Advanced Ocean Modelling: Using Open-Source Software

3.23 Estuaries 83The dynamic instability produces vigorous vertical mixing via the breaking ofinternal waves. Interestingly, at later times of the simulation, velocity shear owing tobottom friction creates a series of clockwise-rotating vortices that “roll” downwardon the slope and interact with previously created vortices that have detached fromthe sea floor.3.22.4 Additional Exercise for the ReaderConduct a reference experiment with a small bottom inclination of, say, 1 degree.Explore whether this slope is sufficient to create dynamical instabilities. If not,increase the bottom inclination in additional experiments until the vertical shearof the near-bottom flow is at a level to produce dynamical instabilities.3.23 Estuaries3.23.1 DefinitionEstuaries are semi-enclosed and narrow bodies of water that are connected to thecoastal ocean, at least intermittently, and in which the salinity is measurably differentfrom that in the adjacent ocean. Estuaries are categorised as two different types(Fig. 3.50).Positive estuaries have a salinity less that of the adjacent ocean. These estuariesare exposed to a humid climate and experience a surplus of freshwater input viarivers over evaporation. Positive estuaries can be characterised by a two-layer structurewith light, low-salinity water flowing toward the ocean in the upper layer andinflow of seawater in the layer underneath. The region of maximum lateral salinitygradients is called brackish water.Negative estuaries, also called inverse estuaries, are situated in arid climaticzones and they experience a net water loss owing to an excess of evaporation overFig. 3.50 Schematic of the circulation in positive and inverse estuaries