3.14 Oceanic Convection
57
Further enhanced forcing provided by a background sea-level gradient of 1.5 cm
over 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 internal
waves at density interfaces despite strong vertical density contrasts.
3.13.4 Additional Exercise for the Reader
Repeat this exercise with different shapes of topographic obstacles and/or different
configurations of the initial density field. For instance, it would be interesting to
explore resultant wave patterns for a system of three superimposed layers of different densities.
3.14 Oceanic Convection
3.14.1 Background
Density-driven convection is a form of instability that is triggered by unstable
density stratification in fluids. Convection produces vigorous vertical mixing and,
hence, aims at reestablishing a statically stable density field. The theory of densitydriven convection can be traced back to early studies of B´ enard (1900) and
Rayleigh (1916).
Convective stirring develops when heating up a fluid from below. The heating
decreases the fluid’s density near the bottom and produces fluid being less dense
than surrounding fluid. The associated buoyancy force triggers individual plumes of
fluid moving upward. Owing to volume conservation, rising fluid has to be accompanied by sinking of ambient fluid. The result is a concert of rising and sinking
fluid parcels, called free convection. In contrast, forced convection is dominated by
shear-flow instabilities, discussed in Sect. 3.10.
Oceanic convection is created by a surface heat loss and associated cooling
and/or by an increase in salinity owing to evaporation or new ice formation. Convection is a common process in the surface-mixed layer of the oceans. It can develop
during night-time cooling. If sustained over a long time, convection in the open
ocean can mix the surface water column to a depth of several hundred metres; in a
few 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 current
knowledge of deep-reaching convection in the open ocean.
3.14.2 Free Convection
In the absence of a vertical shear flow, unstable density stratification only leads
to convection if dynamical disturbances can grow against the effects of vertical
57
Further enhanced forcing provided by a background sea-level gradient of 1.5 cm
over 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 internal
waves at density interfaces despite strong vertical density contrasts.
3.13.4 Additional Exercise for the Reader
Repeat this exercise with different shapes of topographic obstacles and/or different
configurations of the initial density field. For instance, it would be interesting to
explore resultant wave patterns for a system of three superimposed layers of different densities.
3.14 Oceanic Convection
3.14.1 Background
Density-driven convection is a form of instability that is triggered by unstable
density stratification in fluids. Convection produces vigorous vertical mixing and,
hence, aims at reestablishing a statically stable density field. The theory of densitydriven convection can be traced back to early studies of B´ enard (1900) and
Rayleigh (1916).
Convective stirring develops when heating up a fluid from below. The heating
decreases the fluid’s density near the bottom and produces fluid being less dense
than surrounding fluid. The associated buoyancy force triggers individual plumes of
fluid moving upward. Owing to volume conservation, rising fluid has to be accompanied by sinking of ambient fluid. The result is a concert of rising and sinking
fluid parcels, called free convection. In contrast, forced convection is dominated by
shear-flow instabilities, discussed in Sect. 3.10.
Oceanic convection is created by a surface heat loss and associated cooling
and/or by an increase in salinity owing to evaporation or new ice formation. Convection is a common process in the surface-mixed layer of the oceans. It can develop
during night-time cooling. If sustained over a long time, convection in the open
ocean can mix the surface water column to a depth of several hundred metres; in a
few 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 current
knowledge of deep-reaching convection in the open ocean.
3.14.2 Free Convection
In the absence of a vertical shear flow, unstable density stratification only leads
to convection if dynamical disturbances can grow against the effects of vertical