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MOTION IN THE OCEAN. Waves, Tides, and Currents. 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
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MOTIONIN THE OCEAN Waves, Tides, and Currents
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 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
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 • Bulge on opposite side because centr. force > pull of moon • Produced by motions of Earth, sun, & 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
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 • Circulates food, wastes, etc • Trigger spawning (grunion, horseshoe crab)
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 - 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 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