Advanced Ocean Modelling: Using Open-Source Software

150 5 3D Level Modelling5.8.3 ResultsThe gradual density increase in the formation region creates a Deep Western BoundaryCurrent that spreads southward along the western side of the model domain(Fig. 5.19). This boundary current is initiated by the Coriolis force which pushesdeep water against the coast. The resultant upward doming of the density interfaceand geostrophic adjustment lead to steady-state dynamics. Note that an anticycloniceddy appears in bottom layers of the dense-water formation region. Owing to a netinflux of low-density water, the density excess in the formation region reaches asteady value of around 2 kg/m 3 . This implies that density in a formation region isbounded by an upper limit.A return flow establishes in the upper ocean spanning the entire length of theDeep Western Boundary Current (Fig. 5.20). This return flow is created by a dropof the coastal sea level as consequence of the geostrophic adjustment of the densebottom layer. The adjacent ocean remains largely at rest. A counterclockwise rotatingeddy appears in surface layers of the dense-water formation region. Upwellingalong the path of the boundary current leads to entrainment of bottom water into thesurface return flow. There is some indication of meandering of the boundary current,but the inherent dynamics are grossly biased by the coarse lateral grid spacingchosen.In contrast to the Stommel-Arons model, a vertical rather than horizontal overturningcirculation establishes here being concentrated to a narrow zone along thewestern boundary. The final locations of selected Lagrangian floats nicely revealthe boundary current (Fig. 5.21) together with the return flow that establishes in thesurface ocean (Fig. 5.22). This model application does not create any return flows inFig. 5.19 Exercise 24. Distributions of density (shading and contours) and horizontal flow field(arrows) in the lowermost 200 m of the water column domain after 120 days of simulation. Arrowsfor speeds