The Nature of Oceans Chapter 16. Global Ocean? Separate but whole Divisions resulting from...

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