Ocean
Currents
Circulation
of ocean water has profound effects on organisms living in the open ocean and coastal embayments.
Within bays
and estuaries, tidal currents circulate and mix ocean and bay water transporting
larva from offshore to bays and nutrients from bays to ocean.
Currents
Waves break
along the shore at angles and the current flow parallel to the coast.
This is the longshore current or littoral drift
which is the movement of water within the breaker zone and strictly from
breaking waves.
Why
Where and How Rip Currents form
Why Rip
Currents Form
As
waves travel from deep to shallow water, they will break near the shoreline.
When waves break strongly in some locations and weakly in others, this can
cause circulation cells which are seen as rip currents: narrow, fast-moving
belts of water traveling offshore.
Why
Rip Currents are Dangerous
Rip currents are
the leading surf hazard for all beachgoers. They are particularly dangerous for
weak or non-swimmers. Rip current speeds are typically 1-2 feet per second.
However, speeds as high as 8 feet per second have been measured--this is faster
than an Olympic swimmer can sprint! Thus, rip currents can sweep even the
strongest swimmer out to sea.
Over 100 drownings due to rip currents occur every
year in the United States. More than 80% of water rescues on surf beaches
are due to rip currents.
Rip currents can occur at any surf beach with breaking waves, including the
Great Lakes.
When Rip
Currents Form
Rip currents
can be found on many surf beaches every day. Under most tide and sea conditions
the speeds are relatively slow. However, under certain wave, tide, and beach
profile conditions the speeds can quickly increase to become dangerous to
anyone entering the surf. The strength and speed of a rip current will likely
increase as wave height and wave period increase. They are most likely to be
dangerous during high surf conditions as the wave height and wave period
increase.
Where Rip
Currents Form
Rip currents
most typically form at low spots or breaks in sandbars, and also near
structures such as groins, jetties and piers. Rip currents can be very narrow
or extend in widths to hundreds of yards. The seaward pull of rip currents
varies: sometimes the rip current ends just beyond the line of breaking waves,
but sometimes rip currents continue to push hundreds of yards offshore.
How to
Identify Rip Currents
Look
for any of these clues:
a
channel of churning, choppy water
an
area having a notable difference in water color
a
line of foam, seaweed, or debris moving steadily seaward
a
break in the incoming wave pattern
None,
one, or more of the above clues may indicate the presence of rip currents. Rip
currents are often not readily or easily identifiable to the average beachgoer.
For your safety, be aware of this major surf zone hazard. Polarized sunglasses
make it easier to see the rip current clues provided above
How to Avoid and Survive Rip
Currents
Learn
how to swim!
Never swim
alone.
Be cautious
at all times, especially when swimming at unguarded beaches. If in doubt, dont
go out!
Whenever
possible, swim at a lifeguard protected beach.
Obey all
instructions and orders from lifeguards.
If caught in
a rip current, remain calm to conserve energy and think clearly.
Dont fight
the current. Swim out of the current in a direction following the shoreline.
When out of the current, swim towards shore.
If you are
unable to swim out of the rip current, float or calmly tread water. When out of
the current, swim towards shore.
If you are
still unable to reach shore, draw attention to yourself: face the shore,
wave your arms, and yell for help.
If you see
someone in trouble, get help from a lifeguard. If a lifeguard is not available,
have someone call 9-1-1 . Throw the rip current victim
something that floats and yell instructions on how to escape.
Remember,
many people drown while trying to save someone else from a rip current.
Rip Current
Myth
A rip current
is a horizontal current. Rip currents do not pull people under the water-they
pull people away from shore. Drowning deaths occur when people pulled offshore
are unable to keep themselves afloat and swim to shore. This may be due to any
combination of fear, panic, exhaustion, or lack of swimming skills.
In some
regions rip currents are referred to by other, incorrect terms such as rip
tides and undertow. We encourage exclusive use of the correct term rip
currents. Use of other terms may confuse people and negatively impact public
education efforts.
Rip
Currents vs. Rip Tides
Warning sign posted adjacent to tidal inlet in South Carolina. Photo courtesy University of
Delaware Sea Grant College Program
Rip currents
are not rip tides. A specific type of current associated with tides may include
both the ebb and flood tidal currents that are caused by egress and ingress of
the tide through inlets and the mouths of estuaries, embayments,
and harbors. These currents may cause drowning deaths, but these tidal currents
or tidal jets are separate and distinct phenomena from rip currents.
Recommended terms for these phenomena include ebb jet , flood jet , or tidal
jet .
What
is Undertow?
Undertow, an
often misunderstood term, refers to the backwash of a wave along the sandy
bottom. After a wave breaks and runs up the beach face, some of the water
percolates into the sand, but much of it flows back down the beach face
creating a thin layer of offshore-moving water with a relatively high velocity.
This backwash can trip small children and carry them seaward. However, the next
incoming wave causes higher landward velocities, pushing them back up on the
beach.
Undertow does
not pull you under water or out to sea.
More:
Currents
Surface and
wind driven currents carry plankton.
The thermohaline/deep sea circulation brings O2 to abyss and disperses
eggs and young of deep sea creatures and transports heat.
Currents
Deep sea
circulation is driven by temperature and density deficiencies within the ocean.
These are
called thermohaline currents.
As the water
cools it becomes more saline..some
water form ice crystals so salt is left behind.
Currents
Water is
more salty, higher density, sinks to a denser layer, pushes
out polar water toward equator, horizontally moving deep water masses are
deflected by the coriolis bending.
Currents
These currents
help deliver dissolved O2 to the bottom communities where there is no
circulation.
Bottom water
that is devoid of O2 is called anoxic/deoxygenated water.
Currents
The suns
irregular heating patterns of the atmosphere creates
regular wind patterns around the world.
Frictional
drag of prevailing wind along the ocean surface waters causes them to move.
(friction decreases
with depth)
Currents
Coriolis
effect-wind and water are deflected to the right in the northern hemisphere (left
in Southern Hemisphere) due to the earths rotation and
causes water to rotate slowly in circular rivers or gyres.
Coriolis Effects
In the North Pacific the four major,
clockwise currents:
the
North Pacific current flowing west to east
the California
current flowing south, down the west coast of
North
America
the
North Equatorial current flowing east to west
the
Kuroshio current flowing north, up the east coast of Japan.
Coriolis Effects
The Kuroshio
Current is a clockwise, circular flow called the North Pacific gyre. The Alaska
current receives water from the North Pacific current moving in a
counterclockwise gyre in the Gulf of Alaska.
Coriolis Effects
Results:
move warm southern
waters north causing: warm rains and thunderstorms
move
cold northern waters south, producing snowstorms and blizzards.
Currents
Ekman
spiral is formed when wind driving surface currents and the coriolis effect creates
additional deflection of water 100-200m below.
Currents
Ekman
spiral is formed when wind driving surface currents and the coriolis effect creates
additional deflection of water 100-200m below.
Currents
Each
succeeding layer is deflected further to the right.
In some
cases this spiral is responsible for a phenomenon known as UPWELLING.
Currents
Movements of surface water away from the shore enables cold,
dense bottom water to rise and mix with warmer surface water (UPWELLING).
Currents
UPWELLING
carries significant amounts of dissolved minerals to the oceans upper sunlit
limit supplying fertilizer to plants near the surface...
That will
grow food for the fish.
When upwelling stops, the numerous fish disappear (El Nino).
El Niρo
The periodic development of warm ocean
water in the Pacific along the coast of South America, usually during the
winter months
ENSO- El Niρo Southern Oscillation is the
cycling of a Pacific Ocean pattern
During El Niρo- the trade winds die down
in the central and western Pacific leading to a depression of the thermocline in the eastern Pacific, and an elevation of the
thermocline in the west.
El Niρo
Waters warmed
from the western Pacific drift eastward toward the Americas
La Nina is just the opposite and
correlates with stronger than usual trade winds.
Creates
colder surface temperatures in the eastern Pacific.
ENSO & Hawaii
During an El Nino
event, the air and ocean surface temperatures rise, and Hawaii tends to
experience drought like conditions
Hawaii tends to
experience a higher frequency of hurricanes during El Nino events
During a La Nina,
Hawaiis weather is generally cold & wet
WAVES
lWaves are of
practical significance to us...they may swamp a small boat,
lsmash
supertankers,
ldamage
offshore structures,
lforce
commercial vessels to slow their speed,
ldamage
shore structures,
WAVES
lmake
students skip school when they want to surf and
ldetermine
what adaptations organisms along the shoreline must have in order to exist
there.
WAVES
lWaves are
mechanical energy that has been transferred from wind, earthquakes, landslides,
or other waves to the ocean water.
lMost of the
transfer is by wind and waves travel outward from the energy source. As more
energy is supplied, waves become larger.
WAVES
lProperties of
waves
l3 factors
that determine size of wind generated waves.
l1. time of contact
l2. velocity of wind
l3.
fetch-distance over which wind is in contact with water.
WAVES
lAny one of these factors can limit the wave height.
lIf the wind speed is low, it doesn't matter how far and how
long the wind blows over the water, no large waves will be produced.
WAVES
lIf the wind speed is great but short, again, no large waves
will be formed and strong wind over a short area will also not produce large
waves.
lWhen no single limiting factor is present, large waves can
form at sea. (40-50' S).
WAVES
lA typical fetch for a local storm is about 500 miles and with
the storm moving,
land the storm winds circulation around the low pressure area,
WAVES
lThe winds can continue to follow the waves on the side of the
storm which increases fetch and duration of time over which the wind can add
energy to the waves.
lWaves up to 49 feet are not uncommon and the wave lengths can
be between 330-600 feet.
WAVES
lGiant waves over 100' are rare but a Navy Tanker (USS Ramapo)in 1933 encountered a Typhoon and riding on the downside
to ease the ride, was overtaken by waves that when measured against the ships
superstructure by the officer on watch were 112'high.
WAVES
lThe period was 14.8 seconds and the wave speed was 90'/sec (60
mph). While conditions to produce waves of this size occur, none have been well
documented (or have survived).
WAVES
lPARTS OF THE WAVE
lThe portion of the wave that is elevated above the undisturbed
sea surface is the crest.
lThe portion that is depressed below the surface is the trough.
WAVES
lThe distance between two successive crests or two successive
troughs is the length of the wave or wavelength,
land the height of
the wave is the vertical distance from the top of the crest to the bottom of
the trough.
WAVES
lThe amplitude is equal to 1/2 the wave height or the
distance from the crest or trough to the still water or equilibrium surface.
lThe period is the time required for two successive
crests (or troughs) to pass a point in space.
WAVES
lThe relationship between wavelength and wave
periods allow math approximations to be made giving more insight to the
behavior and properties of waves.
WAVES
lWAVE MOTION
lThe particles of water get set into motion when a wave passes
across the water surface.
lThe ocean wave does not represent a flow of water but a flow
of motion or energy from its origin to its eventual dissipation at sea or loss
against land.
WAVES
lAs a wave crest approaches, the surface water particles rise
and move forward.
lImmediately under the crest the particles have stopped rising
and are moving forward at the speed of the crest..
WAVES
lWhen the crest passes, the particles begin to fall and to slow
their forward motion.
lIt reaches a maximum falling speed and zero forward speed when
the midpoint between crest and trough passes.
WAVES
lAs the trough advances, particles slow in falling rate and
start to move backward until at the bottom of the trough they reach the maximum
backward speed and neither rise or fall.
WAVES
lAs the remainder of the trough passes, the water particles
begin to slow their backward speed and start to rise again, until the mid-point
between the crest and trough passes.
WAVES
lNow they start their forward motion and continue to rise with
the advancing crest.
lThe motion creates a circular path or orbit for the
water particles.
lThis is the motion that causes a boat to bob!
WAVES
lThe surface water particles trace an orbit whose diameter is
equal to the wave height.
lThe same motion is transferred to the water particles below
but less energy of motion is found at each succeeding depth.
lThe diameter of the orbits decrease and become smaller and
smaller as the depth increases.
WAVES
lAt a depth of 1/2 the wavelength, the orbital motion has
decreased to almost zero.
WAVES
lWAVE SPEED
lIt is possible to relate the wavelength and period of the wave
in order to determine the wave's speed.
lThe speed of the wave (C) is equal to the length of the wave
(L) divided by the period (T):
lSpeed = length/period or C=L/T
WAVES
lParticles in
the ocean are set into an elliptical motion as wind energy acts on water.
lThe
energy of the particles move (is transferred) through the ocean, not the
particles.
lTheir
movement makes the waves shape.
WAVES
lThe vertical height-from the top of the crest to bottom of
trough is the wave height.
The time between successive crests/troughs passing a fixed
point is the period of a wave.
WAVES
lSharp peaks are called seas and as waves move out of their
area.
lThe crests become rounded forming a swell, a long, low
wave that can travel thousands of miles.
WAVES
lAs the wave
approaches the shallow water, it changes shape.
lthe
wave length decreases.
lthe
height increases as particles encounter resistance from the bottom.
WAVES
lThe pathway
of the particles become more elliptical as it gets closer to the coastline.
l ..bottom resistance
slows the waves.
l..shortens wave length when depth is 1/2 wavelength.
WAVES
lWhen depth
decreases less than 1/2 wavelength (or 1.3x height)
l the frictional drag
along the bottom
land
forward motion of the wave and steepness of the crest causes the wave to break
or collapse against the shore.
WAVES
lStored energy
is released as the water falls against the shore.
WAVES
lBREAKERS
lBreakers are formed in the surf zone because the water
particle motion at-depth is affected by the bottom, slowed down, and compressed
vertically. The orbit speed of the particles near the crest
are not slowed too much so
.
WAVES
lparticles move faster toward the shore than the
wave itself.
lThe crest can curl and eventually break (fall over).
lThere are two types, plungers and spillers.
WAVES
lPlunging breakers are usually found on a steep beach, the
curling crest outruns the rest of the wave curves over the air below it and
breaks with a sudden loss of energy and a splash.
lSpilling waves occur at flatter beaches and consists of
turbulent water and bubbles flowing down the collapsing wave face.
WAVES
lMarine
organisms along the ocean are affected by wave actions.
lSandy and
rocky shores, exposed to the direct assault of strong waves are known as a high
energy environment, as opposed to beaches in protected estuaries, bays
and lagoons which is a low energy environment.
WAVES
lWinter
usually has higher crests and shorter wave lengths than summer thus release
more energy on the shore.
WAVES
lSTORM CENTERS
lMost waves at sea are progressive wind waves.
lThey are build up by the wind,
restored by gravity and travel in a particular direction.
lThese waves are formed in local active storm centers or by
steady winds of the trade wind and westerly wind belts.
WAVES
lAn active storm may be large, with unsteady winds and varying
directions and strength.
lThe winds in the storm flow in a circular pattern around the
low-pressure storm center creating waves that move outward and away from the
storm in all directions.
WAVES
lIn the storm center, the sea surface is jumbled with waves of
all heights, lengths and periods.
lThere are no regular patterns.
lSailors call this a sea.
lAs waves are being generated, they are forced to get larger by
the input of energy forced waves.
WAVES
lDue to variations in the winds of the storm area, energy at
different intensities is transferred to the sea surface at different rates,
resulting in waves with a variety of periods and heights.
WAVES
lOnce a wave is created with its period, the period doesn't
change.
lThe speed may change but the period remains the same.
lThe period is a constant property of the wave until the wave
is lost by breaking at sea, through friction, or crashing against the shore.
WAVES
lWATER TRANSPORT
lWaves transport water toward the beach in the surf zone.
lThere is a drift of water in the direction the waves are
traveling and is intensified in the surf zone and with the waves approaching
the beach at an angle, the transport of water moves both toward and along the
beach.
WAVES
lThis water must flow seaward again and will in a quieter zone
with smaller waves.
lBecause these regions may be some distance apart, and narrow,
the water may flow out quickly forming a
rip
current.
WAVES
lREFRACTION
lWaves usually approach the shore at an angle and when one end
of the crest comes in and feels the bottom and the other end is still in deeper
water, the shallow water end slows and because the deep water part is still
traveling the same speed, the wave crests bend, or refract.
WAVES
lDIFFRACTION
lWhen a wave passes its energy though a narrow opening, some
wave energy will pass through to the other side and once through the energy
radiates out and away from the gap.
WAVES
lREFLECTION
lA steep, vertical barrier in water deep
enough to prevent waves from breaking will reflect the waves..
WAVES
lSTORM SURGES
lPeriods of excessive high water due to changes in the
atmospheric pressure and the wind's action on the sea surface are called storm surges or storm tides.
lThese are not typical waves but share characteristics of
curving sea surfaces and produce like effects to that of tsunamis.
WAVES
lSWELL
lOnce energy or generating forces no longer effect
the waves, the forced waves become free waves moving at speeds due to
their periods and wavelengths.
lSome waves produced have long wavelengths and long periods and
have a greater speed than those with short.
WAVES
lThey gradually move through and ahead of the slower ones and
escape the storm and appear as a regular pattern of undulating crests and
troughs moving across the sea surface.
lOnce away from the storm these waves are called swell.
WAVES
lThey carry considerable energy which they lose very slowly..
lThe movement of the faster through and ahead of the slower
waves is called sorting or dispersion.
lGroups of these faster waves move as wave trains or packets of
similar waves with about the same period and speed (sets).
WAVES
lTSUNAMI
lEarthquakes are often responsible for producing seismic sea
waves or tsunamis. They are called
tidal waves (incorrectly) and formed if in an area the earths crust suddenly
raised or lowered.
WAVES
lEnergy is transferred
to water as the coastal plates shift.
lTravel
through the sea at 100+ mph with a wave length of 100miles when reaching the
coast, wave lengths shorten and heights can increase to 100'.
WAVES
lThe displacement causes a sudden rise or drop of the sea level
and gravity causes the water to quickly fill it in. Waves with long wave
lengths are produced (100-200km) and periods of 10-20 min
WAVES
lHOW A WAVE
FORMS
lTo create a
wave, a generating force is required.
lThis is a
result of a pulse of energy which produces waves (throw a stone etc.)
lThe waves
produced by the generating force moves away from the point of disturbance.
WAVES
lWhen the rock
hits the surface, it disturbs and displaces the water surface.
lAs the rock
sinks, the displaced water flows back into the space from all sides and its
momentum forces it upward resulting in a higher surface.
WAVES
lThe elevated
water falls back causing a depression below the surface, which is filled and
lin
turn sets up a series of waves that move outward and away from the point of
disturbance
luntil
they are dissipated through friction among the water molecules.
WAVES
lThe force
that causes water to return to the level of undisturbed surface is the restoring
force.
lThis has to
do with the surface tension
(elastic quality of the surface due to the cohesive behavior of the water
molecules.)
lThis
affects smaller waves, but larger waves are pulled back by the force of gravity
and are called gravity waves.
WAVES
lThe most
common generating force for water waves is moving air or wind.
WAVES
lAs the wind
blows across a smooth water surface, the friction or drag between air and water
tends to stretch the surface resulting in wrinkles
l. Surface tension acts to restore a
smooth surface.
lThe wind and surface
tension create small waves called ripples or capillary waves.
WAVES
lYou can see
these as wind moves over a smooth surface of a pond or lake, and are called cat's paws as they move across the
surface keeping pace with the wind.
WAVES
lAs the wind blows,
energy is transferred to the water over large areas, for varying lengths of
time, and at different intensities.
lAs waves
form, the surface becomes rougher, and its easier for the wind to grip the
roughened water surface and add energy .
WAVES
lAs the wind
energy is increased, the oscillations of the water surface
becomes larger and the restoring force changes from surface tension to
gravity.
lA wave is a
result of the interaction between a generating
force and a restoring force.
WAVES
lGenerating
forces include any occurrence that adds energy to the sea surface:
lwind,
llandslide,
lsea-bottom
faulting or slipping,
lmoving
ships,
land
even thrown objects.
WAVES
lEPISODIC WAVES
lLarge waves that suddenly appear at sea unrelated to local
conditions are called epsodic waves.
lIt occurs due to the combination of intersecting wave trains,
depths and currents.
WAVES
lNot much is known and when they do occur and swamp ships,
witnesses are often removed.
lThey occur near the continental shelf in water about 600' deep
and in some areas with prevailing wind, wave and current patterns
WAVES
lThere is a maximum height for any given wavelength.
lThis value is determined by the ratio of the wave's height to
the wave's length and is the measure of steepness of the wave:
lSteepness = height/length or S=H/L
WAVES
lIf the height to length ratio exceeds 1:7, the wave is too
steep, the crest angle will be sharper than 120' and the wave is unstable and
will break.
lA wave length of 70m will cause a wave to break when it
exceeds 10m (1:7).
WAVES
lWhitecaps have very short wavelengths(about
1m) and break because the wind increases their height rapidly...quicker than
the wavelength increases.
lAlso when wave trains pass through each other, the quick
increase in height can cause these waves to break (even in the middle of the
ocean).
The rhythmic rise and fall of the oceans
water at a fixed location is known as the tide.
TIDES
Tides
Tides are
long waves moving through the ocean.
When the
crest of the moving tide reaches a location, high tide occurs.
Low tide is
when the trough reaches the location.
Tides
Best known as
the rise and fall of these around the edge of land, the tides are caused by the
gravitational attraction between
the
earth and the sun and between the earth and the moon.
While tides
go unnoticed far out at sea, they are easily observed along the shoreline.
Tides
Three types
of tides occur.
1.
Semi-diurnal 2 high and low tides per day about equal range. Most.EC
2. Diurnal 1
high and 1 low tide per day (24hrs) Gulf of Mexico/Vietnam/Manila
3. Mixed 2
high and 2 low tides per day but different ranges...1 high high,
1 low low 1 high 1 low west coast
Tides
Tides behave
differently in different places.
In some
coastal areas there is a regular pattern of one high and one low tide each day
known as a diurnal tide.
Tides
In other
areas there is a high-low water sequence repeated twice a day...semidiurnal tide and these tides usually reach about the same level at
high and low tides each day.
Tides
The third
type of tide has two high and low tides a day but the tides reach different
high and low levels during a daily rhythm.
Tides
This is
called a semidiurnal mixed tide.
It is caused by
a diurnal (daily) inequality by combining a semidiurnal and diurnal tide.
Tides
In the uniform tidal system (semi and diurnal)
the greatest height to which the tide rises on any day is known as high water
and the lowest point is low water.
In a mixed system it refers to higher high
and lower high water and higher low and lower low waters.
Tides
Tidal
observations made over a period of time are used to calculate the average or
mean tide levels.
Since the
depth of coastal waters is important for navigation, an average low-water
reference is established:
depths
are measured from this level and recorded on navigational charts
Tides
This area is
usually established at mean low water
and the zero reference or tidal datum, is established at this point.
In mixed
tidal areas, mean lower low water is
used as the tidal datum.
Sometimes the
low tide level may fall below the mean value used as the tidal datum producing
a minus tide .
TIDAL CURRENTS
When the tide
is rising, it is called a flooding or flood tide and
when
its falling it is an ebbing tide.
Currents
associated with the rising and falling of the tide in coastal waters is a tidal
current.
TIDAL CURRENTS
They can be
very swift and dangerous.
When the tide
turns, reverse direction, there is a period of slack water during which
the tidal currents slow and then reverse.
MOON TIDE
Water
particles on the side of the earth facing the moon are closest to the moon and
are acted on by the larger moon gravitational force.
Because water
is liquid and deformable,the
force is applied to water particles toward a point directly under the moon.
Tides
It produces a
bulge in the water covering.
At the same
time, the centrifugal force of the earth moon system acting on the water
particles at the earths surface opposite the moon creates a bulge too .
Tides
TIDAL
DAY
The moon and
earth are moving in the same direction along its orbit during a 24 hr period
but the earth must turn an extra 12' and 50 minutes for the moon to be directly
over the same place.
Tides
Therefore the
tidal day is not 24 hours long but 24 hr and 50 minutes explaining why
tides arrive at a location about an hour later each day
Tides
The
tide wave
The
crest of the wave is high water
(tide) and the trough is low water (tide).
The
wavelength of this wave is 1/2 the circumference of the earth and period 12 hr
and 25 min.
Tides
The
SUN TIDE
While the moon plays a greater role in
tide producing, the sun also produces its own tide wave.
Though it is large, the long distance
away means its tide raising force is only 46% that of the moon and the average
time is 24 hours not 24h50m.
Tides
The tide wave
produced by the moon is greater magnitude and continually moves eastward
relative to tide wave produced by the sun and
therefore
the tide forces produced by the moon are greater and more important than that
of the sun (therefore tidal day 24h50m).
Tides
SPRING
AND NEAP TIDES
During
the 29 1/2 days it takes the moon to orbit the earth, the sun, the earth and
the moon move in and out of phase with each other.
Tides
During the
period of the new moon, the moon and sun are lined up on the same side of the
earth so that the high tides, or bulges, produced independently of each,
coincide.
Since the water
level is the result of adding the two wave forms together..
Tides
tides
of maximum height and greatest depression, or tides with the greatest range
between high and low water are produced.
These
are spring tides.
Tides
In a weeks
time, the moon is in its 1st quarter and moved about 12' per day, until its at a right angle to the sun wave.
Now the
crests of the moon tide will coincide with the troughs produced by the sun and
the same is true of the sun's crests and moons troughs.
Tides
They tend to cancel each other out and
the range between high and low water is small.
These are low-amplititude
neap tides.
Tides
At the end of another week, the moon is
full and the sun, moon and earth are again lined up, producing crests that
coincide and tides with the greatest range between
high and low waters, or spring tides.
Tides
These are
followed by neap tides seven days ;
later
and the cycle, 4 weeks, continues with a spring tide every two weeks...etc.
This 4 week
progression creates a tidal cycle of changing tidal amplitude.
Tides
DECLINATION
TIDES
If
the earth and moon are aligned so that the moon stands north or south of the
earth's equator, one bulge is in the Northern Hemisphere and one in the
Southern Hemisphere.
A
point in the middle latitudes passes through only 1 crest and one trough during
the tidal day.
Tides
This
type of diurnal tide is called a declination tide, because the moon is said to
have declination when it stands above or below the equator.
Tides
The
sun also influences these tides as it sits over 23.5N/S at summer and winter
solstice and the variation causes the bulge created by the sun to oscillate
north/south (making a more diurnal sun tide during winter and summer.
Tides
The moons declination is at 28.5N/S and because
the orbit is inclined 5' to the earth sun orbit, it takes 18.6 yrs for the moon
to complete its cycle of maximum declination.
Tides
When the sun and moon coincide, both tide
waves become more diurnal.
Also the moon doesn't move around the
Earth in a perfectly circular orbit or even does the
earth circle the sun at a constant distance.
Tides
TIDE
TABLES
Because of
all the combinations, its not possible to predict the
earths tides from our knowledge of tide raising bodies alone.
But with a
combination of actual local measurements with known astronomical data, tide
predictions are quite accurate.
Tides
Water level
recorders are installed at coastal sites and the rise and fall are measured
over a period of years..
.At least 19
years are needed to allow for the long 18.6 year period of declination of the
moon.
Tide Tables
are published annually by NOAA.
Tides
The tables
give the dates, times, and water levels for high and low water at a primary
tide station.
There are
only 196 of these but consulting a list for other 6000 auxiliary stations
applying corrections to the times and heights of the primary stations helps
more localities get accurate tide predictions.
Tides
Tidal
predictions are based on recorded high measurement from past records which are
used in the future.
Tide Current Tables
Like tides,
these currents are measured at primary locations and data is published similar
to tide tables.
Useful
for moving in and out of estuaries.
Tides
Summary
Without land, 2 bulges
, 1 on side of the earth closest to the moon, 1 on the other side would
appear.
Bulge is high tide from mutual attraction
between earth and moon.
Moon gravitational attraction pulls and
moves the water.
Tides
The opposite
bulge, centrifugal force created as the earth and moon revolve around a common
point...barycenter-4670 km (2900miles) from earths
center.
Tides occur
at different times because the moon goes around the earth in 24h50m or 1 lunar
day. (freq 12h25m)
Tides
Due
to friction between earth and tidal bulges, High tide is given about 50 minutes
after the moon passes over that point. known as a
lunar semidiurnal tide.
Tides
Vertical
height ...high to low water is the tidal range and varies day to day because of
the sun and the moon.
Solar tides are
1/2 the size of lunar tides.
When the sun
and moon line up (2x per month) tides are higher and lower than usual...spring
tide.
Tides
When the moon
is at right angles to the sun, there is less gravitational effect that lessens
the tidal range (neap tides)
Tides
Between these tides the height varies
throughout the month.
Differences in tidal frequencies and
range occur because of the shape of the ocean basins and coastline (Hawaii is a
few cm while the Bay of Fundy is 20m.)
Review Even
Answers
Marine Bio HW#3
2. surface from wind/deep sea from density /temperature
differences
4.
left
6. fetch/wind
speed/wind duration
8. low and high energy environments-shallow offshore and steep
offshore
10. bottom
topography, alignment of moon and sun
12. after
moon pases your area, crest of tide wave moves over
and you experience high tide..trough=low tide///diurnal/semi-diurnal-mixed
Review Questions
2. gulf stream current
4. weakening of trade winds t
6. ekman spiral
8. about 45' to the right
10. ,,
they move so slowly,
12. Coriolis
effect
14. disturbance / energy / medium 14 breaking waves returning to the sea
16.
sea
18. 112
20. deeper
than half of its length
22. fully-developed
sea
24. bottom-oriented
pressure sensors
26. a fast-moving flood of water
28. around
the Pacific Rim
30. coastal estuarine sedimentation, pollutant transport , invertebrate reproduction
32. TWICE
34. TIDAL
CURRENT
36. associated with river inlets, considered to be a true tidal
wave ,exposed to great tidal fluctuation
38. TIDAL
RANGE
Reading---read
it!
