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Ocean Surface Circulation. Motion in the Ocean, Part I, or Why does the ocean have currents, and why do they move in circles?. Jack Barth ([email protected]). NASA web site: http://oceanmotion.org. Two types of Ocean Circulation:. Surface Circulation -- Wind-driven - PowerPoint PPT Presentation
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Ocean Surface Circulation
Ocean Surface Circulation
Motion in the Ocean, Part I, or Why does the ocean have currents, and
why do they move in circles?
Motion in the Ocean, Part I, or Why does the ocean have currents, and
why do they move in circles?
Jack Barth ([email protected])
NASA web site: http://oceanmotion.org
Two types of Ocean Circulation:
Two types of Ocean Circulation:
Surface Circulation -- Wind-drivenDeep Circulation – Density-driven
Density of water is influenced by Temperature and Salinity, so density- driven circulation is often called the“Thermohaline” Circulation
Surface Circulation -- Wind-drivenDeep Circulation – Density-driven
Density of water is influenced by Temperature and Salinity, so density- driven circulation is often called the“Thermohaline” Circulation
Friday’s lecture
Atmospheric CirculationAtmospheric Circulation
Temperature and PressureTemperature and Pressure
As the Earth’s surface is heated, air is warmed, expands and rises (Low Pressure)
Warm air carries water vaporIn the upper atmosphere the air cools and
sinks (High Pressure)Surface winds blow from High Pressure to
Low PressureThis round-trip is called a “cell”
As the Earth’s surface is heated, air is warmed, expands and rises (Low Pressure)
Warm air carries water vaporIn the upper atmosphere the air cools and
sinks (High Pressure)Surface winds blow from High Pressure to
Low PressureThis round-trip is called a “cell”
Things get interesting!Things get interesting!
On a rotating planet, moving objects appear to be deflected
Why is this?
On a rotating planet, moving objects appear to be deflected
Why is this?
Coriolis Deflection Coriolis Deflection Apparent force due to Earth’s rotationDeflection in path of motion when viewed
from a rotating reference frameGustave-Gaspard Coriolis (1835)Familiar from merry-go-roundsSignificant only for large distances
(not toilets and billiards!)
animation
Apparent force due to Earth’s rotationDeflection in path of motion when viewed
from a rotating reference frameGustave-Gaspard Coriolis (1835)Familiar from merry-go-roundsSignificant only for large distances
(not toilets and billiards!)
animation
So, in the frame rotating CCW (like northern hemisphere), unforced particle in motion is deflected to the right.
If frame rotates CW, motion of particle is to the left (reverse film).
velocity
Coriolis Force (northern hemisphere)
velocity
Coriolis Force (southern hemisphere)
Coriolis DeflectionCoriolis Deflection
“During the naval engagement near the Falkland Islands which occurred early in World War I, the British gunners were surprised to see their accurately aimed salvos falling 100 yards to the left of the German ships. The designers of the sighting mechanisms were well aware of the Coriolis deflection and had carefully taken this into account, but they apparently were under the impression that all sea battles took place near 50°N latitude, and never near 50°S latitude. The British shots, therefore, fell at a distance from the targets equal to twice the Coriolis deflection.”
Jerry B. Marion, “Classical Dynamics of Particles and Systems”, 2nd edition, 1971.
Consequences of Coriolis
Consequences of Coriolis
Moving fluids (atmosphere and ocean) turn to the right in the Northern Hemisphere
Moving fluids (atmosphere and ocean) turn to the left in the Southern Hemisphere
Moving fluids (atmosphere and ocean) turn to the right in the Northern Hemisphere
Moving fluids (atmosphere and ocean) turn to the left in the Southern Hemisphere
Global Wind CirculationGlobal Wind Circulation
westerlies
trades
trades
westerlies
Wind-Driven Ocean Circulation
Wind-Driven Ocean Circulation
Steady winds produce waves and set the surface water in motion
Moving water is deflected to the right (N.Hemisphere) or left (S.Hemisphere)
This starts the main “gyre” motion of the surface ocean
Steady winds produce waves and set the surface water in motion
Moving water is deflected to the right (N.Hemisphere) or left (S.Hemisphere)
This starts the main “gyre” motion of the surface ocean
Surface Ocean CirculationSurface Ocean Circulation
Main FeaturesMain Features
Five large gyresAntarctic Circumpolar CurrentEquatorial CountercurrentVelocities vary -- fastest are
meters/sec
Five large gyresAntarctic Circumpolar CurrentEquatorial CountercurrentVelocities vary -- fastest are
meters/sec
Ocean Surface Current Speed
Ocean Surface Current Speed
cm/second
How fast is a cm/second?100 centimeters in a meter; 1000 meters in a kilometer
so 100,000 centimeters per kilometer24 hrs x 3600 sec/hr = 86,400 sec~100,000 seconds per day
1 cm/second = 1 km/dayR. Lumpkin (NOAA/AOML)
106 m3/sec (Sverdrup) = all the rivers
106 m3/sec (Sverdrup) = all the rivers
Gulf Stream - Benjamin FranklinGulf Stream - Benjamin Franklin
1760sSailing times
to and from Europe
1760sSailing times
to and from Europe
Gulf Stream from satelliteGulf Stream from satellite
So, do the gyres just follow the winds?
So, do the gyres just follow the winds?
Not exactly! But the winds get the motion in the ocean started
The oceans respond by flowing and turning
Water piles up in the center of gyres -- several meters high
Not exactly! But the winds get the motion in the ocean started
The oceans respond by flowing and turning
Water piles up in the center of gyres -- several meters high
Global Wind CirculationGlobal Wind Circulation
westerlies
trades
trades
westerlies
Ekman Transport -- moves water 90°to the winds
Ekman Transport -- moves water 90°to the winds
Ekman (1905)
Geostrophic CurrentsGeostrophic Currents
Coriolis deflection plus the Pressure Gradient steers the currents around the
gyres
Coriolis deflection plus the Pressure Gradient steers the currents around the
gyres
Northern Hemisphere Gyres
westward intensification
Northern Hemisphere Gyres
westward intensification
~1000meters
Surface CirculationSurface Circulation
Upwelling and Oregon’s Ocean
Upwelling and Oregon’s Ocean
Winter winds from the south -- downwelling
Summer winds from the north -- upwelling
Winter winds from the south -- downwelling
Summer winds from the north -- upwelling
Winter SummerWinter Summer
Oregon’s Summer Oregon’s Summer
Thanks to Alan Dennis (COAS/OSU)
Cold, nutrient-rich water near the Oregon coast: leads to
phytoplankton blooms
Cold, nutrient-rich water near the Oregon coast: leads to
phytoplankton blooms
Barth (2007)
T(ºC)
chl (mg/m3)
Equatorial Divergence Equatorial Divergence
Equatorial Divergence Equatorial Divergence
Antarctic Circulation Antarctic Circulation
How do we track ocean circulation?
How do we track ocean circulation?
Fixed Buoys -- measure current speed and direction
Drifters -- travel with the currents and transmit their location
Fixed Buoys -- measure current speed and direction
Drifters -- travel with the currents and transmit their location
Beach Swap Meets!Beach Swap Meets!
Tracking Currents:The Story of the Lost Nikes
Tracking Currents:The Story of the Lost Nikes 1: 60,000 shoes
spilled, May 1990 2-8: 1990-’91 9: 1993 10: 1994
1: 60,000 shoes spilled, May 1990
2-8: 1990-’91 9: 1993 10: 1994
Marine Debris: Pacific Trash
Marine Debris: Pacific Trash
What about the debris from the recent Japanese tsunami?What about the debris from
the recent Japanese tsunami?
US Navy photo
AFP-Getty Images
How long before debris might reach the US west coast?
How long before debris might reach the US west coast?
North Pacific Current
~ 10 cm/s ~ 10 km/day
~7300 km
Courtesy of N. Maximenko & J. Hafner(UH)
about 2 years for the first of it … but much will sink and enter the North Pacific Garbage Patch
Ocean Surface CirculationOcean Surface Circulation• surface currents driven by winds• Coriolis and pressure forces result in
oceanic gyres• wind-driven currents reach down
several 100s of meters up to 1km• speeds of 10-100 cm/s (0.1-1.0 m/s
~ 0.2-2 knots); strongest on western sides of ocean basins
• Ekman flow away from coast leads to coastal upwelling and plankton blooms
• surface currents driven by winds• Coriolis and pressure forces result in
oceanic gyres• wind-driven currents reach down
several 100s of meters up to 1km• speeds of 10-100 cm/s (0.1-1.0 m/s
~ 0.2-2 knots); strongest on western sides of ocean basins
• Ekman flow away from coast leads to coastal upwelling and plankton bloomsNASA web site: http://oceanmotion.org