Ocean Currents

Current: large mass of continuously moving ocean water

• These waters make up about 10% of all the water

in the ocean.

• These waters are the upper 400 meters of the ocean.

• Wind driven (friction between water & wind)

• Horizontal

• These waters make up the other 90% of the ocean

• Caused by density and temperature differences

• Affect living organisms by mixing water

• Vertical

Surface Currents--Wind Circulation

Deep Water Currents--Thermohaline Circulation

Model of Circulation

Global Deep Ocean Circulation

Primary Forces -- start the water movingThe primary forces are:

1. Solar heating

2. Winds

3. Gravity

4. Coriolis

• South Equatorial Current - east to west; south of the equator

Examples of Currents

Gulf Stream:• East coast of US• South to north from Gulf of Mexico and across

Atlantic• Avg speed 8km/hr• 160km wide/100m deep• Surface temps 24C or 75°F• First charted by Ben Franklin and

nephew for colony trade with England

Becomes North Atlantic Current

• Warms coastal waters of England & Ireland by 15C

• Warmer summer/milder winters

• Warm moist air meets northern colder air = rain/fog for those countries

California Current:• Flows north to south• Colder waters

Water currents are deflected due to the Coriolis Effect

Eckman Spiral

• Lower layers of the ocean pushed by the friction

between the water layers above.

• The lower layer moves slower than the layer

above.

• With each successive layer down in the water

column the speed is reduced.

•This leads to the Spiral Affect

•The net movement of water is 90o to the right of the wind direction (in the northern hemisphere).

The North Atlantic Gyre

Continents and Coriolis Effect steer ocean currents

Causes them to move in large circles called Gyres

In North, moves in clockwise, in South, counter-clockwise

Importance to Marine Life:

Move plankton thousands of km across ocean

Tropical organisms found at higher latitudes on the western side of ocean basins, cooler water organisms can be found further down on eastern side (ex. Galapagos Islands)

Galapagos

Gyre Flow in the North Atlantic—sea surface height at center of gyre is slightly higher in left side of

Atlantic (west)—faster, narrower, warmer current

Western and Eastern Parts of Ocean Basins

North Atlantic Circulation

Note the fast current between Cuba and south

Florida. This is why rafters from Cuba

wind up off the Florida coast near

Miami!

Vertical Currents:

Caused by salinity and temperature differences

Globally - thermohaline circulation

Salinity

Ex. Gibraltar Current

Mediterranean Sea – 39ppt salinity, very dense

Atlantic Ocean – 35ppt salinity, less dense

39%

35%

Mediterranean water sinks; Atlantic water flows into surface of Mediterranean

During WWII – subs used current to drift undetected into the Atlantic

Temperature:

Cold water sinks, warm water rises (convection)

Water sinks at the poles and rises at the equator

Along with wind, helps to transfer uneven solar heating

Upwelling:• Vertical current rises to surface• Nutrient rich…

• Phosphates and nitrates (important for plankton growth)

• Excellent fishing grounds (lots of plankton)• Ex. Peruvian Coast (west coast of South America)• Opposite of Upwelling is Downwelling

Equatorial Upwelling

Additional Currents

Turbidity Current:• Fast current down continental slope• Up to 80km/hr• Cloudiness due to silt, mud, and clay• Due to steep slope• Cause of submarine canyons• Ex. Hudson and Baltimore canyons off Atlantic

coast

Wave and Tide Induced Currents

Undertow: • Returning current (backwash) as waves break on beach• Can cause you to lose footing and be pulled under

Rip Current:• Sand eroded by wave action• Sand bar forms parallel to beach• Acts like a dam holding accumulated water• Too much water accumulates and sandbar breaks• Water rushes in a narrow current seaward

El Nino: • Spanish name for a male child• Initially referred to a weak, warm current appearing

annually around Christmas time along the coast of Ecuador and Peru

• Not good for their fishing industry and guano birds (cormorant, booby, and pelican)

• Can produce significant economic and atmospheric consequences worldwide

• Occur every 3-7 years, lasting about one year• Event in 1997-1998 was largest ever recorded

El Nino - the eastward movement of warm water from the western equatorial pacific to the eastern equatorial pacific.

Sea Surface Temperatures (SSTs): (During a normal year)

• A large pool of warm water is in the western Pacific

• Low pressure dominates in the western Pacific

over the warm water, high pressure dominates in

the eastern Pacific over the colder water

• The trade winds blow east to west, piling water westward and pushing water away from the equator.

• ~ 0.5 meter difference in sea level height from west to east.

• Cold nutrient-rich water is closer to the surface in a normal year due to equatorial upwelling.

• Cold tongue of water along the equator off of the coast of Peru (upwelling zone) which extends into the Pacific Ocean – highly productive area.

Eastern Pacific Ocean: “Normal” Conditions

Sea Surface Temperatures (SSTs): (During an El Nino year)

• SSTs in the eastern Pacific become warmer than normal

• High surface pressure shifts from the eastern Pacific to the western Pacific

• Low pressure shifts from the western Pacific to the eastern Pacific

Eastern Pacific: El Niño Conditions•Wind convergence pattern changes.•The surface low migrates eastward into the Pacific Ocean away from Australia.•The pressure gradient which normally drives the trade winds no longer exists, the trade winds ease, sometimes stop or reverse direction.•The sea surface tilt (high in Darwin, low in Tahiti) relaxes.

Eastern Pacific: El Niño Conditions

• The cold nutrient-rich tongue of water off the coast of Peru disappears.

• The thermocline in the eastern Pacific deepens, cutting off the upwelling currents along the coast of Peru and North America.

• Precipitation forms farther east – tropical western coast of Central America is wet, the western Pacific is dry (drought in Australia).

This shift in surface pressure is called the

El Nino Southern Oscillation

(ENSO)ENSO has a large impact on the precipitation

distribution around the Pacific basin

1982-1983: Strong El Niño• In November of 1982, 20,000 nesting Frigate birds were

counted on islands off the coast of Peru.

• In March of 1983, there were only 100 nesting Frigate birds on the same islands.

• Dead adult Frigate birds began to wash up on the coast of South America in the spring of 1983.

• In 1982-1983, all of the seal pups born on the Galapagos Islands died due to lack of food.

World-Wide Effects: El Niño “Teleconnections”

• Drought and brush fires in Australia

• Crop failure due to lack of rain/human starvation in Indonesia and the Philippines

• Floods and land slides in Peru and Ecuador

• Drought and water shortages in India and Sri Lanka (Ceylon)

• Increased number/strength of Pacific cyclones and a decrease in the number/intensity of Atlantic hurricanes

• Fires in Borneo and Sumatra

• Coral reef deaths across the western Pacific

• Downpours and flooding in the USA gulf coast states

• Drought, disease, malnutrition in South Africa

• Flooding, mud slides in the US Colorado River Basin

World-Wide Effects: El Niño “Teleconnections”

Global Precipitation & Temperature Anomalies due to El Nino

• After the El Nino phase, La Nina follows

• La Nina refers to the unusually cold water that is

found in the eastern Pacific ocean

• In the U.S., winter temperatures are warmer than normal in the Southeast, and cooler than normal in the Northwest.