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 O2 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