steady state general ocean circulation
DESCRIPTION
Steady State General Ocean Circulation. “steady state” means: constant in time, no accelerations. or. Sum of all forces = 0. Outline:1. Ekman dynamics (Coriolis~Friction) 2. Geostrophic dynamics (Coriolis~Pressure gradients) 3. Ekman+Geostrophy with Coriolis as f=f 0 + b y . - PowerPoint PPT PresentationTRANSCRIPT
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Steady State General Ocean Circulation
“steady state” means: constant in time, no accelerations
or
Sum of all forces = 0
Outline: 1. Ekman dynamics (Coriolis~Friction)
2. Geostrophic dynamics (Coriolis~Pressure gradients)
3. Ekman+Geostrophy with Coriolis as f=f0+y
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The subtropical
gyre circulation
is a geostrophic flow (with
many eddies)
From WHP Pacific Atlas (Talley, 2007)
http://www-pord.ucsd.edu/whp_atlas
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Geostrophy
Coriolis balances pressure gradient
Geostrophic Degeneracy
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Ekman balance: Coriolis ~ Friction QuickTime™ and a decompressorare needed to see this picture.
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from Stewart, 2005
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a-1=(2Az/f) is a vertical decay scale~ 20m-60m
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Ekman velocity spiral• Surface velocity to the right of the wind (northern hemisphere, due to Coriolis)• Surface layer pushes next layer down slightly the right, and slightly weaker current• Next layer pushes next layer, slightly to right and slightly weaker current• Producing a “spiral” of the current vectors, to right in northern hemisphere,
decreasing speed with increasing depth• Details of the spiral depend on the vertical viscosity (how frictional the flow is, and
also whether “friction” depends on depth)
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Ekman transportThe wind stress on the ocean surface is the vector
= ( (x) , (y) )
Integrate the Coriolis/friction balances in the verticalx: -fv = /z(AVu/z) -> -fVEK= AVu/z = (x) /y: fu = /z(AVv/z) -> fUEK= AVv/z = (y) / •UEK and VEK are the “Ekman transport” ∫udz, ∫vdz
•Ekman “transport” is exactly to the right of the wind stress (northern hemisphere ).
•Ekman transport does not depend on the size or structure of AV (but the detailed structure of the spiral DOES depend on it)
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Ekman layer “transport”• “Transport”: 90° to wind, to right in northern hemisphere • UEk= /f (units are m2/s, not m3/s so technically this is not a transport;
need to sum horizontally along a section to get a transport).
• Typical size: for wind stress 0.1 N/m2, UEk= 1 m2/s. Integrate over width of ocean, say 5000 km, get total transport of 5 x 106 m3/sec = 5 Sv.
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Ekman layer depth• Depth: depends on eddy viscosity AV (why?)
Dek = (2AV/f)1/2
• Eddy viscosity is about 0.05 m2/sec in turbulent surface layer, so Ekman layer depth is 20 to 60 m for latitudes 80° to 10°.
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Ekman layer velocity• Velocity: spirals with depths and magnitude depends on
eddy viscosity. If AV is constant, surface velocity is 45° to wind
• For eddy viscosity 0.05 m2/sec, and wind stress of 1 dyne/cm2 (.1 N/m2), surface velocity is 3 cm/sec at 45°N.
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Observations of Ekman layer
Direct current measurements in California Current region revealed excellent Ekman-type spiral (Chereskin, JGR, 1995)
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Global surface wind velocity
Westward Eastward
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Ekman divergence (Ekman upwelling) at equator and at land boundaries
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Equatorial Land boundary(southern hemisphere, like Peru)
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Ekman transport convergence and divergence
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Ekman transport convergence and divergence
Vertical velocity at base of Ekman layer: order (10-4cm/sec)
(Compare with typical horizontal velocities of 1-10 cm/sec)
EkmanPumping
Geostrophicflow
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Pacific winds (mean) • Ekman pumping Gray: downwelling White: upwelling
from Talley 2006
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Ekman Pumping
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continuity
vertical integral
Ekman transports
Convergence inEkman transport
Vertical velocityEkman pumping
Recall MEx = y/f Ekman transport 90 degrees to windMEy = -x/f