Advanced Ocean Modelling: Using Open-Source Software

5.4 Exercise 21: Eddy Formation in a Strait 135net. The General Bathymetric Chart of the Oceans (GEBCO) found at http://www.gebco.net would be a useful data source. Instead of this, the author wishes to demonstratethe reader an alternative way to create an idealised bottom topography usingbathymetric charts as visual template. Section 5.4.4 details the method creating thebathymetry shown in Fig. 5.7.The model domain is 360 km long and 180 km across resolved by a lateralisotropic grid spacing of 3.6 km. The objective here is to be able to just resolve theeddy scale of 30 km with a sufficient number of grid points while the model domainis large enough to capture several eddies. The islands Hokkaido and Sakhalin appearas rectangular blocks, which is sufficient for the purpose of the study. The model’sSoya Strait has a width of about 60 km and a maximum depth of 60 m. Water depthin the Sea of Okhotsk is limited to 200 m to allow for relatively large numericaltime steps. The vertical grid spacing is set to 20 m. The Coriolis parameter is set to1 × 10 −4 s −1 , corresponding to an inertial period of 17.45 hrs.For simplicity, density is assumed uniform and the existence of sea ice is ignored.The model is forced via prescription of an inflow from the Japan Sea with anarbitrarily chosen speed of 20 cm/s. This speed is applied to velocity componentsdirected normal to the open boundary. Parallel velocity components are kept at zerovalue. To avoid initial disturbances, the inflow speed is gradually adjusted to its finalvalue over the first 2 days of simulation. Zero-gradient conditions are used for othervariables at inflow boundaries. At the downstream open boundary, zero-gradientconditions are used for all variables.Lateral momentum diffusion with uniform eddy diffusivity of A h =5m 2 /s isused in conjunction with no-slip boundary conditions for flow parallel to coastlines.This condition is implemented via appropriate settings of the velocity value on thefirst inland grid point (see Fig. 5.8 for an example). To avoid problems in the oceaninterior, this condition is only used in grid cells adjacent to coastlines. No-slip conditionsare used adjacent to steps in bathymetry. The Kochergin scheme is employedfor parametrisation of vertical turbulence using the same parameter settings as inExercise 20. For simplicity, bottom friction is disabled.Fig. 5.7 Model geometry for Exercise 21. Thick solid lines denote closed boundaries. Arrows indicatethe inflow boundaries. The dashed line indicates a boundary used for the initial prescriptionof Eulerian tracer concentration