Advanced Ocean Modelling: Using Open-Source Software

4.3 Exercise 17: Tidal-Mixing Fronts 109fronts) tend to become dynamically unstable and break up into mesoscale eddiesinducing vigorous lateral mixing across the front. This cannot be simulated with the2.5d model.4.3.4 Additional StudyPrognostic advection-diffusion equations for Eulerian tracer concentration fields areadded to the code to quantify the sources of water that make up the frontal zone. Tothis end, the author decided to use three separate tracer fields (Fig. 4.9). One field isallocated an initial concentration of 100% in the well-mixed regime and zero valueselsewhere. The other two tracer fields mark the surface and bottom layers of thestratified regime.Fig. 4.9 Exercise 17: Initial distribution of three separate Eulerian tracer fields4.3.5 Results and DiscussionAfter 24 hrs of simulation, the stratified regime has contributed 19% of near-bottomwater and 13% of near-surface water to the frontal zone (Fig. 4.10). The well-mixedregime outside the front has contributed 23% to the frontal zone. The remainder45% of water stems from other sources. The timescale of semi-diurnal tidal variations,mimicked here via variation of turbulence levels, is shorter than the inertialperiod (which is about 17.5 hrs for the model configuration), so that the geostrophicadjustment process remains incomplete. Instead of this, each period of enhancedtidal stirring is followed by a phase of gravitational adjustment. During this process,surface and bottom water of the stratified regime are displaced a few kilometerscloser to the shore and previously mixed water becomes drawn into the pycnoclineof the stratified regime. The result of this gravitational adjustment is a pumpingof both surface and bottom water from the stratified regime into the frontal zone.Convergence-induced upwelling of near-bottom water supports this process (seemiddle panel in Fig. 4.10). Findings, shown here, confirm the injection of nutrient