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 (barth@coas.oregonstate.edu)
NASA web site:
http://oceanmotion.org
Two types of Ocean Circulation:
Surface Circulation -- Wind-driven
Deep Circulation – Density-driven
Density of water is influenced by
Temperature and Salinity, so densitydriven circulation is often called the
“Thermohaline” Circulation
Friday’s lecture
Atmospheric Circulation
Temperature and Pressure
As the Earth’s surface is heated, air is
warmed, expands and rises (Low Pressure)
Warm air carries water vapor
In the upper atmosphere the air cools and
sinks (High Pressure)
Surface winds blow from High Pressure to
Low Pressure
This round-trip is called a “cell”
Things get interesting!
On a rotating planet, moving objects
appear to be deflected
Why is this?
Coriolis Deflection
Apparent force due to Earth’s rotation
Deflection in path of motion when viewed from
a rotating reference frame
Gustave-Gaspard Coriolis (1835)
Familiar from merry-go-rounds
Significant 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).
Coriolis Force
(northern
hemisphere)
velocity
Coriolis Force
(southern
hemisphere)
velocity
Coriolis 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
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 Circulation
westerlies
trades
trades
westerlies
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
Surface Ocean Circulation
Main Features
Five large gyres
Antarctic Circumpolar Current
Equatorial Countercurrent
Velocities vary -- fastest are meters/sec
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 kilometer
24 hrs x 3600 sec/hr = 86,400 sec~100,000 seconds per day
1 cm/second = 1 km/day
R. Lumpkin (NOAA/AOML)
106 m3/sec (Sverdrup) = all the rivers
Gulf Stream - Benjamin Franklin
1760s
Sailing times
to and from
Europe
Gulf Stream from satellite
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
Global Wind Circulation
westerlies
trades
trades
westerlies
Ekman Transport -- moves
water 90°to the winds
Ekman (1905)
Geostrophic Currents
Coriolis deflection plus the Pressure Gradient
steers the currents around the gyres
Northern Hemisphere Gyres
westward intensification
~1000
meters
Surface Circulation
Upwelling and Oregon’s Ocean
Winter winds from the south -downwelling
Summer winds from the north -upwelling
Winter
Summer
Oregon’s Summer
Thanks to Alan Dennis (COAS/OSU)
Cold, nutrient-rich water near the
Oregon coast: leads to
phytoplankton blooms
chl
(mg/m3)
T
(ºC)
Barth (2007)
Equatorial Divergence
Equatorial Divergence
Antarctic Circulation
How do we track ocean
circulation?
Fixed Buoys -- measure current
speed and direction
Drifters -- travel with the currents
and transmit their location
Beach Swap Meets!
Tracking Currents:
The Story of the Lost Nikes
 1: 60,000 shoes
spilled, May 1990
 2-8: 1990-’91
 9: 1993
 10: 1994
Marine Debris: Pacific Trash
What about the debris from
the recent Japanese tsunami?
AFP-Getty Images
US Navy photo
How long before debris might
reach the US west coast?
about 2 years for the first of it … but much will sink
and enter the North Pacific Garbage Patch
Courtesy of N. Maximenko & J. Hafner(UH)
Ocean Surface Circulation
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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
NASA web site:
http://oceanmotion.org