Advanced Ocean Modelling: Using Open-Source Software

106 4 2.5D Vertical Slice Modelling4.3 Exercise 17: Tidal-Mixing Fronts4.3.1 BackgroundTidal currents, if energetic enough, can trigger vertical mixing of the entirewater column. This tidal mixing only occurs in shallower regions of swift tidal flows,whereas the density stratification in adjacent deeper portions of the sea and weakertidal flows remains stratified. A density front establishes as a consequence of thisdifferential mixing and marks the transition zone between stratified and unstratifiedwater (Fig. 4.6). The resultant horizontal pressure gradient force gives rise to flowsthat, on time scales of several days, become subject to the geostrophic adjustmentprocess. The typical width of tidal-mixing fronts is a few kilometers.Tidal-mixing fronts, such as those in the Irish Sea, are known for their enhancedbiologic productivity and they are generally rich in fish and seabird abundance. Bottomwater of the stratified regime contain typically more nutrients than surface waterand the mixed regime. It is believed that the flux of high-nutrient near-bottom waterfrom the stratified side of the front into the well-mixed region contributes to this(Mann and Lazier, 1996). Most tidal mixing fronts only exist during the warmerseasons of the year given that solar heating is required for establishment of thermalstratification of the water column. Tidal mixing fronts are usually found closer tothe shore during spring tides than during neap tides.The ratio between local water depth h and the cube of the average speed U oftidal flows is often used as an indicator of the location of a tidal-mixing front (Mannand Lazier, 1996). Advanced methods consider effects of intensified wind-inducedmixing and air-sea heat fluxes. A discussion thereof is beyond the scope of thisbook.Fig. 4.6 Schematic of the density distribution created by enhanced tidal mixing in shallower water4.3.2 Task DescriptionThe model domain is 10 km in length. Total water depth linearly decreases from100 m at one side to 50 m at the other side of the domain (Fig. 4.7). Lateral boundariesare closed. The horizontal grid spacing is set to Δx = 100 m and a vertical