the oceanic general circulation
Post on 02-Jan-2016
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The Oceanic General Circulation
Regardless of hemisphere or ocean basin, there is an intense current on the western boundary
Gulf stream
Subtropics
Subtropics
Wind-driven ocean circulation
• Key questions:
– How does the wind drive upper ocean circulation?
– How does the interior ocean respond?
– Why is there a Gulf Stream?
Surface forcing: annual mean pattern
Upper ocean response to wind stress
δ ~ 10-100 m
Wind-driven upwelling keeps the SST low at eastern boundaries
Equatorial Ekman upwelling keeps the SST low right along the equator
Pumping and suction in the upper ocean
- Wind stress curl controls vertical motion
Global wind stress curl
The Taylor column model
• Interior ocean flow can be modeled as flow organized into rigid columns.
• Model properties:– Velocity vector cannot vary
with depth– Column cannot get wider or tilt– Responds to Ekman pumping
by expanding in length– Responds to Ekman suction
by contracting
Pumping at surface
Column gets longer
Taylor column tank demo
If there is no Ekman pumping or suction (as above), fluid columns must flow around obstacles
Taylor columns on a sphere
• Columns must stay parallel to Earth’s rotation axis (they cannot tilt)
• For spherical geometry, columns near the equator are longer
• Therefore, columns move equatorward as they stretch
Subtropical ocean gyres can be explained by Sverdrup theory
1)Wind-driven Ekman pumping pushes down the water column at the surface2)Due to the spherical geometry, equatorward flow dominates the interior ocean3)Return flow occurs at the western boundary, generating an intense northward flow
Why a western boundary current?
• Symmetric gyres are unstable on a rapidly rotating, spherical planet– One side of the gyre must
have a stronger boundary current than the other
• The general sense of circulation must match the anticyclonic sense of driving wind stress
Antarctic Circumpolar Current
Vertical structure of the Antarctic Circumpolar Current
The Southern Ocean is the only place on Earth where deep water is directly upwelling from a depth of 2000 m to the surface
End of lecture
Next week, we will continue to discuss ocean general circulation, focusing on the thermohaline (i.e. buoyancy-driven) circulation
New homework set on T-square!Due next Friday (9/28)
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