Transcript Lecture 14 Oceans and Coastal Processes u

Oceans and Coastal Processes
Four Major Oceans plus Arctic Ocean and nine or so Seas
Flashlight
Globe
Recall Ocean
Depths
depend on the
Plate Tectonic
Province:
(0-150
meters)
(less than 3 km)
~4 km
7 to 11
km
(4-7
miles)
Sea Level Changes
• Rapid seafloor spreading expands the
mid-ocean ridge displacing water and
raising sea level.
• Sea level slowly drops during Ice Ages
when substantial volumes of ocean water
are locked up in ice sheets.
• Sea Level rises "suddenly" when glaciers
melt
Ocean Salinity
• Dissolved “Salts” are ions from weathering
• Salinity in surface water averages ~35 parts
per thousand.
• Salinity variation more extreme in coastal
waters. River input, evaporation in lagoons
• Salinity and temperature change with depth rapid change salinity (saltiness) at the halocline.
• Salt water is more dense than fresh water
• Ice is less dense than liquid water (so it floats)
• Ice is freshwater, recall most fw in ice caps
Ocean Temperatures
• Ocean temperatures of 27oC are typical of
tropical surface
• Waters and temperatures of 2oC are
typical for deep ocean waters.
• Cold water is more dense than warm
water. It will move under less dense
water.
Salinity varies w/ temperature and
ocean current
mixing
low salinity of arctic – isolated low evap cold, freshwater rivers and snow
40 ppt Red Sea
ocean: 33-37 ppt high 36-37 at +/- 30o, high evap, low eq 34 trop rain
Salinity shown in cross-section of ocean in ppt
Southern Hemisphere Summer
Saltiest at 23.5S South Latitude
Sun overhead, more evaporation
Solar Heating
• The equator receives 2.5 times more sunlight
(insolation [photons / m2 ]), incoming solar
radiation, than the poles.
• Highest average annual ocean surface
temperatures (~27oC) at equator
• Lowest 0oC at
high latitudes
23.5
Demo, Flashlight
Globe
Heat versus Temperature
• When heat something, its temperature rises.
• Heat is the total energy of molecular motion in a
•
•
substance. Heat energy depends on the speed of the
particles, the number of particles (the size or mass),
and the type of particles in an object.
Temperature is a measure of the average energy of
molecular motion in a substance. Temperature does not
depend on the size or type of object.
For example, the temperature of a small cup of water
might be the same as the temperature of a large
bathtub of water, but the tub of water has more heat
because it has more water and thus more total thermal
energy.
Water thermal properties 1
• 1. Heat capacity Cp water very high. Water can store
huge amounts of heat without raising temperature.
The bonds vibrate faster, but the water molecules H2O
need not speed up.
•
• Cp water = 4.2 x Cp air
Heat Capacity: heat to raise 1 cm3 substance 1oC
• Ocean heat storage plays a crucial role in controlling
global climate patterns.
http://hendrix.uoregon.edu/~stanm/phys162s2003/PHYS162/Heat_Capacity.html
Water thermal properties 2
• Density r is mass/unit volume
• Salt water is denser than fresh water
• Cold water denser than warm water.
• Freshwater Ice forms at -2oC from
saltwater. Less dense than Saltwater,
floats
• Remaining Seawater is saltier, denser
Average Annual Sea-Surface Temperature
~30C Equator, 0C Poles, average temp for a
latitude maintained by currents
Poles should be much colder, but currents carry warm equatorial water poleward
Temperature and Currents
• Surface waters (<300 m) warmed by Sun
• Currents cause thermal mixing: results in
relatively uniform surface temperature by
latitude.
• The impact of current activity on climate
diminishes with depth
Temperature and Depth
• Temperatures exceed 20oC over tropical
ocean's surface, 2oC below 2,000 m.
• The depth zone in which
temperature decreases rapidly is called the
thermocline
1000 m
2000 m
Continents and Climate
• Over two-thirds of Earth’s surface is
ocean.
• Currents started by prevailing winds
• Currents deflected by continent positions
and Earth’s rotation
• Climates caused by current and wind
circulation
Ocean Currents
• Again, ocean currents started by winds.
• Winds are ultimately caused by equator to
poles insolation (heat from Sun) differences
• Winds drag surface waters as surface currents
• Winds and currents are influenced by Earth's
rotation (Coriolis effect).
• Currents influenced by distribution of
continents
Coriolis Effect
Air masses at rest above the
equator are moving much
faster than air masses at rest
over us in NJ.
Both must rotate once per
day, but the equatorial air
goes much further.
Equatorial Air has faster spin
it still has it , so it is
Coriolis Effect
Air in Northern Hemisphere
deflects to the right, reverse in Southern H.
as it moves toward poles
faster than land below
DEMO: Coriolis Clip
Winds Start Currents
• Air molecules in wind push water molecules
3-4% of wind speed.
• Wind moving nearly parallel to latitude as a
•
•
•
result of Coriolis deflection.
West-pushed ocean currents form near equator,
east-pushed ocean currents form about 50o
Latitude
Continents deflect or prevent current flow
Results in circular current called a gyre.
Example: Generation of
North Pacific Gyre
The major wind cells
Convection
Maximum
heat from
Sun
Define “Insolation”
= radiation from sunlight
DEMO: globe,
flashlight if not done previously
Surface Currents
(Influence Climate)
Winds push currents
East pushed flow near 50 degrees
Gyre CW
West pushed flow near equator
West pushed flow near equator
Gyre CCW
East pushed flow near 50 degrees
Note Warm Currents not past
S 60o Latitudes
Western Boundary Currents
• Winds cause water to pile up along the western
•
•
•
subtropical sides of major oceans.
Generate fast-flowing currents that redistribute
warm tropical waters toward the poles.
Gulf Stream, Kuroshio, Brazil currents are “West
Side” marine rivers, relatively narrow (less than
100 km across) but deep water masses.
Flow at speeds of 100 to 200 km/day for
thousands of kilometers, from equator to high
latitudes
Eastern Boundary Current
• Eastern boundary currents (Canary,
•
•
•
•
California, Peru) complete the eastern leg of
each gyre
Wider, carry less water, and move more slowly.
The Canary current, nearly 1,000 km (625 miles)
wide but very shallow
1/3 water of Gulf Stream and travels at 30
km/day
2/3 continues as cold deep salty current
WIND
North Atlantic Deep Water
(NADW) down here
Evaporation and Ice
formation increases
saltiness and density. Arc shallowing lifts
Dense water
sinks cold deep water
forms NADW
Winds blow
surface
waters west,
forcing deep
water up to
surface
where it
warms
ThermoHaline
Conveyer Belt AKA
Wallace Broecker
Global Ocean
Columbia University
Conveyer Belt
Shores and Coastal Processes
• Shorelines are places where bodies of water
meet dry land
• Coasts are landward of ocean shorelines
• Beach: a narrow strip of land, washed by
waves or tides .
• Ordinary Waves are caused by WIND
– Waves are produced when wind drag causes the
surface water of oceans/lakes to rise and fall
Waves get refracted on approaching shoreline
Typical Coast
Waves are caused by _____________?
Parts of A Wave
Fetch – length of unobstructed wind
Southern Ocean inc. Straits of Magellan
Dana ‘Two Years Before the Mast’
Oscillatory and Translatory Motion
Wave Refraction
Still a small ‘longshore component
Longshore Currents (Swash and
Backwash)
Longshore current is produced as waves
reflect from coastline. Swash and
backwash
Rip Currents
Cut in Bar
Sand Bar
Rip Currents are produced when storms cut holes in sandbars just offshore.
Tides
• Daily rise/fall of surfaces of oceans/lakes due
to gravitational pull of the Moon/Sun on the
Earth– also due to force created as Earth spins
on its axis
• Flood tides- elevate sea surface that cause
shoreline to move inland
• Ebb Tides- low sea surface that cause
shoreline to move seaward
Tidal Bulges
A mass in circular motion
accelerates toward the axis
of rotation. This centripetal
acceleration is exerted on
the mass by some other
object,say a string held in
my hand. The mass exerts
an equal and opposite force
on the object, the centrifugal
force.
Tides mostly caused by the Moon
Affect of the Sun Much less
Why?
The Effect of Tides On Shorelines –
River Hebert in Nova Scotia
Tidal Bore flows upstream
Source: Clyde H. Smith/Peter Arnold, Inc.
Large Tidal Extremes
•Bay of Fundy tides reach16 m (about
53 ft) at the head of the bay. The 12.4
hour period of the twice daily lunar tides
is close to the natural back and forth
sloshing period of the bay.
Source: William E. Ferguson
Rising Tide at Bay of Fundy
Maximum
Source: William E. Ferguson
Coastal Erosion (Crashing Surf)
Wave erosion occurs when
deep water waves hit the shore
with full force. Air and water are
forced into cracks at high
pressure
Crashing Surf, Oregon Coast
Fast Erosion
Source: Criag Tuttle/The Stock Market
Erosional Coastal Landforms
Submerged Shallow
Area Bends Waves
Erosional Coastal Landforms
Sea Caves on Cape Kildare, Prince Edward Island, Canada)
Source: John Elk/Bruce Coleman
Deposition of A Tombolo
A Tombolo Landward of A Sea Stack – Big Sur,
California
Source: Cliff Wassmann
Coastal Protection
• Riprap/Seawall- protects shore lines
• Breakwaters – Stabilize beaches
• Jettys – Keep inlets clear
Beach-Protection Structures - Riprap
Source: Jack Dermid /Photo Researchers, Inc.
Beach-Protection Structures –
Seawall Along the Gulf Coast of Louisiana
Source: Martin Miller
Breakwaters
Breakwaters off Cape May, New
Jersey
Source: John S. Shelton
Jetties
Jetty in Miami Beach, Florida
Source: Townsend P. Dickinson
Components of A Typical Beach
•A beach is a dynamic narrow
segment of coast washed by
waves/tides and covered with
sediments
•Foreshore is the area
between low tide and high
tide
•Backshore is the area
between high tide and sea
cliff or inland vegetation
line
•Beach face is the
steepest part of Foreshore
•Berm is a horizontal
Transport and Depositional
Features
• Recall Longshore Drift sand moved by
LSC
• Spit is a finger-like ridge of sand deposited
where Longshore drift encounters deeper
water
• Hook is a curved spit
• Baymouth is a spit that covers the access
to a bay – fills with sediment
Deposition Spits, Hooks, and Baymouth
Bars
Organic Coasts
• Fringing Reef- initially surround land, grow
seaward
• Barrier Reef- separated from coast by a
lagoon
• Atoll- circular structure from great depth that
encloses shallow lagoon
• Mangroves
Evolution of Coral Atolls
Remember Guyots?
Charles Darwin
coral larvae are planktonic
Wake Island, a Coral Atoll
Source: William E. Ferguson
Typical Mangrove Coast,
Florida
“Make your own island”
Source: S. J. Krasemann/Peter Arnold, Inc.
Steep Shore
Steep Cliffs may occur at new Divergence (Red Sea)
or near a Trench (Oregon Coast)
Headlands, benches, sea arches, stacks, tombolos
All other things
being the same,
steep cliffs
easier to erode
than gentle
slope
Source: William Boyce/Corbis
Define Active Margin
Thailand
Island Arcs protect coastline
allow deltas to form
Cambodia
Laos
(Chao Phraya River)
Delta Protected from
Typhoons and Tsunamis
December 26, 04
Tsunami
Sumatra,
Indonesia
Source: Tom Van Sant / Geoshere Project
Santa Monica/Science Photo Library
Passive Margin
coast far from plate margins
Define Passive Margin
Active Margins
coasts close to plate boundaries