Geoscience Reference
In-Depth Information
8
OCEAN CIRCULATION SYSTEMS
There are two fundamental drivers of ocean circulation systems: surface wind
and density variations.
8.1 THE WIND-DRIVEN CIRCULATION:
EKMAN DYNAMICS
Surface ocean currents are driven by surface winds, which transfer momentum
into the ocean through frictional acceleration. The uppermost layers of the
ocean then drag along the layers below. This frictional acceleration weakens
with increasing depth, and the depth at which it is zero is the base of the ocean
mixed layer. The essential physics of this process was developed by Swedish
oceanographer V.W. Ekman, and the depth over which wind-driven ocean cur-
rents are generated is known as the
Ekman layer
in his honor.
Picture a quiescent initial state in the Northern Hemisphere, with no wind
and an ocean surface at rest. Now imagine that a westerly wind begins to
blow and it activates the surface waters. As soon as a parcel of water begins to
move eastward (
u
> 0) under this frictional acceleration, it attains a southward
(negative) meridional acceleration component due to the Coriolis acceleration,
/ ~
dv dt f
< 0. As a result, the ocean surface current flows to the right of the
surface wind's direction for this Northern Hemisphere example (
f
> 0), as il-
lustrated in
Figure 8.1.
In the Southern Hemisphere (
f
< 0), the surface current
is directed to the left of the surface wind.
Now, surface waters drag along the water immediately below, and the ve-
locity of this lower layer is directed to the right of the surface water in the
Northern Hemisphere, and to the left in the Southern Hemisphere, due to Co-
riolis acceleration. As deeper layers are accelerated by the layers above them,
(a) Northern hemisphere
(b) Southern hemisphere
Friction force
(wind stress)
Current
direction
Coriolis force
Coriolis force
Current
direction
Friction force
(wind stress)
Figure 8.1. Directions of surface currents relative to surface winds in the
(a) Northern and (b) Southern Hemispheres.
