Geoscience Reference
In-Depth Information
which the winds made their mark on the water mass. It was Ekman, in
1905, who rationalized the surface motions in the ocean following
observation of the trajectories of icebergs in the Arctic. He noted
indeed that the trajectory of the icebergs, whose center of mass is
essentially underwater, did not follow the axis of the wind, but veered
to the right of the windward direction. The wind drags the surface of
the ocean by a force of stress, in the axis of the wind, which spreads
by vertical diffusion in the layers beneath. In the interior ocean, the
diffusion forces are in equilibrium with Coriolis acceleration, and
when the full system of equations is resolved in an ocean with a
constant vertical diffusion, the resulting motion is characterized by a
spiral disappearing into the ocean while turning to the right, and
decreasing exponentially toward the deep ocean. This is Ekman's
spiral (Figure 2.8), which is the solution to the system of equations:
f
k
x
u
h
= ∂ {
A
v
∂
u
h
/∂
z
} /∂
z
with, as a boundary condition at the surface
z
=
s
:
A
v
∂
u
h
/∂
z
=
τ
/
ρ
,
where
A
v
designates the coefficient of vertical diffusion, and
τ
the
wind stress vector at the surface.
Northern hemisphere
Northern hemisphere
wind
wind
surface
current
surface current
integrated
transport
integrated
t
t
transport
surface
current
integrated
transport
Southern
hemisphere
Figure 2.8.
a) Diagram illustrating the oceanic Ekman spiral in response
to the wind stress at the surface; b) diagram illustrating the surface current
and the Ekman transport integrated in each hemisphere (from [FIE 10])
