Oceanography set 3

What is a “rogue wave”?

A rare wave with a height greater than twice the significant wave height (Hs)

Where was the first documented rogue wave recorded?

At the Draupner Platform in the North Sea in 1995

What causes rogue waves?

Constructive interference between multiple waves that align their crests

What is the “significant wave height”?

The mean height of the largest one third of waves

What is wave height?

The vertical distance between a wave’s trough and crest

What does “Hs” stand for?

Significant wave height

What is the “90th percentile” in wave measurement?

The tail end of the distribution representing the largest waves

What happens during constructive interference?

Wave crests align, doubling the wave height

What happens during destructive interference?

Crests and troughs cancel each other, resulting in flat seas

What is mixed interference?

A mix of constructive and destructive interference that produces irregular wave heights and rhythms

Why do ocean waves come in a range of sizes?

Because mixed interference is common in the ocean

What was the height of the largest wave ever reported?

34 meters (112 feet)

Who reported the largest wave and when?

The crew of the USS Ramapo in 1933 during a Pacific typhoon

How did the USS Ramapo crew calculate wave height?

Through visual observation and geometry

What happens to a wave as it approaches shallow water?

Amplitude increases, wavelength and speed decrease, and wave height and steepness increase

When does a wave break?

When its steepness exceeds 1/7

How does speed relate to water depth in shallow waves?

Speed is proportional to the water depth

How does particle motion change from deep to shallow water?

It transitions from circular (deep) to elliptical (shallow) to nearly horizontal (very shallow)

What is a spilling breaker?

A wave that breaks over a gentle slope, dissipating energy gradually over a large area

What is a plunging breaker?

A wave that forms over a moderately sloping seafloor, ideal for surfers

What is a surging breaker?

A wave that occurs over a steep seafloor, dumping energy suddenly onto the beach

What causes a tsunami?

Vertical displacement of the seafloor during an underwater earthquake

Why is “tidal wave” an incorrect term for a tsunami?

Because tsunamis are not caused by tides but by seismic activity

What major tsunami occurred on December 26, 2004?

The M9.2 Sumatra earthquake, which killed about 228,000 people

What type of wave is a tsunami considered to be?

A shallow-water wave (because its wavelength is much longer than the water depth)

What is the formula for tsunami speed?

S=3.13rootd (where d = water depth in meters)

How fast can tsunamis travel in deep water?

Around 220 m/s (about 500 mph)

What happens to a tsunami as it approaches land?

Wavelength and speed decrease, while wave height and steepness increase

Why do tsunamis not “break” like normal waves?

Because they behave as shallow water waves with mostly horizontal motion instead of vertical crests

What are tides?

Predictable, rhythmic variations in sea level caused by gravitational and inertial forces

What type of wave are tides considered?

Extremely long-wavelength waves with a period of about 12 hours

What do the crest and trough of a tide correspond to?

Crest = high tide; Trough = low tide

How many tides does Boston experience each day?

Two high and two low tides per day (semidiurnal tides)

How much later does each high tide occur daily in Boston?

About 50 minutes later each day

What is the average tidal range in Boston?

About 10 feet (3 meters)

Are both daily high tides the same height?

No, they usually differ in height

What are tidal predictions based on?

A combination of tidal constituents, gravitational effects from the Earth, Moon, and Sun

What are diurnal, semidiurnal, and longer-period constituents?

• Diurnal: once per day

• Semidiurnal: twice per day

• Longer-period: weeks, months, or years

What affects the amplitude of tidal constituents at a given location?

The ocean basin’s geometry and coastline shape

Why are tides significant to the D-Day invasion of 1944?

The English Channel has large tides, the Allies timed the invasion just after low tide to avoid German traps

What mistake did the Germans make in predicting the Allies’ landing time?

They expected a high-tide landing

Why was tide prediction difficult at the time?

Location-specific predictions were hard to compute without modern computers

Who invented the first tide-predicting machine?

Lord Kelvin in 1871

How did Lord Kelvin’s tide machine work?

It used gears representing tidal constituents and produced tide height predictions when cranked

How was the machine updated for D-Day?

Expanded to 26 tidal constituents for greater precision in 1942

Which law explains the forces behind tides?

Newton’s Universal Law of Gravitation

What is the equation for gravitational force?

F=G(m1xm2)/r^2

In what work did Isaac Newton publish this law and propose the Equilibrium Tide Model?

Principia (1687)

What two forces cause tides according to the Equilibrium Tide Model?

The Moon’s gravitational attraction and the inertia of the Earth-Moon system

Why are there two tidal bulges on Earth?

One faces the Moon (gravitational pull), and the other is on the opposite side (due to inertia)

What is the barycenter?

The shared center of mass that both the Earth and Moon orbit around

What causes the bulge on the side opposite the Moon?

Water being “flung” away from the barycenter by centripetal force

What does Fg represent in tidal forces in Newton’s Model?

Gravitational force directed toward the Moon

What does Fr represent in Newton’s Model of tides?

Inertial (centrifugal) force directed away from the Moon

What is the tide-generating force in Newton’s model of tides?

The difference between Fg and Fr

Do tidal bulges form because water is lifted upward?

No, they form because water moves horizontally along Earth’s surface toward the bulges

Which has a stronger effect on tides: the Moon or the Sun?

The Moon, because it’s much closer to Earth

What is the main tidal constituent?

The lunar semidiurnal constituent

What secondary effect does the Sun create?

A smaller solar semidiurnal tide

What are spring tides?

Tides with the greatest range, when the Earth, Moon, and Sun are aligned (Full or New Moon)

What are neap tides?

Tides with the smallest range, when the Moon and Sun are at right angles to Earth

How often do spring and neap tides occur?

Twice each month

What type of tidal constituent do spring and neap tides represent?

A longer-period constituent (weeks)

Why does Earth experience semidiurnal tides?

Because it rotates through two tidal bulges each day

Why is a tidal day 24 hours and 50 minutes long?

The Moon’s orbit around Earth causes the bulges to move slightly ahead each day

Are neap tides caused by constructive or destructive interference?

Destructive interference

Are spring tides caused by destructive or constructive interference?

Constructive interference

What causes tides on Earth?

Gravitational interactions between the Earth, Moon, and Sun

What is the difference between a wave and a tide?

Waves move energy, not water; tides are large-scale movements of water driven by gravity

How long is a tidal day?

About 24 hours and 50 minutes

Why is a tidal day longer than a regular day?

Because the Moon moves slightly in its orbit each day, so Earth must rotate an extra 50 minutes to “catch up”

How long does it take the Moon to orbit the Earth?

About 30 days

What is a spring tide?

When the Moon and Sun line up (during new or full moon), creating the largest tidal range

What is a neap tide?

When the Moon and Sun are at right angles (first or third quarter), creating the smallest tidal range

During which moon phases do spring tides occur?

New moon and full moon

During which moon phases do neap tides occur?

First quarter and third quarter

What does the Equilibrium Tide Model assume?

That oceans cover the entire Earth and can move freely under the Moon’s gravity

How many tidal bulges exist according to the Equilibrium model?

Two: one facing the Moon and one on the opposite side

What causes the two tidal bulges?

One is caused by the Moon’s gravity, and the other by inertia as Earth moves

What is lunar declination?

The angle between the Moon’s orbit and Earth’s equator, it causes unequal tides

What is a limitation of the Equilibrium Model?

It doesn’t account for continents, coastlines, or ocean depth, which affect real tides

What does the Dynamic Tide Model include that the Equilibrium Model doesn’t?

The effects of continents, ocean basin shape, coastlines, and seafloor depth (bathymetry)

Why can’t tides move freely around Earth?

Because continents block the tidal bulge’s movement

What are amphidromic points?

Locations in the ocean where there is no tidal range (high and low tide occur at the same time)

What are cotidal lines?

Lines that connect places experiencing high tide and low tide at the same time

What do red and blue colors indicate on a global tidal range map?

Red = large tidal range, Blue = small tidal range

What are tidal currents?

Horizontal movements of water caused by rising and falling tides

What is a flood tide?

When water moves in toward the shore

What is an ebb tide?

When water moves out to sea

What is slack tide?

When there is no current, occurring at high or low tide

When is the flood current speed the fastest?

At mid-tide

How does depth affect current speed?

Deeper water = faster current

What are the main factors influencing tides?

The gravity of the Moon and Sun, the rotation of Earth, and the shape of ocean basins

How often do most coastal areas experience high and low tides each day?

Two high tides and two low tides every 24 hours and 50 minutes

Why are some high tides higher than others?

Because of lunar declination, which tilts the tidal bulges away from the equator

Why is the Dynamic Model more accurate than the Equilibrium Model?

Because it accounts for real-world geography and ocean dynamics

Why are marine sediments important?

They provide habitat for benthic organisms, contain economic natural resources (like oil, gas, coal, and polymetallic nodules), and serve as archives of Earth’s history for paleontology and paleoclimatology

What are lithogenic (clastic) sediments?

Small fragments of pre-existing rocks delivered to the ocean (dirt, mud, sand, etc.)

What are the main transport methods for lithogenic sediments?

Rivers, wind (Aeolian sediment), and ice (ice-rafted detritus)

What does “well-sorted sediment” mean?

Sediment where all grains are similar in size, usually due to wind transport