the nature of oceans chapter 16. global ocean? separate but whole divisions resulting from...
TRANSCRIPT
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The Nature of Oceans
Chapter 16
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Global Ocean?
• Separate but whole• Divisions resulting
from– Continents– Island arcs– Straits– Political boundaries
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Salinity
• Total dissolved salts (all dissolved ions)
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Oceans as buffers
• Dissolved gasses– CO2 and O2
• Atmospheric gasses and dissolved ocean gas concentrations
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Salinity and Latitude (in ppt)
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Other salinity factors
• Coastlines
• Fresh water inflow—rivers
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“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
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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
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Surface Currents• Benjamin Franklin and mail
• Driving force– Friction between wind and water—circular
ocean currents--gyres
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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
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Effect on Oceans
WIND
Wind effect on Water with Depth
Coriolis Effect and Water Depth (collectively named Ekman Transport)
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Surface Currents
• Can also profoundly affect climate
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Deep-sea Currents
• Caused by cooling temperature and rising salinity
• Thermohaline circulation)
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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
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UpwellingElkman transport
Nitrogen, phosphates and silica—Support plankton populations—base of Food chain
Fishing industry
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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
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What Causes Tides?– Second tide—when moon on opposite side of
Earth--Why?Actual E/M system
Common conception of E/M system
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Tide Times
• Tides do not occur at same time everyday– Tot offset by 53
minutes/day
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Spring Tide
Neap Tide
~ Seven Days Between…
Types of tides
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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
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Narrow inlets
• Bay of Fundy, Canada– Shape of bay creates dramatic changes
between normal and high tidesHT
LT
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Storm Surge
• A low pressure effect
• Stronger waves, more erosion
• Decrease beach slope
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Coastlines
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Coasting along…
• Coasts– Complex Interaction between
• Atmosphere, hydrosphere, geosphere, biosphere
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Ocean Waves
Waves don’t affect us
past ½ the wavelength
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Nothing Like a Day at the Beach…
• Beach– Accumulations of sand
and gravel
• Sediment for beaches– Sea cliff erosion– river transport
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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
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Sediments Supplied by Rivers
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Constructive WavesSummer
Leaves sandy beach
Spilling Breaker
Powerful Swash(pushes material up beach)Weak Backwash
(little erosion)
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Destructive WavesWinter
Leaves gravelly beach
Crashing breaker
Weak swash
(little material brought in)
Powerful Backwash
(lots of erosion)
AHHHHHHHHHHH!
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Wave Refraction
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Longshore Current, Drift
• 1 million m3/yr moved S just at Santa Monica, CA
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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
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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
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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
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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…
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A “Kick” in the Groin
• Recall longshore drift– Sand is carried along
the beach by waves at angle to beach
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Good and Bad Results
• Bad– Manasquan, NJ
• Good
Argh, my beaches
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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
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What else can we do?
• Offshore dredging below wave base– Cost– Larger storms
• Nourishment– Dumping sand offshore
• Longshore drift– Inlets and dredging
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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
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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”
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Galveston, TX—Hurricane Ike