Geoscience Reference
In-Depth Information
It was at the end of the 1940s that Sverdrup was able to understand
the internal behavior of the ocean as he put forward a theory that
would later carry his name. Sverdrup was interested in the circulation
of the interior ocean, beneath the mixed layer. He considered a basin
stretching from the tropics to mid-latitudes with, in its northern part,
winds from the west and in its southern part, winds from the east,
which corresponds to a negative vorticity, and causes a downward
Ekman pumping, at the interface between the surface layer and the
interior ocean. Taking account of the principle of conservation of
the total vorticity on the sphere - the total vorticity is the sum of the
relative vorticity of the fluid and the planetary vorticity
f
, this supply
of negative vorticity imposes a southward displacement on the fluid,
that is to say toward a latitude where the planetary vorticity
f
is
weaker. He showed that in the wind field of negative vorticity, the
fluid masses of the interior ocean move toward the Equator and
conversely, in a wind field of positive vorticity, they move toward the
Poles.
R
EMARK
.- What is vorticity? It is the rotation capacity of a particle.
Its formal expression is written as:
ζ
=
curl u
h
.
k
= ∂
v
/∂
x
- ∂
u
/∂
y
The total vorticity is the sum:
ζ
t
=
ζ
+
f
One important consequence of Sverdrup's theory concerns the
large western boundary currents (Figure 2.13). If, in an interior basin,
the fluid mass moves southward, as the total mass is conserved at a
given latitude, it is indeed necessary to find a northward movement to
compensate and this is ensured by the western boundary currents such
as the instance of the Gulf Stream, which rises in a longshore current,
the equivalent of all the mass displaced southward in the
entire interior basin. This western boundary current is made possible
by the very energetic character of its interactions with the western
shore, which engender the positive vorticity necessary for this
displacement.
