Waves, tides, and currents revised

Waves, Tides, and CurrentsWaves, Tides, and Currents

Chapter 16.2Chapter 16.2

MOTIONMOTION IN THE IN THE OCEANOCEAN

WavesWaves A disturbance which A disturbance which

moves through or moves through or over the surface of a over the surface of a fluidfluid

Mostly caused by Mostly caused by windswinds

(Also earthquakes, (Also earthquakes, volcanoes, grav. volcanoes, grav.

pull)pull)

Form of great energyForm of great energy

Wave CharacteristicsWave Characteristics

Parts of a WaveParts of a Wave CrestCrest = high point = high point

TroughTrough = low point = low point

HeightHeight = vertical = vertical distance from crest to distance from crest to troughtrough

WavelengthWavelength = = Horizontal distance Horizontal distance between crest to crest between crest to crest or trough to troughor trough to trough

Wave period : time for 2 crests to pass fixed point (T) sec

Wave speed (C) : C = wavelength / T (m/s)

Wave steepness : H / wavelength

When H / wavelength = 1/7 or angle at crest 120 or less = Breaker

Size of Wind Generated Size of Wind Generated WavesWaves

Depends on 3 thingsDepends on 3 things:: Wind SpeedWind Speed Wind Duration (length of Wind Duration (length of

time wind blows)time wind blows) ““Fetch” Extent of open Fetch” Extent of open

water across which the water across which the wind can blowwind can blow

Water Motion in WavesWater Motion in Waves

Water travels in vertical Water travels in vertical circular orbitscircular orbits

Wave moves, particles don’t!Wave moves, particles don’t!

Importance of WavesImportance of Waves

Shaping Shaping CoastlinesCoastlines Erode cliffsErode cliffs Grind rock into sandGrind rock into sand

EcologyEcology Returns OReturns O

2 2 to waterto water Stir up food for filter Stir up food for filter

feedersfeeders

Types of WavesTypes of Waves

CHOP – Short period (back bays)

SWELL – Long period (boat rolls; seasickness)

SWASH – water up beach BACKWASH – back down

Caused by undersea quake or volcano

• Wavelength = ~150 mi. Wave height = 6” – 1’

Can NOT perceive in boat Speed > 500 mph

Slows down to ~25 mph at shore; water builds up to ~65+ ft

TSUNAMI “TIDAL WAVE”

Tsunami Waves

Creation of a TsunamiCreation of a Tsunami

TidesTides The rhythmic rise and The rhythmic rise and

fall of the ocean’s waterfall of the ocean’s water

High tide = rising, incoming tide, flow

Low tide = receding, outgoing tide, ebb

Slack tide = vertical movement stops

Tides are very long, Tides are very long, slow wavesslow waves

They have a wave They have a wave period of 12 hours 25 period of 12 hours 25 minmin

Tidal day is 24 hours Tidal day is 24 hours 50 min50 min

NJ has 2 high and 2 NJ has 2 high and 2 low tides dailylow tides daily

1. Gravitational pull of 1. Gravitational pull of sun & moon on Earthsun & moon on Earth

What Causes Tides?What Causes Tides?

• Moon closer, therefore > effect

• Like magnet, pulls water away from surface = TIDAL BULGE

2. Centrifugal Forces

• Produced by motions of Earth, sun, & moon

• Bulge on opposite side because centr. force > pull of moon

Types of TidesTypes of Tides

2x’s/month2x’s/month

•Spring Tide - Moon and sun are in direct

line with one another- Results in unusually

high tidal range

-Tidal Range = vertical distance between high & low tides

Neap TideNeap Tide sun and moon are at sun and moon are at

right angles right angles

Pulls cancel each Pulls cancel each other out – causes a other out – causes a weak pullweak pull

unusually low tidal unusually low tidal rangerange

2 x’s / month2 x’s / month

Spring vs. Neap TidesSpring vs. Neap Tides

Distance bet. Moon & Distance bet. Moon & EarthEarth

Perigee TidesPerigee Tides• Moon closest to earth, very high tides (causes Moon closest to earth, very high tides (causes

flooding)flooding)

Apogee TidesApogee Tides• Moon farthest away from earth, very low tidesMoon farthest away from earth, very low tides

Types of Tides ContinuedTypes of Tides Continued Diurnal TidesDiurnal Tides

1 high & 1 low / day1 high & 1 low / day Parts of Gulf of Mexico and AsiaParts of Gulf of Mexico and Asia

Semi-Diurnal TidesSemi-Diurnal Tides 2 high & 2 low / day2 high & 2 low / day Atlantic coasts of North America and EuropeAtlantic coasts of North America and Europe

MixedMixed 2 high & 2 low / day (height varies)2 high & 2 low / day (height varies) Pacific coastPacific coast

Importance of TidesImportance of Tides

• Expose & submerge orgs

• Circulate water in bays & estuaries

• Trigger spawning (grunion, horseshoe crab)

• Circulates food, wastes, etc

CurrentsCurrents• What are currents?

- “Rivers” of circulating water

• Causes- Wind

- Rotating Earth

- Density Changes

Surface Ocean CurrentsSurface Ocean Currents

• Broad, slow drifts; never cross equator

• Wind generated; circular gyres

• Coriolis Effect

- N. Hemis – clockwise; Right

- S. Hemis – counterclockwise; Left

• Gulf Stream

- Brings warm water from equator north along east coast of N. A.

- N. Atlantic

-Sometimes form eddies – circulating water that pinches off from the current

MIGRATION NAVIGATION

WEATHER

Localized Surface Localized Surface CurrentsCurrents

Longshore CurrentLongshore Current..

Flows parallel to shore; move sedimentFlows parallel to shore; move sediment

RIP CURRENT

- Caused by converging longshore currents

- Very dangerous ; Red Flag

- DO NOT fight rip current; swim parallel to shore to get out of channel

Deep Ocean CurrentsDeep Ocean Currents

Separated from surface Separated from surface currents by boundary currents by boundary called a “called a “ThermohalineThermohaline” ” (diff in densities)(diff in densities)

Flow beneath surface; cross Flow beneath surface; cross equatorequator

Move North to SouthMove North to South

Importance Of Deep Importance Of Deep CurrentsCurrents UpwellingUpwelling

• Brings deep water to surf.Brings deep water to surf.• Circulates nutrients upCirculates nutrients up• Moves plankton & larvaeMoves plankton & larvae