Waves, Tides, and Currents

MOTION IN THE OCEAN

Waves A disturbance which

moves through or over the surface of a fluid

Mostly caused by winds

(Also earthquakes, volcanoes, grav.

pull)

Form of great energy

Wave Characteristics

Parts of a Wave Crest = high point

Trough = low point

Height = vertical distance from crest to trough

Wavelength = Horizontal distance between crest to crest or 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 Waves

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

time wind blows) “Fetch” Extent of open

water across which the wind can blow

Water Motion in Waves

Water travels in vertical circular orbits

Wave moves, particles don’t!

Importance of Waves

Shaping Coastlines

Erode cliffs Grind rock into sand

Ecology Returns O

2 to water

Stir up food for filter feeders

Types of WavesCHOP – 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 Tsunami

Tides The rhythmic rise and

fall 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, slow waves

They have a wave period of 12 hours 25 min

Tidal day is 24 hours 50 min

NJ has 2 high and 2 low tides daily

1. Gravitational pull of sun & moon on Earth

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 Tides

2x’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 Tide sun and moon are at

right angles

Pulls cancel each other out – causes a weak pull

unusually low tidal range

2 x’s / month

Spring vs. Neap Tides

Distance bet. Moon & Earth

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

flooding)

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

Types of Tides Continued Diurnal Tides

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

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

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

Importance of Tides• Expose & submerge orgs

• Circulate water in bays & estuaries

• Trigger spawning (grunion, horseshoe crab)

• Circulates food, wastes, etc

Currents• What are currents?

- “Rivers” of circulating water

• Causes- Wind

- Rotating Earth

- Density Changes

Surface 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 Currents

Longshore Current.

Flows 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 Currents

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

Flow beneath surface; cross equator

Move North to South

Importance Of Deep Currents Upwelling

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