Waves and Tides

Anatomy of a Wave What is a wave? -Transmission of energy through matter; matter moves back and forth or rotates, but then returns to its original position Three types of progressive waves: 1) Longitudinal: when matter moves back and forth in same direction energy travels 2) Transverse: motion of matter is perpendicular to the direction in which the whole wave is moving 3) Orbital: transmit through fluids only; main waves that concern us with respect to the ocean; energy moves fluid in a circular motion

Wave Terminology Wavelength: measured as horizontal distance between identical points on two waves; crest to crest Crest: highest point above the avg water level Trough: lowest point Height: vertical distance from the trough to the crest Period: time it takes for the same spot on two waves to pass a single point Frequency: number of waves that pass a fixed point in one second

Forces that Cause Waves Disturbing forces (cause waves): 1. Winds (most common) 2. Changes in gravity 3. Seismic activity: eartquakes and volcanic eruptions (tsunamis) Restoring forces (resist waves): 1. Gravity (main force here for large waves) 2. Coriolis Effect: good for resisting tides 3. Surface tension: resists surface distrubances in tiny waves

Deep Water Waves vs. Shallow Water Waves Deepwater Waves: - Occur in water that is deeper than half their wavelength - No interaction with the bottom floor can change any wave characteristics - The orbital motion progression is also unaffected Shallow-water Waves: - Occur in water that is shallower than one- fourth the wavelength - Bottom creates a drag that affects the orbital motion - Flattens the circular motion Both can exist at the same time!

What causes internal waves? Occur within different density layers Can be more than 30 m tall, but move very slowly Scientists don’t really know what causes these waves; likely that they get their energy from wind, gravity, or seismic forces (like surface waves)

Breaking Waves What makes a wave break? -waves break when the height exceeds the wavelength by a certain ratio; the wave breaks as white caps What are the three basic types of wave breaks? 1) Plunging breakers: curl as the top of the wave pitches through the air before splashing into the bottom; occur on steep beaches 2) Spilling breakers: occur on gentle slope beaches; top of the wave tumbles and slides down the front of the wave 3) Surging breakers: occur on very steep beaches; little to no bottom contact, waves don’t slow down, but surge forcefully unbroken; can be very destructive

Refraction, diffraction, deflection Refraction: bending of light rays; bend the ocean waves as well when waves begin to approach the ocean shore at an angle Diffraction: occurs when waves pass an obstacle, like a jetty; energy shift within a wave, allowing a new wave pattern to form Reflection: occurs when waves hit an abrupt obstacle that is nearly perpendicular in the water, like a sea wall; in this case, the wave retains much of its energy and bounces back towards open waters

What is a standing wave? Vertical oscillation in which water rocks back and forth, rising and falling at the ends, but relatively motionless near the center Like coffee sloshing back and forth after you bump the cup

Destructive Waves: storm surges, seiches, tsunamis What causes a storm surge? - This is a destructive wave that forms when high winds push water against the shore, where it piles up What causes seiches? - This is a form of standing wave that can be destructive; result from a strong wind that pushes the water level up on one side of a lake or basin What causes tsunamis? -results from sudden water displacement caused by a landslide, an iceberg falling into sea from a glacier, a volcanic eruption, or an earthquake (most common)

What causes the tides? Tides: variations in the ocean’s level Result from the gravitational pull of the moon and (to a lesser degree) the sun They pull the ocean into a huge wave with a wave length the size of an ocean basin Sun and moon create two bulges on opposite sides of the Earth; position of the sun and moon change slowly, causing the bulge to rotate around the Earth As a coastline rotates into the bulge, the tide rises; as it rotates out, the tide falls

What influences besides lunar and solar gravity affect the tides? The imperfect sphere of the Earth The season Time of the month The shape of the ocean basin Coriolis effect **All influence the tides!**

Types of tides: vary with location Diurnal: single high and low tide that occurs daily in certain locations; Gulf of Mexico Semidiurnal: have two roughly equal high and low tides daily; happens on east coast of US Mixed: consists of two unequal high and low tides daily; happens in Pacific coast of the US

What are tidal currents and a tidal bore? Tidal Currents: the daily tides create a current that flows into and out of bays, rivers, harbors, and other restricted places -Inflow= flood current -Outflow= slack current Tidal Bore: when the incoming tide produces a wave that flows into a river, bay, or other relatively narrow area; also known as a true tidal wave that can be several meters high

Sun, Moon, and Types of Tides The influence of the moon on the tides is about twice the influence of the sun 1) Spring Tides: when the sun and the moon are aligned on the same side of Earth (new moon) AND when the sun and moon are aligned on opposite sides of Earth (full moon) 2) Neap Tides: when the sun’s gravitation pulls to the side of the moon’s tidal bulge and tends to raise low tides and lower the high tide