Ocean Currents - · PDF filePhysical properties of the atmosphere: Density • Warm, low...
Transcript of Ocean Currents - · PDF filePhysical properties of the atmosphere: Density • Warm, low...
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