Oceans - Exam 2

Disrupting Wave Forces

Wind - Disrupting force creates the waves

Restoring Wave Forces

Gravity for wind waves, and Surface Tension for capillary waves.

Wave Displacement

Disturbance of the water surface does not give rise to a permanent displacement. Individual water particles are displaced from and return to equilibrium position as each wave passes.

Factors of Wave Generation

Wind Speed - Must be faster than speed of wave crests

Wind Duration - Longer periods of time will lead to larger waves

Fetch - The uninterrupted distance waves go without changing direction

Sea

Water surface fluctuations that occur within the region of wave generation, where water motions are irregular and appear disorganized. 

Swell

A type of wave. Covering a broad region of wave propagation, swell waves are organized, long period (lower frequency).

Swell waves are able to travel thousands of miles without changing period or height.

Surf

Region adjacent to the shoreline where waves shoal, transform, and release energy in the process of breaking and run-up on the shoreline.

Swell Dispersion

Swell (long period, organized waves) are generated by distant winds and their organization is partly due to the distance the waves travel across the ocean. Swell dispersion occurs because of the relationship between wavelength and wave speed in deep water.

Deep Water vs. Shallow Water

Deep Water - H > L/2. C=L/T. Speed depends on wavelength. L=gT²/ 2pi

Shallow Water - Speed depends on depth. C = square root of gh.

L/2

This is the point at which a wave will “feel” the seafloor.

(L)

Wavelength

(T)

Wave Period

(H)

Wave Height

(C)

Celerity. Wave Speed

(a)

Wave Amplitude

(H) / (L)

Wave Steepness

(h) or (d)

Water deepness

1 / (T)

Wave Frequency

Wave Orbital Motion

• Individual particles move in circular patterns in deep water 

• The size of the circles decreasing exponentially with water depth until L/2

• The wave form propagates through the medium (fluid/water)

• Bottom of wave orbital motion is also called wave base

When waves approach the shore…

• Waves slow down

• Wave Period (T) does not change 

• Waves closer together (wavelength decreases)

• Wave height increases

• Wave breaks

Wave Refraction

• Begins when shoaling begins

• energy is distributed equally along the length of the wave crest

• Waves lose little energy as they travel in deep water 

• Wave is traveling at equal speed (C=L/T)

• wave rays (red) are distributed equally along wave crest

Wave Refractions - Headlands

Wave crests bend and converge into more shallow areas (such as headlands). Reduces the length of the wave crest and concentrates the total energy = more energy per length of wave 

Expend energy at the shoreline.

Converging wave crests = more energy  

Wave Refractions - Bays

Wave crests are stretched along broader areas (bays) – increase the length of the wave crest and reduces the energy per length.

Expend energy at the shoreline. 

Stretched wave crests = less energy 

Longshore Currents

• Waves typically approach at angle to beach due to refraction and regional wind climate. 

• Waves break and move sediment up the beach at an angle.

• Backwash returns to the sea perpendicular to beach 

• Creates a zig-zag pattern of swash and backwash 

• Longshore drift gradually moves sediment along shoreline in the direction of wave breaking 

• Builds and erodes beaches and other coastal landforms

Wave Diffraction

When part of the wave is blocked by a solid object and the remaining part of the wave spreads out after passing the object. 

Tide Causes

Relationship between the mass of the moon or sun as well as their distance from Earth

Gravitational Force Equation

F = G x (M1 x M2)/D²

(F) = Force

(G) = Gravity constant 6.673 × 10^-11 m x kg^-1 x s

(M1 and M2) = Mass of the two objects

(D) = Distance

The sun exhibits 0.5 times the pull of the moon, this is because the sun is farther away than the moon.

Equal Tidal Bulges

On the side of Earth, opposite from the position of the moon, the centripetal force (away from moon) > gravitational force (toward moon), and creates a second bulge of water. 

Factors Affecting Tidal Properties

The sun

The moon does not traverse exactly along the equator

Tidal Day

24 hours and 49 minutes.

There is an extra 49 minutes because the moon rotates slower than the earth (1 month to 1 day).

Amphidromic System

• Water particles move in a flattened elliptical orbit (shallow water wave) 

• When tides are positioned in a basin with the correct configuration & dimensions, tides can form a standing wave with a node (central point; tide = 0) and antinodes (high tide) 

• Coriolis causes the wave to be deflected, creating a rotary motion around the central node (the amphidromic point) 

Tidal Ranges

Macro-tidal:  > 4 m 

Meso-tidal: 2- 4 m 

Micro-tidal: < 2 m 

Tidal Classifications

Diurnal - 1 High and 1 Low Tide every tidal day

Period: 24 hrs 49 mins  

Semi-diurnal - 2 High and 2 Low Tides every tidal day with ~equal heights

Period: 12 hrs 24.5 mins

Mixed - 2 High and 2 Low TIdes - the elevation of the high and lows in each tidal day are different 

Most common tide type 

Spring Tides

Occur twice a lunar cycle (new and full moon) when the gravitational force of the sun combines with the moon creating larger than normal tidal range.

Highest high tides and lowest low tides 

Neap Tides

Occur twice in a lunar cycle when at first and third quarters of the moon when the gravitational force of the sun opposes the moon creating a reduced tidal range.

Dampened tidal range 

Tides at the Coast

• very shallow water, 

• constrictions at inlets and mouths of estuaries, 

• mixing of fresh water, 

• friction at seabed

Tidal Prism

The volume of water in an estuary or bay between mean high tide and mean low tide. The volume of water leaving an estuary at ebb tide.

Flood Tides

The tidal current flows into harbors, bays, and estuaries from the sea 

Ebb Tides

The tidal current flows out of harbors, estuaries, bays to sea 

Slack Tides

Occur during the transition between incoming high and outgoing low tides, when there is no net water movement.

Necessary Conditions for Hurricane Formation

Tropical Wave or Disturbance

Conditional Instability (Cold and Warm air)

Warm Sea Surface Temp (>26.5C)

Little to Zero Vertical Wind Sheer

Displacement by at least 5 degrees from the equator

Hurricanes

A warm core (or center) low pressure system, that develops over tropical or subtropical waters, and has circular wind patterns around a well-defined center (eye)

Hurricane Eye

Dry, descending air. Calmer winds, little overhead cloud cover

Hurricane Eye Wall

• Warm air rises violently 

• Strongest winds are located immediately outside of the eye wall

• Strong thunderstorms and tornadoes

Hurricane Rainbands

• Heavy rain, tornadoes

• Gaps between the spiraling bands 

Why do hurricanes spin?

• Air flows toward the low pressure center

• Air gets deflected to the right in the northern hemisphere due to the coriolis effect 

• Results in counterclockwise spin of cyclonic storms in NH. 

• Southern hemisphere cyclonic storms spin clockwise. 

Where are hurricanes most likely to form?

Atlantic Basin

When is hurricane season?

June - November

Saffir-Simpson Scale

Grades hurricanes based on maximum sustained wind speed. 1-5

Future of Hurricanes

• More frequent 

• More Intense 

• New research indicates that intensification is happening more rapidly 

Storm Surge

The rise of sea level produced by water being pushed toward shore by the force of winds swirling around the storm. Measured as the height of the water above the normal predicted astronomical tide. 

Storm Tide

The total observed seawater level during a storm, resulting from the combination of storm surge and the astronomical tide. As a result, the highest storm tides are often observed during storms that coincide with a new or full moon.  **Wind driven waves are additional to the storm tide.

Storm Surge Factors

- Intensity 

Stronger storm = More storm surge 

• Size (Radius of Maximum Winds) 

Larger storm = More storm surge 

• Forward Speed 

Slower storm = Storm surge farther inland 

• Angle of Approach 

Alters focus of storm surge 

• Width and Slope of Shelf (Bathymetry) 

Gradual shelf = More storm surge

Sea Level Rise

• Sea level is the elevation where the sea surface meets land.

• Globally, 8/10 of the world’s largest cities are located near the coast. 

• Sea level rise threatens infrastructure, economies, and the safety of people who live on the coast. 

Main Contributors of Sea Level Rise - Short Term

Thermal Expansion

Melting glacial ice sheets

Thermal Expansion

Warmer, less dense water occupies a greater volume even without adding any water to the oceans

Melting Ice Sheets

Melting water from ice sheets and alpine glaciers contributes to the total water in the oceans

Factors contributing to Sea Level Rise - Eustatic Sea Level

Plate Tectonics

CO2 Levels

Plate Tectonics - SLR

Requires land masses at the poles to aggregate ice sheets. Vary over 10s of millions of years 

CO2 Levels - SLR

CO2 traps heat in the atmosphere and increases global average temperatures

Have decreased since the Cretaceous (65 my) associated with significant global cooling 

Vary slowly through geologic time. 

Milankovitch Cycles

Three components are associated with the tilt, wobble, and orbit of the Earth around the Sun.

• Eccentricity

• Obliquity

• Precession

Eccentricity

• ~100 ky cycle

• Variation in the shape of Earth’s orbit 

Obliquity

• Changes in the axis of rotation (tilt)

• Varies on 41 ky cycles between 21.8° and 24.4°

Precession

• 23ky cycle 

• Currently northern hemisphere summer is toward the sun

• In 11,500 years, this will switch

Chemical Proxies

Methods that use chemical or isotopic analyses of marine sediments, fossils, or other materials to reconstruct past sea levels

Geomorphic and Sedimentary

Use of features in the landscape or sedimentary record as indicators of past sea level position and elevation

Impacts of Sea Level Rise

SLR + Erosion has resulted in conversion of marsh land to open water in Southern Louisiana  

LA is in the midst of a land loss crisis that has claimed nearly 2,000 square miles of land since the 1930s.

The state could lose up to another 3,000 square miles

Sea Level Rise will amplify level and duration of coastal flooding during storm events

Nuisance Flooding: 

• High-tide flooding 

• Not associated with a storm system or rain 

• 300% to more than 900% more frequent than it was 50 years ago.