Western intensification
149
Summary
For a constant density ocean, the dominant (Sverdrup) balance in the
interior of the domain is given by
β 0 v ∗ =
1
ρD
(
∂τ
y
∗
∂x ∗
−
∂τ x
∗
∂y ∗
)=
f 0
D
w E∗ ,
This is a vorticity balance where fluid parcels move north-south such
that the β-induced vorticity change of this motion is compensating the
wind-induced vorticity input.
The continental boundary layers are only able to compensate the Sverdrup transport in the western part of the basin. The boundary layer
thicknesses are either
δ I =(
U
β 0
)
1/2 ,δ S =
δ E f 0
2Dβ 0
,δ M =(
A H
β 0
)
1/3 .
depending on the dominant vorticity balance in the western boundary
layer. Here U is set by the Sverdrup velocity scale
U =
τ 0
ρDβ 0 L
The boundary layer thicknesses δ M and δ S are set by lateral and bottom friction and are called the Munk and Stommel boundary layer
thickness, respectively.
The barotropic quasi-geostrophic theory of the wind-driven ocean circulation is a first step to understand the midlatitude gyres and the western intensification of boundary currents. It explains that the β-effect is
responsible for the strong east-west symmetry of the midlatitude ocean
currents and that inertial effects can induce north-south asymmetry in
the flow.