Advanced Ocean Modelling: Using Open-Source Software

3.14 Oceanic Convection 57Further enhanced forcing provided by a background sea-level gradient of 1.5 cmover the length of the domain triggers massive internal waves of a height of 40 m(Fig. 3.29b). This demonstrates that swift flows can trigger large-amplitude internalwaves at density interfaces despite strong vertical density contrasts.3.13.4 Additional Exercise for the ReaderRepeat this exercise with different shapes of topographic obstacles and/or differentconfigurations of the initial density field. For instance, it would be interesting toexplore resultant wave patterns for a system of three superimposed layers of differentdensities.3.14 Oceanic Convection3.14.1 BackgroundDensity-driven convection is a form of instability that is triggered by unstabledensity stratification in fluids. Convection produces vigorous vertical mixing and,hence, aims at reestablishing a statically stable density field. The theory of densitydrivenconvection can be traced back to early studies of Bénard (1900) andRayleigh (1916).Convective stirring develops when heating up a fluid from below. The heatingdecreases the fluid’s density near the bottom and produces fluid being less densethan surrounding fluid. The associated buoyancy force triggers individual plumes offluid moving upward. Owing to volume conservation, rising fluid has to be accompaniedby sinking of ambient fluid. The result is a concert of rising and sinkingfluid parcels, called free convection. In contrast, forced convection is dominated byshear-flow instabilities, discussed in Sect. 3.10.Oceanic convection is created by a surface heat loss and associated coolingand/or by an increase in salinity owing to evaporation or new ice formation. Convectionis a common process in the surface-mixed layer of the oceans. It can developduring night-time cooling. If sustained over a long time, convection in the openocean can mix the surface water column to a depth of several hundred metres; in afew regions such as the Greenland Sea and the Labrador Sea even to great depths> 2,000 m. Marshall and Schott (1999) give a comprehensive review of the currentknowledge of deep-reaching convection in the open ocean.3.14.2 Free ConvectionIn the absence of a vertical shear flow, unstable density stratification only leadsto convection if dynamical disturbances can grow against the effects of vertical