Advanced Ocean Modelling: Using Open-Source Software

5.5 Exercise 22: Exchange Flow Through a Strait 139water and to an inflow of surface water from the ambient ocean (Fig. 5.10b). TheEurafrican Mediterranean Sea, the Red Sea and the Persian Gulf are examples ofconcentration basins. The Baltic Sea and the Arctic Ocean are examples of dilutionbasins.The Coriolis force comes into play in this exchange circulation when the width ofthe entrance to a mediterranean sea exceeds the internal Rossby radius of deformation.In this situation, geostrophic adjustment triggers a predominantly horizontalexchange circulation. In additional to this, geostrophic frontal flows often lead todynamic instabilities and shedding of eddies, which makes the exchange circulationcomplex and difficult to predict. On the other hand, relatively narrow entrancesconstrain the circulation to a purely vertical overturning circulation.5.5.3 Task DescriptionThe model domain is 200 km in length and 100 km across, resolved by lateral gridspacings of Δx = Δy = 4 km (Fig. 5.11). Included are two separate “deeper” oceanbasins, 100 m in depth, being connected by a strait of 50 m in depth. The verticalgrid spacing is set to Δz = 10 m. The strait is 40 km in length and 20 km in width.Initially, water density is uniform at 1,028 kg/m 3 and there are no currents. Theexchange circulation is created by gradually increasing the density of the entirewater column in the left half of the model domain by Δρ = 2 kg/m 3 over the first 2days of the simulation. This is done via the flux condition:ρ i, j,k = ρ i, j,k + ΔtT Δρwhere T is taken as 2 days (expressed in seconds), and Δt is the numericaltime step. This type of forcing allows for density anomalies to evolve during theFig. 5.11 Model configuration for Exercise 22