Geoscience Reference
In-Depth Information
The surface current is at 45° to the right of the windward direction
in the northern hemisphere, and at 45° to the left of the windward
direction in the southern hemisphere. More than the surface current,
we are often interested in the current integrated in the surface layer
where the wind dragging effect is evident - this is Ekman's layer, a
few tens of meters deep. The current within this layer is therefore at
90° of the windward direction and is called Ekman transport,
perpendicular to the wind at surface. It characterizes the main motions
of the surface layers of the ocean. Its expression is given by:
T
E
= ∫
He->
s
u
h
dz
where
T
E
designates the Ekman transport, integrated between the base
of the Ekman layer He to the surface
s
. The equation for the
momentum in the Ekman layer is integrated in this layer:
∫
He->
s
f
k
x
u
h
dz
= ∫
He->
s
∂ {
A
v
∂
u
h
/∂
z
} /∂
z
dz
From where:
f
k
x
T
E
=
τ
/
ρ
0
which allows us to calculate the Ekman transport:
T
E
=
τ
x
k
/
ρ
f
.
This expression of Ekman transport confers important properties
on it: Ekman transport is produced in the surface layer of oceans, it is
perpendicular to the windward direction, on the right in the
northern hemisphere and on the left in the southern hemisphere, and
its amplitude is inversely proportional to the Coriolis parameter
f
.
Thus, for a given wind, the Ekman transport is even more substantial
close to the Equator (Figure 2.8).
We will now focus on some particular properties resulting from
spatial variations in Ekman transport. To illustrate the ocean's
reaction, we will take account of the vertical structure of the ocean,
with a relatively thin and light surface layer, separated from the interior
ocean by a region very stratified in density or in temperature, with a
strong gradient called the thermocline (we will return to this notion
later in more detail). The surface ocean is reactive to the wind field,
