Ocean Currents

Earth Science

Ocean Currents

• Currents- water in a stream – Surface currents- currents that move on or near

the surface & are caused by wind – Deep current- move very slowly beneath the

surface of the ocean & are caused by density differences in water

Ocean Currents: Surface Currents

• Driving energy source for surface currents is wind

• Surface currents extend down to a depth of 100m

• 3 controlling factors in surface currents – Wind belts– Earth’s rotational effects– Location of the continents

Ocean Currents: Surface Currents

• Wind belts – Most directly affect the surface currents – Trade winds & westerlies• Trade winds at latitudes north & south of the equator • Westerlies are located in the middle latitudes • North of the equator the trade winds blow from the

northeast (Northern Hemisphere) & are usually warm • Southern Hemisphere trade winds blow from the south

east – In both hemispheres they push water to the west

• Wind Belts – Northern Hemisphere westerlies blow from the

southwest…moves currents eastward across the Atlantic & Pacific Oceans

– Southern Hemisphere westerlies blow from the northwest…moves currents eastward • Produce the largest current on earth…West Wind Drift

– Travels the entire circumference of the earth and is located in the southern part of all 3 major oceans

Ocean Currents: Surface Currents

• Coriolis Effect – Deflection of the earth’s winds and ocean currents

based on the rotation of earth – Coriolis effect along with the earth’s winds cause

gyres • Gyres- huge circle of moving water • Northern hemisphere- gyre moves clockwise • Southern hemisphere- gyre moves counterclockwise

Ocean Currents: Surface Currents

• Continents – The surface current flows against a landmass and

is divided and deflected • Equatorial Current– Warm currents – 2 of them both move in a westward direction

Ocean Currents: Surface Currents

• North Pacific & Atlantic Currents– North Atlantic• North Atlantic Equatorial Current

– Warm current that moves water north along the east coast of North America called the Gulf Stream

– Gulf Stream widens & slows and becomes the North Atlantic Drift » Drift- weak current

– All of those currents form the North Atlantic Gyre

Ocean Current: Surface Currents

• North Pacific– Similar to the North Atlantic – Japan Current is equivalent to the Gulf Stream • Flows northward & turns into the North Pacific Drift

when it spreads out & slows down as it flows towards North America

Ocean Currents: Surface Currents

• Currents in the Southern Hemisphere – Currents here also flow in gyres but in a

counterclockwise direction– Southern regions of all three major oceans

constant westward winds produce the West Wind Drift • West Wind Drift is the largest current in the world • Because there is no continents to stop it the West wind

drift completely circles Antarctica

Ocean Currents: Surface Currents

• Currents in the Southern Hemisphere cont…– Indian Ocean currents follow 2 patterns• Southern part the currents follow a counterclockwise

gyre• Northern part the currents are governed by monsoons

– Monsoons change directions with the seasons

Ocean Currents: Deep Currents

• Cold, dense currents that flow beneath the surface of the ocean – Much slower moving than surface currents

• Produced as cold, dense water of the polar regions sinks & flows beneath warmer ocean water toward the equator – Movements are the result of density differences – Higher density of polar waters is also a result of an increase in

salinity • Deep-current layer rises only when winds blow the

surface water out of the way

Ocean Currents: Deep Currents

• Antarctic Bottom Water – The densest & coldest ocean water in the world – Dense, cold water sinks to the bottom & very

slowly move north forming the Antarctic Bottom Water current • Moves to north to about 40 degrees & takes several

hundred years to make the trip

Ocean Currents: Deep Currents

• North Atlantic Deep Water– South of Greenland the water is exceptionally cold

& has high salinity causing it to sink • Forms a current that travels south underneath the Gulf

Stream • Flows southward all the way to the Antarctic & over the

Antarctic Bottom Water

– Deep Atlantic currents also are found near the Mediterranean Sea

Ocean Currents: Deep Currents

• Turbidity Currents – Occur when large masses of sediment that have

accumulated along a continental shelf or continental slope suddenly break loose and slide downward • Causes the water to become more dense than the

surrounding water • Believed to cause submarine canyons to deepen

Ocean Waves

• Wave- periodic up and down movement of water – Transfer energy

• 2 basic parts of a wave: – Crest- highest point of a wave– Trough- lowest point of a wave

• Wave characteristics– Wave height- vertical distance between a crest & a trough of a

wave – Wavelength- horizontal distance between 2 crests of a wave – Wave period- time it takes for one complete wavelength– Wave speed= wavelength/period

Ocean Waves

• Wave Energy– Main source of energy for waves is wind

• The more energy that is transferred the larger the wave becomes

– Because of surface area…• Large waves tend to become larger because the wind has

more area to push on • Smaller waves tend to die out because there is a very

small surface area for the wind to push on

– Swell- group of long, rolling waves that are the same size

Ocean Waves

• Water Movement in A Wave – Only the energy of wave moves forward, the water

itself moves very little • The water particles within the wave move in a circular

motion – Circle traced by each water particle in a wave has a diameter equal

to the height of the wave

– Energy received by a wave decreases as depth increases because the waves receive their energy from the wind • Diameter of the circle traced by each water particle

decreases as depth increases

Ocean Waves

• Wave Size– 3 factors that affect wave size:

• Speed of the wind• Length of time the wind blows • Fetch of a wave

– Distance that the wind can blow across open water– Long fetch produces very large waves

» Likely to occur during storms

– Size of a wave will only increase to a certain height to length ratio before it collapses

– Whitecaps- occur when high wind speeds blow the crest off of a wave

Ocean Waves: Waves & the Shore

• Breakers– Height of a wave changes as the wave approaches the

shore – As a wave moves into shallow water, the bottom of

the wave is slowed by friction but the top of the wave continues to move at its original speed

– The top of wave gets farther & farther ahead of the bottom & eventually it topples over & forms a breaker. • Height of the wave when it topples over is one to two time

the height of the original wave

Ocean Waves: Waves & the Shore

• Breakers cont…– Size & force of breakers are determined:• Original wave height• Wavelength• Steepness of the ocean floor close to shore

– More steep = rapid increase of wave height = breaker with greater force » Pacific Coast

– Less steep = slower increase of wave height = breaker with less force » Atlantic Coast

Ocean Waves: Waves & the Shore

• Undertows & Rip Currents – Undertow- water carried onto a beach is pulled

back into deeper water by an irregular current • Create problems only along shores with steep drop-offs

– Rip current- form when water from large breakers returns to the ocean through channels in underwater sand bars that are parallel to the beach • Flow perpendicular to shore

Ocean Waves: Waves & the Shore

• Longshore Currents – Form sandbars – Forms as waves approach the beach at an angle – Flow parallel to the shore

Ocean Waves: Wave & the Shore

• Tsunamis – Most destructive waves in the ocean– Energy that forms them comes from seismic sea

waves not the wind • Usually caused by earthquakes on the ocean floor, but

also by volcanic eruptions & underwater landslides

– Have very long wavelength, wave period is about 15 minutes, and speed of 725 km/hr

– Entire depth of the water is involved in wave motion – Height increase greatly as they approach shore

Tides

• Tides- daily changes in the level of the ocean surface – The gravitational pull of the moon on the earth

and its waters is the major cause of tides – Low tides are formed halfway between two high

tides

Tides: Behavior

• Tidal movement is due to the rotation of the earth and the pull of the moon. – Earth rotates from west to east, so tidal bulges

appear to move westward around the earth. • Because there are two tidal bulges most locations on

the ocean have two high tides & two low tides

– Tidal range- difference in the level of high tide and low tide at specific locations • Can vary widely from place to place

Tides: Behavior

• During period of new moon & full moon the high tides are the highest and the low tides the lowest– This is because the sun, moon, & earth are all aligned – These are called spring tides

• During 1st quarter & 3rd quarter phases the daily tidal range is the smallest – This is because the moon & the sun are at right

angles to each other in relation to the earth – These tides are called neap tides

Tides: Variations

• Tidal patterns are greatly influenced by the size, shape, depth, & location. – Atlantic Coast: tides are semidiurnal, which means

twice a day• Have a fairly regular tidal range

– Gulf of Mexico: diurnal, which means once a daily – Pacific Coast: follow a mixed pattern & have an

irregular tidal range • Very high followed by very low, then a lower high tide

followed by a higher low tide

Tides: Variations

• Tidal oscillations- caused by the slow rocking motions of ocean water that occur as the tidal bulges move around the ocean basins – Produce the world’s greatest tidal range in the Bay

of Fundy (tidal ranges is larger than 15m)

Tides: Tidal Currents

• Tidal current- occurs as the ocean water rises and falls with the tides, it flows toward & away from the coast. – Flood tide- when tidal current flows toward the coast– Ebb tide- when tidal current flows away from the

coast– Slack water- time between flood tide and ebb tide

with no tidal currents. • Tidal bore- surge of water that rushes upstream

where a river meets an ocean

Tide Videos

• http://youtu.be/kJOvxQElfLc• http://youtu.be/5W2sM1Ma7YA