The Nature of Oceans
Chapter 16
Global Ocean?
• Separate but whole• Divisions resulting
from– Continents– Island arcs– Straits– Political boundaries
Salinity
• Total dissolved salts (all dissolved ions)
Oceans as buffers
• Dissolved gasses– CO2 and O2
• Atmospheric gasses and dissolved ocean gas concentrations
Salinity and Latitude (in ppt)
Other salinity factors
• Coastlines
• Fresh water inflow—rivers
“Ogres…I mean, Oceans are like onions…”
• Thermal layering—think oil/water
Epipelagic zone—surface to 200-450 mConstant temp—stirring by wind/waves
Thermocline—layer of rapidly decreasingtemperature
Here be strange critters—FRIGG’N COLD
Polar seas—exception--no layering
Ocean Currents
• Surface currents in upper 400 m– Involve 10% of water in oceans– Driven by wind horizontally
• Deep-sea currents caused by changes in density below 400 m– Vertical and horizontal changes– Driven by density
Surface Currents• Benjamin Franklin and mail
• Driving force– Friction between wind and water—circular
ocean currents--gyres
Notice Something Odd?
• Currents do not always flow in direction of prevailing wind—why?
• Coriolis Effect– All free standing objects in N. hemisphere are
deflected to the right--opposite in S. hemisphere
Effect on Oceans
WIND
Wind effect on Water with Depth
Coriolis Effect and Water Depth (collectively named Ekman Transport)
Surface Currents
• Can also profoundly affect climate
Deep-sea Currents
• Caused by cooling temperature and rising salinity
• Thermohaline circulation)
Something cold with salt?
• Wind near poles = cold– Wind causes
evaporation• Water salinity increases• Water temp decreases• Density of water at
poles = high
• Water sinks at poles and rises near the equator
UpwellingElkman transport
Nitrogen, phosphates and silica—Support plankton populations—base of Food chain
Fishing industry
Tides
• Gravitational pull of moon and sun causes Earth’s oceans to swell at given times
• Two tides per day—12 hours apart
• Place directly beneath moon—strongest pull
• Only part of story
What Causes Tides?– Second tide—when moon on opposite side of
Earth--Why?Actual E/M system
Common conception of E/M system
Tide Times
• Tides do not occur at same time everyday– Tot offset by 53
minutes/day
Spring Tide
Neap Tide
~ Seven Days Between…
Types of tides
Who Cares?
• Tides (whether high or low) can affect wave height/erosion– Especially in narrow inlets
• Its an additive effect
• Think about:– Waves– Storm surge
Narrow inlets
• Bay of Fundy, Canada– Shape of bay creates dramatic changes
between normal and high tidesHT
LT
Storm Surge
• A low pressure effect
• Stronger waves, more erosion
• Decrease beach slope
Coastlines
Coasting along…
• Coasts– Complex Interaction between
• Atmosphere, hydrosphere, geosphere, biosphere
Ocean Waves
Waves don’t affect us
past ½ the wavelength
Nothing Like a Day at the Beach…
• Beach– Accumulations of sand
and gravel
• Sediment for beaches– Sea cliff erosion– river transport
Sea Cliff Erosion
• Storms• Power of waves
– Sea cliffs and spaceships—hydraulic fracture
– Waves carry sand, gravel
• Liquid sandpaper• Can create sea
caves
– Salt cracking
Sediments Supplied by Rivers
Constructive WavesSummer
Leaves sandy beach
Spilling Breaker
Powerful Swash(pushes material up beach)Weak Backwash
(little erosion)
Destructive WavesWinter
Leaves gravelly beach
Crashing breaker
Weak swash
(little material brought in)
Powerful Backwash
(lots of erosion)
AHHHHHHHHHHH!
Wave Refraction
Longshore Current, Drift
• 1 million m3/yr moved S just at Santa Monica, CA
In the Old Days
• Temporary shelters
• Ramshackle cabins
• By the 1700s:– Built behind barrier islands– Built houses on stilts
• By the 1850s:– Resorts, bridges, summer homes, retirement
homes
Riprap and “Claptrap”
• Changing the natural order– Boulders, seawalls– Requests to local government
• An increase in sedimentation—”olde days”• wide deforestation• railroads• population increase• land cultivation• overgrazing
• Beach enlargement
Everything Has a Price: Seawalls
• Act just like sea cliffs– Small sand is removed– Bigger waves approach
closer to shore– With no beach, waves
can undermine barrier
A Decline In Beaches
• Modern changes…– Dams as sediment traps– Mining sand from river bottoms—building
material– Better land use practices– Beach erosion accelerated
• Commonly 5-10 m/yr, up to 200 m/yr in Nile
• Need to stop erosion…
A “Kick” in the Groin
• Recall longshore drift– Sand is carried along
the beach by waves at angle to beach
Good and Bad Results
• Bad– Manasquan, NJ
• Good
Argh, my beaches
Beach Replenishment
• In areas already “screwed up” can’t we just dump more sand?– Dump truck = 10 m3 of sand– If person’s lot is 30 m wide with 65 m of beach
(~2,000 m2) = 0.5 cm thick sand– 200 truck loads to raise the beach 1 m at a
cost of $60,000 / home– Need to also dump sand offshore– Where do we get all this sand? Peter or Paul?
• Inland finer-grained sand
What else can we do?
• Offshore dredging below wave base– Cost– Larger storms
• Nourishment– Dumping sand offshore
• Longshore drift– Inlets and dredging
Barrier Islands• Offshore bars or barrier islands
– Parallel to shoreline, migrate with waves– Common on East and Gulf coasts– 0.4 – 4 km wide, usually < 3 m asl– Lagoons act as quiet waterways for boats
Island Migration
• Migrate with gradual rise of sea level– Sea level rises about 30 cm (1 ft) a century– Migration inland by 100-150 m or more
• Immediate danger during large “storms”
Galveston, TX—Hurricane Ike