Advanced Ocean Modelling: Using Open-Source Software

144 5 3D Level Modellinggeostrophic flow follows bathymetry contours and circulates smoothly around thebay. Onshore flow in the bottom Ekman layer results in coastal upwelling in whichdeeper water reaches the sea surface after about 5 days of simulation.The surface flow attains speeds of up to 80 cm/s along the upwelling front thatgradually moves away from the southern coastline of the headland. Owing to thisdeparture from the coast, the upwelling jet looses its bathymetry control and producesan anticyclonic eddy of about 25–30 km in diameter (Fig. 5.16, top panel).This scale is 4–5 times the internal Rossby radius of deformation in agreement withbaroclinic instability theory (e.g., Cushman-Roisin, 1994).An upwelling jet also establishes downstream the bay along the northern coastlineof the model domain. Interaction of the eddy-induced return flow with this jetleads to formation of a counter-rotating vortex pair (Fig. 5.16, bottom panel). Whenrecalling the direction of Ekman drift in the bottom Ekman layer, it is not difficult toconclude that the anticyclonic eddy creates downwelling and suppresses upwellingin its centre, whereas the cyclonic eddy gradually lifts deeper water to the surface.Hence, headlands can operate as agents of baroclinic eddy formation which locallyeither suppress or enhance upwelling.Fig. 5.16 Exercise 23. Snapshot surface distributions of tracer concentration (color shading) andflow field (arrows, averaged over 4 × 4 grid cells. Red shading refers to dense subsurface waterbeing upwelled to the surface. Bottom panel: Thecircle marked “H” (for high-pressure centre)indicates a cyclonic eddy that operates to suppress upwelling. The circle marked “L” (for lowpressurecentre) refers to an anticyclonic eddy