Advanced Ocean Modelling: Using Open-Source Software

3.24 Exercise 14: Positive Estuaries 89where A is grid-averaged age, Adv(A) and Diff(A) denote the advection and diffusionterms, and the constant unity represents ageing. In a semi-enclosed body ofwater such as an estuary, entrainment of younger ambient seawater leads to establishmentof an equilibrium age distribution (see Fig. 3.52). As flushing times, thespatial distribution of age indicates the marine connectivity in a region of interest.The advantage of predicting age rather than flushing time is that it does not requireany threshold value and that it allows for continuous prediction without the need torestart the model in different seasons of the year.Fig. 3.52 Entrainment ofyounger water leads toestablishment of a steadyvalue of age3.24 Exercise 14: Positive Estuaries3.24.1 AimThe aim of this exercise is to simulate the circulation and density structure of apositive estuary subject to varied magnitudes of freshwater inflow and exposed totidal flows of a given tidal range. To make the simulation more realistic, variablechannel width is implemented in the vertical ocean-slice model.3.24.2 Task DescriptionWe consider a river channel of 120 km in length and 50 m in width opening up tothe sea (Fig. 3.53). The channel’s sidewalls are for simplicity vertical. The channelwidens to 500 m at its opening to the sea. Channel depth increases linearly from 5 to20 m from head to mouth. We can employ the 2d vertical ocean slice model with afew modifications (see below) under the assumption that the flow is uniform acrossthe channel width. A vertical grid spacing of 1 m together with a horizontal gridspacing of 2 km are used.