Waves, Currents, and Tides MOTION IN THE OCEAN Waves, Currents, and Tides

Waves A disturbance which moves through or over the surface of a fluid Mostly caused by winds (Also earthquakes, volcanoes, the pull of gravity). Form of great energy

Wave Characteristics Parts of a Wave Crest = high point Trough = low point Height = vertical distance from crest to trough Amplitude = ½ of height. Crest to midline or trough to midline Wavelength = Horizontal distance between crest to crest or trough to trough Midline or Still Water Level = No waves or mid point of wave height between the crest and The trough

Breaking Waves; Breakers Waves break near the shore because the water becomes shallow. Close to the shore the ground beneath the water interferes with the circular motions of the water molecules as they participate in a passing wave. Sandbars further off shore can have the same effect, much to the delight of surfing enthusiasts like Bart.

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!

Wave from deep water reaching shallow water.

Waves reaching a sandbar or reef underwater

Breakers at the Beach

Importance of Waves Shaping Coastlines Ecology Erode cliffs Grind rock into sand Ecology Returns O2 to water Stir up food for filter feeders

Types of Waves CHOP – Short period (back bays) SWELL – Long period (boat rolls; seasickness) SWASH – water up beach BACKWASH – back down

TSUNAMI “TIDAL WAVE” 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 Waves

Creation of a Tsunami

Receding Ocean

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

Currents What are currents? Causes - “Rivers” of circulating water - Wind - Rotating Earth Density Changes Topography of the ocean floor Layout of the Continents

Surface Ocean Currents Broad, slow drifts; never cross equator Wind generated; circular gyres The Ocean’s Conveyor Belt

Coriolis Effect - N. Hemis – clockwise; Right - S. Hemis – counterclockwise; Left

Gulf Stream - N. Atlantic - Brings warm water from equator north along east coast of N. A. Sometimes form eddies – circulating water that pinches off from the current

IMPORTANCE OF SURF. CURRENTS NAVIGATION MIGRATION 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 Flow beneath surface; cross equator Separated from surface currents by boundary called a “Thermohaline” (diff in densities) Move North to South

Importance Of Deep Currents Upwelling Brings deep water to surf. Circulates nutrients up Moves plankton & larvae

Tides

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 NC has 2 high and 2 low tides daily

What Causes Tides? 1. Gravitational pull of sun & moon on Earth Moon closer, therefore > effect Like magnet, pulls water away from surface = TIDAL BULGE

2. Centrifugal Forces Bulge on opposite side because centr. force > pull of moon Produced by motions of Earth, sun, & moon

Types of Tides 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 2x’s/month

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

Importance of Tides Expose & submerge orgs Circulate water in bays & estuaries Circulates food, wastes, etc Trigger spawning (grunion, horseshoe crab)