Advanced Ocean Modelling: Using Open-Source Software

70 3 Basics of Nonhydrostatic Modelling3.17.4 ResultsThe surface heat loss triggers convective mixing in the surface layer (Fig. 3.38).Convective plumes extend to the sea floor in shallow regions and penetrate into thepycnocline in deeper portions of the model domain. They create internal waves atthe pycnocline that operate to entrain pycnocline water into the surface mixed layer.The aspect ratio of convection cells is of the order of 2. An exception are theshallow regions where the aspect ratio approaches values of 4. This bias is presumablycaused by the relatively coarse grid spacing chosen, which does not adequatelyresolve the length scale of convection cells in this region.Owing to limited penetration, the density gain in shallow water exceeds thatin deeper water, so that a density contrast between shallower and deeper regionsdevelops over time. With the vanishing surface heat flux, the dense water formed inthe shallow regions forms of a gravity current flowing downward on the slope. Thistriggers a return flow in upper parts of the water column, which is a requirement ofvolume conservation, driven by a lateral pressure gradient owing to an inclined seasurface.Fig. 3.38 Exercise 10. Snapshots of density distribution (shading and contours) at selected timesof the simulation