140
5 3D Level Modelling
adjustment period. The Coriolis parameter is set to f = 1 × 10
−4 s
−1 (Northern
Hemisphere). This configuration gives an internal deformation radius of approximately 7 km, implying that geostrophic adjustment can take place within length
scales imposed by the strait’s geometry. Coriolis effects can certainly not be ignored
here. The lateral grid spacing only marginally resolves dynamic processes on length
scales on the internal deformation radius and some bias in the dynamics is to be
expected.
Lateral eddy viscosity and eddy viscosity are given a constant value of A h = K h =
1 m
2 /s. The no-slip condition is used along coastlines. Vertical eddy viscosity and
eddy diffusivity, assumed equal, are diagnosed from Kochergin’s turbulence closure
scheme. The bottom-friction parameter is set to r = 0.001. The total simulation
time is 20 days with data outputs at every 4 hrs. Data outputs consist of surface
and bottom distributions of density and lateral flow fields together with a vertical
transect of these variables across the middle of the strait at x = 100 km. The freesurface version of the model is used with a time step of Δt = 10 s, which satisfies the
CFL criterion for fast-propagating surface gravity waves (Eq. 5.14). The pressure
accuracy for the S.O.R. simulation is set to = 0.01 Pa.
5.5.4 Results
The density forcing creates a lateral density contrast across the strait. This initiates a
bottom-arrested density-driven current moving denser bottom water from the western basin into the eastern basin (Fig. 5.12). Volume transports associated with this
current lower the sea level in the western basin and lift the sea level in the eastern
basin. The resultant sea-level gradient, in turn, triggers a westward surface return
flow through the strait.
The Coriolis force becomes a dominant force within a timescale of a few days.
Geostrophic adjustment along density fronts follows. As a result of this adjustment, the bottom flow of denser water turns to the south as it enters the eastern
basin, whereas the returning surface flow turns to the north as it enters the western
basin.
Later in this process, nonlinear interaction produces transient eddies inside the
strait, and a stationary barotropic eddy forms in the outflow region (Fig. 5.13). The
dynamics and density structure in the strait are highly transient varying from horizontally aligned density surfaces to situations of vertically aligned density surfaces.
The reason behind this intermittency are transient pulses of dense water outflows,
dynamical instabilities, and a complex combination of gravitational and geostrophic
adjustments. Figure 5.14 shows a situation which agrees with expectations. At other
times, however, the outflow can be intermittently concentrated near the surface with
return flows occurring along both sides of the strait. Overall, the outflow of dense
water gives rise of an anti-clockwise circulation pattern in the eastern basin. In
reality, dense outflows can travel vast distances along the continental slope such
5 3D Level Modelling
adjustment period. The Coriolis parameter is set to f = 1 × 10
−4 s
−1 (Northern
Hemisphere). This configuration gives an internal deformation radius of approximately 7 km, implying that geostrophic adjustment can take place within length
scales imposed by the strait’s geometry. Coriolis effects can certainly not be ignored
here. The lateral grid spacing only marginally resolves dynamic processes on length
scales on the internal deformation radius and some bias in the dynamics is to be
expected.
Lateral eddy viscosity and eddy viscosity are given a constant value of A h = K h =
1 m
2 /s. The no-slip condition is used along coastlines. Vertical eddy viscosity and
eddy diffusivity, assumed equal, are diagnosed from Kochergin’s turbulence closure
scheme. The bottom-friction parameter is set to r = 0.001. The total simulation
time is 20 days with data outputs at every 4 hrs. Data outputs consist of surface
and bottom distributions of density and lateral flow fields together with a vertical
transect of these variables across the middle of the strait at x = 100 km. The freesurface version of the model is used with a time step of Δt = 10 s, which satisfies the
CFL criterion for fast-propagating surface gravity waves (Eq. 5.14). The pressure
accuracy for the S.O.R. simulation is set to = 0.01 Pa.
5.5.4 Results
The density forcing creates a lateral density contrast across the strait. This initiates a
bottom-arrested density-driven current moving denser bottom water from the western basin into the eastern basin (Fig. 5.12). Volume transports associated with this
current lower the sea level in the western basin and lift the sea level in the eastern
basin. The resultant sea-level gradient, in turn, triggers a westward surface return
flow through the strait.
The Coriolis force becomes a dominant force within a timescale of a few days.
Geostrophic adjustment along density fronts follows. As a result of this adjustment, the bottom flow of denser water turns to the south as it enters the eastern
basin, whereas the returning surface flow turns to the north as it enters the western
basin.
Later in this process, nonlinear interaction produces transient eddies inside the
strait, and a stationary barotropic eddy forms in the outflow region (Fig. 5.13). The
dynamics and density structure in the strait are highly transient varying from horizontally aligned density surfaces to situations of vertically aligned density surfaces.
The reason behind this intermittency are transient pulses of dense water outflows,
dynamical instabilities, and a complex combination of gravitational and geostrophic
adjustments. Figure 5.14 shows a situation which agrees with expectations. At other
times, however, the outflow can be intermittently concentrated near the surface with
return flows occurring along both sides of the strait. Overall, the outflow of dense
water gives rise of an anti-clockwise circulation pattern in the eastern basin. In
reality, dense outflows can travel vast distances along the continental slope such