Currents, Waves, & Tides Highways in the Sea

Causes of Currents What forces are responsible for currents? Wind: transfers energy through friction on surface; causes currents and waves Sea level: steeper slope=greater pressure gradient=drives a larger, faster current Water density: greater density in one area causes pressure gradient to drive a current below the surface

Causes of Currents What influences the direction and nature of these currents? Ocean boundaries force currents in various directions; Trade winds and the westerlies account for most of the wind’s energy that drives currents

Western VS Eastern How do Western Boundary Currents differ from Eastern Boundary Currents? Eastern Currents: carry cool water towards equator; tend to be wide and shallow; not common to have eddies spin off these currents - Western Currents: much stronger; western intensification: Coriolis effect is a major contributor; Trade winds also contribute to the strong western currents

Upwellings & Downwellings Wind-driven currents that flow vertically Upwelling: upward vertical current that brings deep water to the surface Downwelling: downward vertical current that pushes surface water deep into the ocean What kind of biological effect does this have? -nutrients to shallow waters and to deeper waters -weather patterns

El Nino Buildup of warm water in the Central and Eastern Equatorial Pacific Tremendously affects world weather patterns El Niño = warm surface current in equatorial eastern Pacific that occurs periodically around Christmastime

La Nina Powerful upwellings bring deep cooler waters to the surface This colder than normal condition is called La Nina (opposite of El Nino) Scientists still trying to determine what exactly causes these warm and cold phases

Ocean Conveyor Belt The interconnected flow of currents that redistribute heat Earth’s “air conditioner” Moderates the world’s climate Ocean Conveyor Belt Animation: http://bcs.whfreeman.com/thelifewire/content/chp58/5802003.html

How do we study currents? Drogue: float that determines the drift of currents Argo Float: float that transmits data to satellites Various Flow Meters: measure and record current speed and direction (Eulerian study method) Doppler Acoustic Current Meter: determines current direction and speed Flotsam method: accidental opportunities to study currents; ships losing cargo that wash up on beaches and can improve computer models of currents

Waves & 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: Longitudinal: when matter moves back and forth in same direction energy travels Transverse: motion of matter is perpendicular to the direction in which the whole wave is moving Orbital: transmit through fluids only; main waves that concern us with respect to the ocean; energy moves fluid in a circular motion

Wave Terms: 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): Winds (most common) Changes in gravity Seismic activity: eartquakes and volcanic eruptions (tsunamis) Restoring forces (resist waves): Gravity (main force here for large waves) Coriolis Effect: good for resisting tides Surface tension: resists surface distrubances in tiny waves

Deep Waves VS Shallow 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!

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

Destructive Waves 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)

TIDES

What causes 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

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

Sun, Moon, and Types of Tides The influence of the moon on the tides is about twice the influence of the sun 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) 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