waves travel)
Properties of waves
The 4 factors that determine the size of a
wind generated waves: (3 D’s and a V)
• duration (time) wind is in contact with
the water
• distance (fetch) over which the wind is
above the water
• the direction in which the wave is
traveling
• the velocity(speed) of the wind
FETCH: AREA IN WHICH THE WIND IS
BLOWING
• In the fetch, many different groups of waves of
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various sizes are generated and interfere with each
other.
As they move away from the area, the waves become
more regularly sized and spaced.
– This is because the speed of a wave in open water is closely
related to its wavelength.
• The largest, fastest moving waves are found at the
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front, the smaller slower moving ones fall behind.
This produces a regular wave pattern, or swell.
– A swell is a series of large, evenly spaced waves, often
observed hundreds of miles away from the storm that
spawned them.
Arrival on Shore
• As waves approach a shore, the motion they
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generate deep down begins to interact with the sea
floor.
This slows the waves down and causes the crests in
a series of waves to bunch up, an effect called
shoaling.
The period of the waves does not change, but they
gain height as the energy each contains is
compressed into a shorter horizontal distance, and
eventually break.
Waves can also refract (change in direction) as they
reach a coastline. This concentrates wave energy
onto headlands and shapes some types of beaches.
Shoaling
Waves gain height as their energy is
compressed, and eventually break
Spilling Breakers
Plunging Breakers
•Occur on flatter shores
•Occurs on steeper shores
•Crests break and cascade
down the front, as they near
the shore
•Crest curls and falls over the
front of the advancing wave
•Energy dissipates gradually
•The whole wave collapses at
once
Yucky math….
• Frequency – the number of waves that pass a fixed point in a
certain amount of time
F = number of waves
Time
EX: 14 pass by in 7 seconds. What is the frequency?
F = 14 waves = 2 waves/sec
7 sec
• Period – the time between successive crests or troughs passing a
fixed point
P = 1/frequency
EX: Given a frequency of 2 waves/sec, what is the period?
P = ½ = .5 sec
Tides
• Regular rises and falls in sea level, along with
horizontal flows of water
• Caused by the gravitational pull of the moon,
sun, and earth
• Most noticeable near coasts, but occur
everywhere
• The basic daily pattern of high and low tides is
caused by the moon’s influence on Earth.
• Variations in the range between high and low
tides over a monthly cycle are caused by the
combined influence of the Sun and Moon.
• The Sun and Moon pull on the Earth, the water, even
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you!
But gravitational attraction depends on distance and
mass.
– For example, you have very little mass and you're very
close to the Earth, so the Sun and Moon can't just yank you
off the planet.
– The Sun is extremely massive, but it is an average of 93
million miles (150 million km) from Earth, compared with
about 238,866 miles (384,400 km) from here to the Moon.
– And since the Moon is nearly 400 times closer to our
planet, its influence on our oceans is twice as strong as the
Sun's.
HIGH/LOW TIDES
• As the earth spins and the moon orbits the earth, two
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different forces are created at the earth’s surface:
– a gravitational pull of the earth toward the moon,
– A centrifugal force directed away from the moon.
These forces combine to produce two bulges in the earth’s
oceans:
– one toward the moon
– the other away from it.
As the earth spins on its axis, these bulges sweep over the
surface, producing high and low tides.
The cycle repeats every 24 hours and 50 minutes (one lunar
day) rather than every 24 hours (one solar day), because
during each cycle, the moon moves around a little in its orbit.
• The timing of tides is determined by the
Earth's rotation and the Moon's orbit
around the Earth.
• As the Earth rotates once about its axis in
24 hours, the Moon is moving 1/30th of
the way around in its orbit.
• It takes a given location on Earth about 50
minutes to "catch up with" the orbiting
Moon, so a particular tide returns in
approximately 24 hours and 50 minutes.
High and Low Tides
Tide Patterns
• Factors that cause tide patterns:
– Landmasses interfere with the movement of
tidal bulges
– The moon orbit tilts to the plane of the
equator
• If the Earth were a perfect sphere without
large continents, all areas on the planet
would experience two equally proportioned
high and low tides every lunar day.
• The large continents on the planet, however,
block the westward passage of the tidal
bulges as the Earth rotates.
• Unable to move freely around the globe,
these tides establish complex patterns within
each ocean basin that often differ greatly
from tidal patterns of adjacent ocean basins
or other regions of the same ocean basin
• Three basic tidal patterns occur along the Earth’s major
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shorelines.
In general, most areas have two high tides and two low tides
each day.
When the two highs and the two lows are about the same
height, the pattern is called a semidiurnal tide.
– If the high and low tides differ in height, the pattern is
called a mixed semidiurnal tide.
Some areas, such as the Gulf of Mexico, have only one high
and one low tide each day.
– This is called a diurnal tide.
– The U.S. West Coast tends to have mixed semidiurnal
tides, whereas a semidiurnal pattern is more typical of the
East Coast.
• http://www.onr.navy.mil/focus/ocean/motion/
tides1.htm
Diurnal Tides
• One high and one low tide a day
Monthly Tide Cycles
• The sun and the moon combined caused
monthly changes in tide patterns
Spring Tide
• Especially strong tides, have nothing to do with the
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“spring season”.
High tides are really high, and low tides are really
low
Occur during the full moon (when the sun, moon,
and earth are in a line); and the new moon.
Happens twice a month
Neap Tides
• Especially weak tides
• High and Low tides are very small
• Occurs at the first and last quarter moon; when the
gravitational forces of the Moon and the Sun are
perpendicular to one another ; we see a half moon
– The effects of the sun and moon partly cancel
out
Tidal Currents
• Caused by the horizontal flow of water
with the changing tides
• Currents run fastest halfway between the
high and low tide
• Currents slow down at the end of one
tide, and reverse direction at the
beginning of the other
What Affects Tidal Currents?
• Shape of coastlines
• Bottlenecks to flow (narrow channels)
• Underwater obstructions
Tidal Races
• Occur at bottle necks
• Produce very powerful currents
Whirlpools
• Where flowing water meets underwater
obstructions
• Causes spiraling, funnel-shaped
disturbances
Japan – new whirlpool popped up
on March 11, 2011
Old Sow (Deer Island, Canada)
• Largest whirlpool in the Western
Hemisphere (2nd only to Maelstrom)
• Formed when rising tide passes both sides
of Indian Island and takes a sharp right
turn around the tip of another island’s
point
The Naruto whirlpools (naruto
Strait, Japan)
Maelstrom (off the coast of
Norway) – Largest Whirlpool
Eddies
• Larger, flatter, circular currents
Rip Tides
• Areas of high turbulence caused when
multiple currents meet
EL NINO & LA NINA
• El Nino and La Nina are large climatic disturbances
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caused by abnormalities in the pattern of sea surface
temperature, ocean currents, and pressure systems.
They occur in the tropical parts of the Pacific Ocean.
El Nino and La Nina have important impacts for
weather patterns throughout the Pacific and beyond.
Most scientists regard El Nino and La Nina as extreme
phases of a complex global weather phenomenon
called the El Nino-Southern Oscillation (ENSO).
EL NINO
• Spanish term “el nino” means “the little boy”
or “Christ child”.
• El Nino events typically last from 12 to 18
months and occur cyclically, but somewhat
unpredictably.
• On average, they occur about 30 times per
century, with intervals that are sometimes as
short as two years and sometimes as long as
10 years.
• Their underlying cause is not understood.
EL NINO PATTERN
• The pressure systems that normally
develop in the Pacific, and the southeast
trade winds, weaken or reverse.
• The pool of warm surface water extends
from the western Pacific into the central
and eastern Pacific.
EL NINO EFFECTS
• An El Nino event causes wetter-than-normal conditions, and
floods, in countries on the western side of South America
(particularly Ecuador, Peru, and Bolivia).
– These conditions may also extend to the southeastern United States.
• In other parts of the world, it causes drier conditions.
– Drought and forest fires become more common in the western Pacific,
mainly in Indonesia and Australia, East Africa and northern Brazil.
• The warmer waters in the eastern Pacific cause a reduction in
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the Peru Current and reduced upwelling near the coast of
South America.
– This reduces the level of nutrients in the seawater, which
has a negative impact on fish stocks.
Other effects include a quieter Atlantic hurricane season and an
increase in the extent of sea ice around Antarctica.
– Japan, western Canada, and the western US experience more storms
and warmer weather than normal.
LA NINA
• Spanish for “the little girl.”
• Is the reverse of an El Nino event.
• It is characterized by unusually cold ocean temperatures in the
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eastern and central equatorial Pacific, and by stronger winds
and warmer seas to the north of Australia.
La Nina conditions sometimes follow closely on an El Nino. La
Nina causes increased rainfall in some world regions and
drought in others.
– India, Southeast Asia, and eastern Australia are drenched by rains, but
the southwestern US experiences higher temperatures and low rainfall.
Meanwhile, northwestern states of the US experience colder, snowier
winters.
• La Nina is also associated with an increase in Atlantic hurricane
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activity.
Overall, the effects of a La Nina event often tend to be strongest
during winters in the Northern Hemisphere.
HURRICANES & TYPHOONS
• Hurricanes and typhoons are names used in
different parts of the world for similar weather
phenomena.
• They are characterized by violent winds moving
in a circular pattern over the ocean, dense
bands of clouds, and rainfall.
• In the Atlantic, they are known as hurricanes.
• Those in the west Pacific are called typhoons.
• They start as a low-pressure systems over
warm oceans in the tropics, And occur mainly
in late summer (late June to early November in the
Gulf).
HURRICANES
• All hurricanes develop from the effects of the sun warming the
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surface of a broad area of ocean , and the air above it.
This heating causes masses of warm, moist air to rise, creating a
region of low pressure at the surface, and dense clouds above it.
The low pressure sucks in more air, which spirals to the center,
creating a circular wind system.
As it grows stronger, it is pushed westward by the prevailing
Trade Winds.
In the Atlantic, a storm attains hurricane status once its winds
exceed 74 mph.
Eventually, most of the sea storms move away from the equator
to the north in the northern hemisphere.
When one reaches land, it begins to lose energy because it is no
longer fed by warm water, and winds are slowed by features on
the land.
HURRICANE DEVELOPMENT
• A fully developed hurricane is about 185-370 miles wide and 6•
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9 miles high.
A region of low atmospheric pressure, called the eye,
develops in the middle of the hurricane.
Within the rest of the storm, winds spiral in a counterclockwise
direction in the northern hemisphere, and a clockwise direction
in the southern hemisphere (because of the Coriolis Effect)
Within An area surrounding the eye, called the eye wall, the
air spins upward, forming dense clouds.
The eye stays calm because the winds that spiral in never
reach the center.
Rain bands radiate out from the eye.
HURRICANE EFFECTS
• As it moves across the ocean, the low-
pressure eye of the storm sucks seawater up
into a mound.
• When it hits land, the water in this mound
surges over the coast in what is called as
storm surge.
– This ends up flooding homes, destroying roads and
bridges, and seriously eroding the coastline.
• The high winds topple unstable buildings and
uproot trees. Water movements can also
devastate coral reefs.