Winddriven Circulation

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Wind-driven Circulation

• Ekman layer
• Divergence/convergence
• Geostrophy
• Major ocean currents

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Heat enters ocean-atmosphere in tropics, goes out
at poles
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Winds on a NON-rotating earth flow north-south
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(Pinet)
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(Pinet)
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Major ocean current systems
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Averaged global ocean topography from satellite
observations
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Coriolis Force
(Mann and Lazier)
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• Coriolis force is an apparent force
• Coriolis force ? 2? sin? u, where
• ? is the Earths rotation rate, i.e.,
• ? ? 2?/86400 sec-1 ,
• ? is the latitude, and
• u is the speed of the object
• Coriolis parameter f 2? sin?
• Coriolis force depends on latitude and speed of
  particle

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Northern hemisphere situation
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Inertial Motion in Baltic Sea Inertial Period of
14 hours (time it takes to complete a
circle) Inertial Period T2?/f
Latitude ?57.8oN f 2? sin?
2 (2?/86400) sin(57.8) 1.23 x 10-4 sec-1
Inertial Period 2?/f 14.18
hours In 7 days (15 tick marks) there are 11.8
inertial periods (count the crossings of the
trajectory)
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Wind Stress Direct effect of the wind does NOT
extend beyond the top 50-100 meters of the water
column
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Vectors not collinear Vectors still not
collinear Vectors collinear Wind drag Water
drag Coriolis 0
Initially, the velocity is in the direction of
the wind stress (or wind drag) But because the
forces are not balanced the water moves to right
of wind stress, until the vector addition of
forces is zero, at which time the velocity is at
45o to the right (in the Northern hemisphere) of
the wind stress (or wind drag)
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DE
Ekman layer depth DE?50-100m
(Pinet)
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Idealize the conditions in the red box...
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The red box...
Ekman Transport
Westerlies
Southward transport Northward transport
Convergence
Trades
At the convergence, water piles up and
sinks, thus depressing the thermocline and
deepening the nutricline!
Ekman transport is proportional to wind stress ?
greater transport for greater wind stress
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Convergence (sea surface pile-up)
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Sea surface
Pressure gradient force
Coriolis force
Velocity into page
Geostrophic Balance Pressure gradient force
balanced by Coriolis force requires that the
velocity be into the page, along the pressure
lines, not across them (opposite to our daily
experience of a ball rolling downhill).
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Geostrophy a frictionless balance between the
pressure gradient And the Coriolis acceleration
generates currents that move Around the hill
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Measured average topography of the North Atlantic
(red-high)
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• Important points to note
• The sea level pile-up is a result of the
  convergence of the Ekman transport
• The Ekman layer is ONLY 50-100 meters thick
• The resulting pressure gradient is felt
  throughout the water column
• Thus the geostrophic current occurs over a MUCH
  greater depth than the depth of the wind-driven
  layer, as much as the top 200 to 500 meters

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(Pinet)
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Major ocean current systems
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Atlantic temperature and salinity
500m
Thicker warm water layer at 30oN (also in
Southern Hemisph.)
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