Ocean Currents

Why is Ocean Circulation Important?

• Transport ~ 20% of latitudinal heat– Equator to poles

• Transport nutrients and organisms

• Influences weather and climate

• Influences commerce

Convection cell model

Non-rotating Earth

Add rotation and add landmasses unequal heating and cooling of the Earth

Physical properties of the atmosphere: Density

• Warm, low density air rises

• Cool, high density air sinks

• Creates circular- moving loop of air (convection cell)

Physical properties of the atmosphere: Water vapor

• Cool air cannot evaporate as much water vapor, so is typically dry

• Warm air can evaporate more water vapor, so is typically moist

• Water vapor decreases the density of air

Physical properties of the atmosphere: Pressure

ITCZ intertropical convergence zone= doldrumsLow pressure, wet climate

High pressure, dry climate

Low pressure, wet climate

30o

30o

60o

60o

90o

90o

0o

High pressure, dry climate

The Coriolis effect• The Coriolis effect

– Is a result of Earth’s rotation– Causes moving objects to follow

curved paths:• In Northern Hemisphere, curvature

is to right• In Southern Hemisphere, curvature

is to left– Changes with latitude:

• No Coriolis effect at Equator• Maximum Coriolis effect at poles

The Coriolis effect on Earth• As Earth rotates,

different latitudes travel at different speeds

• The change in speed with latitude causes the Coriolis effect

equator

Quito

Buffalo

79oW

North Pole

South Pole

N

Quito

Buffalo

equa

tor

Buffalo moves 783 mphQuito moves 1036 mph

15o

A) Idealized winds generated by pressure gradient and Coriolis Force.  B) Actual wind patterns owing to land mass distribution..

Surface Currents

The upper 400 meters of the ocean (10%).

Deep Water Currents

Thermal currents (90%)

Ocean Currents

Surface CurrentsForces

1. Solar Heating (temp, density)

2. Winds

3. Coriolis

Wind-driven surface currents

Wind-Driven and Density-Driven Currents

• Wind-driven currents occur in the uppermost 100 m or less

• Density differences causes by salinity and temperature produce very slow flows in deeper waters.

Sailors have know about ocean currents for centuries

Sailors have know that “rivers” flow in the seas since ancient times. They used them to shorten voyages, or were delayed by trying to stem them.

If navigators do not correct to deflection by currents, they may be far away from where they think they are and meet disaster.

Ben Franklin and the Gulf Stream

Matthew Fontaine MauryThe first systematic study

of currents was done by Maury based on logbooks in the US Navy’s Depot of Charts and Instruments.

His charts and “Physical Geography of the Sea” assisted navigators worldwide.

http://www.npg.si.edu/exh/brady/gallery/97gal.html

Winds and surface water

• Wind blowing over the ocean can move it due to frictional drag.

• Waves create necessary roughness for wind to couple with water.

• One “rule of thumb” holds that wind blowing for 12 hrs at 100 cm per sec will produce a 2 cm per sec current (about 2% of the wind speed)

Top-down drag

• Wind acts only on the surface water layer.• This layer will also drag the underlying water,

but with less force.• Consequently, there is a diminution of speed

downward.• Direction of movement is also influenced by

the Coriolis Effect and Ekman Spiral

Ekman spiralEkman spiral describes

the speed and direction of flow of surface waters at various depths

• Factors:– Wind– Coriolis effect

Ekman transportEkman transport is

the overall water movement due to Ekman spiral

• Ideal transport is 90º from the wind

• Transport direction depends on the hemisphere

Ekman Transport

Water flow in the Northern hemisphere- 90o to the right of the wind directionDepth is important

Currents in the “Real” Ocean

Currents rarely behave exactly as predicted by these theoretical explanations due to factors such as

• Depth—shallow water does not permit full development of the Ekman spiral

• Density—deeper currents moving in different directions influence the overlying surface movement

Geostrophic Flow Surface currents generally mirror average

planetary atmospheric circulation patterns

Gyres are large circular-moving loops of waterFive main gyres (one in each ocean

basin):• North Pacific• South Pacific• North Atlantic• South Atlantic• Indian

• Generally 4 currents in each gyre• Centered about 30o north or south

latitude

Current GyresCurrent Gyres

Geostrophic flow and western intensification

• Geostrophic flow causes a hill to form in subtropical gyres

• The center of the gyre is shifted to the west because of Earth’s rotation

• Western boundary currents are intensified

Figure 7-7

Western intensification of subtropical gyres

• The western boundary currents of all subtropical gyres are:– Fast– Narrow– Deep

• Western boundary currents are also warm• Eastern boundary currents of subtropical gyres

have opposite characteristics

Boundary Currents in the Northern Hemisphere

Type of Current General Features Speed Special Features

Western boundary Currents warm swift sharp boundary Gulf Stream, Kuroshio narrow w/coastal circulation,

deep little coastal upwelling

Eastern Boundary Currents cold slow diffuse boundariesCalifornia, Canary broad separating from coastal

shallow currents, coastal upwelling common

Pacific Ocean surface currents

“Hills and Valleys” in the Ocean• A balance between the

Ekman transport and Coriolis effect produces “hills” in the center of the gyres and “valleys” elsewhere

• Gravitational effects from sea floor features also produce variations in sea surface topography

http://earth.usc.edu/~stott/Catalina/Oceans.html

What do Nike shoes, What do Nike shoes, rubber ducks, and rubber ducks, and hockey gloves have to hockey gloves have to do with currents?do with currents?

Lost at Sea

•January 1992 - shipwrecked in the Pacific Ocean, off the coast of China

•November 1992 - half had drifted north to the Bering Sea and Alaska; the other half went south to Indonesia and Australia

•1995 to 2000 - spent five years in the Arctic ice floes, slowly working their way through the glaciers2001 - the duckies bobbed over the place where the Titanic had sunk

•2003 - they were predicted to begin washing up onshore in New England, but only one was spotted in Maine

•2007 - a couple duckies and frogs were found on the beaches of Scotland and southwest England.

Duckie Progress

• “Great Pacific Garbage Patch”

• Estimate: 46,000 pieces of floating garbage/mi2.

North Pacific Subtropical Gyre

North Pacific Subtropical Gyre

135° to 155°W and 35° to 42°N

North Pacific Subtropical Gyre

http://marinedebris.noaa.gov/info/patch.html#6

Great Pacific Garbage Patch- Good Morning America 2010 http://www.youtube.com/watch?v=uLrVCI4N67M&feature=player_embedded

Eddy

Warm core ring1. Rotates clockwise2. Found on the landward side of the current

Cold core ring (cyclonic eddy)1. Rotates counterclockwise2. Forms on the ocean side of the current

A circular movement of water formed along the edge of a permanent current

In an average year, 10-15 rings are formed

150-300 km in diameter

Speed 1 m/sec

Sargasso Sea

Upwelling and downwellingVertical movement of water (� )

– Upwelling = movement of deep water to surface

• Hoists cold, nutrient-rich water to surface• Produces high productivities and abundant

marine life– Downwelling = movement of surface water

down• Moves warm, nutrient-depleted surface water

down• Not associated with high productivities or

abundant marine life

upwelling

downwelling

Langmuir Circulation

Surface and Deep-Sea Current Interactions

Unifying concept: “Global Ocean Conveyor Belt”

http://seis.natsci.csulb.edu/rbehl/ConvBelt.htm

Heat Transport by Currents

• Surface currents play significant roles in transport heat energy from equatorial waters towards the poles

• May serve as “heat sources” to cooler overlying air, “heat sinks” from warmer

• Evaporation and condensation participate in latent heat exchanges

Matter Transport and Surface Currents

• Currents also involved with gas exchanges, especially O2 and CO2

• Nutrient exchanges important within surface waters (including outflow from continents) and deeper waters (upwelling and downwelling)

• Pollution dispersal• Impact on fisheries and other resources

Global ocean circulation that is driven by differences in the density of the sea water which is controlled by temperature and salinity.

                                     

                                                                                                                   

White sections represent warm surface currents. Purple sections represent deep cold currents

What effect does global warming play in

thermohaline circulation?

http://www.youtube.com/v/MZbsMlr9WRI?version=3

North Atlantic regional cooling

Global climate interconnections

CO2 fossil fuel combustion

Atmospheric and ocean temp

1 2 3 4

6 5

Subtropical evaporation

High latitude precipitation & runoff

Deep water formation & thermohaline circulation

Nordic seas salinity & deep convection

Potential feedback of increased

tropical salinity