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. Non-rotating Earth. Convection cell model. Add rotation and add landmasses
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Why is Ocean Circulation Important? • Transport ~ 20% of latitudinal heat • Equator to poles • Transport nutrients and organisms • Influences weather and climate • Influences commerce
Non-rotating Earth Convection cell model
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 hold much water vapor, so is typically dry • Warm air can hold more water vapor, so is typically moist • Water vapor decreases the density of air
90o High pressure, dry climate 60o Low pressure, wet climate High pressure, dry climate 30o ITCZ intertropical convergence zone= doldrums Low pressure, wet climate 0o 30o 60o 90o
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
North Pole Buffalo moves 783 mph Quito moves 1036 mph Buffalo 15o N equator Quito Buffalo equator 79oW Quito South Pole
Idealized winds generated by pressure gradient and Coriolis Force. • Actual wind patterns owing to land mass distribution..
Ocean Currents • Surface Currents • The upper 400 meters of the ocean (10%). • Deep Water Currents • Thermal currents (90%)
Surface Currents • Forces • Solar Heating (temp, density) • Winds • Coriolis
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.
Matthew Fontaine Maury The 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 spiral Ekman spiral describes the speed and direction of flow of surface waters at various depths • Factors: • Wind • Coriolis effect
Ekman transport Ekman 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 direction Depth 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
Current Gyres • Gyres are large circular-moving loops of water • Five 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
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 boundaries California, Canary broad separating from coastal shallow currents, coastal upwelling common
“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, rubber ducks, and hockey gloves have to do with currents?
Duckie Progress • 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.
2004-2007 Barber’s Point
North Pacific Subtropical Gyre • “Great Pacific Garbage Patch” • Estimate: 46,000 pieces of floating garbage/mi2.
North Pacific Subtropical Gyre 135° to 155°W and 35° to 42°N
North Pacific Subtropical Gyre Great Pacific Garbage Patch- Good Morning America 2010 http://www.youtube.com/watch?v=uLrVCI4N67M&feature=player_embedded http://marinedebris.noaa.gov/info/patch.html#6
Eddy • 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 • Warm core ring • Rotates clockwise • Found on the landward side of the current • Cold core ring (cyclonic eddy) • Rotates counterclockwise • Forms on the ocean side of the current
Upwelling and downwelling Vertical 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
Satellite Observations • TOPEX/Poseidon, Jason 1, and other satellites have observed patterns of change over the past few years • Animation of seasonal and climatically-influence shifts available at http://seawifs.gsfc.nasa.gov/OCEAN_PLANET/MOVIES/Topex_Dynamic_Ocean_Topography.mpg
El Niño-Southern Oscillation (ENSO) • El Niño = warm surface current in equatorial eastern Pacific that occurs periodically around Christmastime • Southern Oscillation = change in atmospheric pressure over Pacific Ocean accompanying El Niño • ENSO describes a combined oceanic-atmospheric disturbance
El Niño • Oceanic and atmospheric phenomenon in the Pacific Ocean • Occurs during December • 2 to 7 year cycle • Sea Surface Temperature • Atmospheric Winds • Upwelling