Advanced Ocean Modelling: Using Open-Source Software

140 5 3D Level Modellingadjustment period. The Coriolis parameter is set to f = 1 × 10 −4 s −1 (NorthernHemisphere). This configuration gives an internal deformation radius of approximately7 km, implying that geostrophic adjustment can take place within lengthscales imposed by the strait’s geometry. Coriolis effects can certainly not be ignoredhere. The lateral grid spacing only marginally resolves dynamic processes on lengthscales on the internal deformation radius and some bias in the dynamics is to beexpected.Lateral eddy viscosity and eddy viscosity are given a constant value of A h = K h =1m 2 /s. The no-slip condition is used along coastlines. Vertical eddy viscosity andeddy diffusivity, assumed equal, are diagnosed from Kochergin’s turbulence closurescheme. The bottom-friction parameter is set to r = 0.001. The total simulationtime is 20 days with data outputs at every 4 hrs. Data outputs consist of surfaceand bottom distributions of density and lateral flow fields together with a verticaltransect of these variables across the middle of the strait at x = 100 km. The freesurfaceversion of the model is used with a time step of Δt = 10 s, which satisfies theCFL criterion for fast-propagating surface gravity waves (Eq. 5.14). The pressureaccuracy for the S.O.R. simulation is set to ɛ =0.01Pa.5.5.4 ResultsThe density forcing creates a lateral density contrast across the strait. This initiates abottom-arrested density-driven current moving denser bottom water from the westernbasin into the eastern basin (Fig. 5.12). Volume transports associated with thiscurrent lower the sea level in the western basin and lift the sea level in the easternbasin. The resultant sea-level gradient, in turn, triggers a westward surface returnflow 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 easternbasin, whereas the returning surface flow turns to the north as it enters the westernbasin.Later in this process, nonlinear interaction produces transient eddies inside thestrait, and a stationary barotropic eddy forms in the outflow region (Fig. 5.13). Thedynamics and density structure in the strait are highly transient varying from horizontallyaligned 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 geostrophicadjustments. Figure 5.14 shows a situation which agrees with expectations. At othertimes, however, the outflow can be intermittently concentrated near the surface withreturn flows occurring along both sides of the strait. Overall, the outflow of densewater gives rise of an anti-clockwise circulation pattern in the eastern basin. Inreality, dense outflows can travel vast distances along the continental slope such