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)