Upper ocean currents, Coriolis force, and Ekman Transport

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Upper ocean currents, Coriolis force, and Ekman
Transport

• In the open ocean mixed layer vertical structure
  was the key to biological productivity
• Mixing, light, stratification, critical depth
• In coastal regions and the equator, wind-driven
  horizontal currents can cause upwelling of water
  with high nutrients leading to sustained
  production over a season, or longer
• Wind stress Coriolis force giveEkman currents
  and upwelling

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From Lalli and Parson, Biological Oceanography
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Some physics
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Coriolis force In a fixed frame of reference the
ball travels in a straight line (Newtons
laws) In a rotating frame of reference (on the
table, or Earth), the ball appears to turn. In
the example, the merry-go-round is turning
clockwise and the ball turns toward the left.
This is the Southern Hemisphere effect. In the
Northern Hemisphere the local rotation is
counter-clockwise, and Coriolis force deflects
motion to the right.
N
S
http//marine.rutgers.edu/dmcs/ms320/coriolis.mov
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Figure 9.1 Inertial currents in the North Pacific
in October 1987 (days 275-300) measured by
holey-sock drifting buoys drogued at a depth of
15 meters. Positions were observed 10-12 times
per day by the Argos system on NOAA
polar-orbiting weather satellites and
interpolated to positions every three hours. The
largest currents were generated by a storm on day
277. Note these are not individual eddies. The
entire surface is rotating. A drogue placed
anywhere in the region would have the same
circular motion. From van Meurs (1998).
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• V0 is 45 to the right of the wind (in the
  northern hemisphere)
• V0 decreases exponentially with depth as it
  turns clockwise (NH)
• At depth z -DE the flow speed falls to e-p
  0.04 times the surface current and is in the
  opposite direction (typical DE is 20 to 40 m)

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Figure 9.3. Ekman current generated by a 10m/s
wind at 35N.
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Eastern Boundary Current program Progressive
vector diagram. Apr-Oct 1993
water
wind
Progressive vector diagram, using daily averaged
currents relative to the flow at 48 m, at a
subset of depths from a moored ADCP at 37.1N,
127.6W in the California Current, deployed as
part of the Eastern Boundary Currents
experiment. Daily averaged wind vectors are
plotted at midnight UT along the 8-m relative to
48-m displacement curve. Wind velocity scale is
shown at bottom left. (From Chereskin, T. K.,
1995 Evidence for an Ekman balance in the
California Current. J. Geophys. Res., 100,
12727-12748.)
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Equator-ward winds on ocean eastern
boundaries Pole-ward wind on ocean western
boundaries
Pole-ward winds on ocean eastern
boundaries Equator-ward wind on oceanwestern
boundaries
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http//marine.rutgers.edu/cool/research/upwelling.
html
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Estimating the upwelling velocity from NJ coast
data
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Wind speed and along-shelf currents at various
depths along the continental shelf off northwest
Africa
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