0.6 0. Density ratio
Fig. 5.115 a, b Diapycnal diffusivity of temperature (solid line), salinity (dashed line), and density (dotted line) as function of density ratio Rp. Note that salt fingering is represented by the domain of the right panel (b) where Rp > 1; while diffusive layering corresponds to the domain of Rp < 1 (Zhang etal, 1998).
of gravitational potential energy associated with salt Angering in the world's oceans can be estimated from Eqns. (5.171) and (5.172). Assuming that the background value of Kœ = 0, the total amount of gravitational potential energy released in the global oceans is estimated as 8 GW. The horizontal distribution of this energy conversion was included in Figure 3.18. This is much smaller than the energy associated with wind stress input and tidal dissipation; however, this energy is most concentrated in the subtropical basin interior, where it is much larger than the corresponding part of tidal dissipation. Therefore, energy release from salt fingering may play a dominant role in regulating the structure of the main thermocline and circulation in the subtropical gyre interior.
We emphasize that salt fingering itself does not create mechanical energy; instead, the double diffusive process can only release the gravitational potential energy stored in the mean state of the circulation system, which in turn is created by the external sources of mechanical energy from wind stress and tidal dissipation. In addition, the separation of water from seawater implies a certain amount of equivalent mechanical energy and the removal of entropy produced through salty and fresh water mixing. The energetics related to the maintenance of double diffusion in connection with global-scale circulation remains unclear.
5.4 Theories for the thermohaline circulation 5.4.1 Conceptual models for the thermohaline circulation
Thermal overturning circulation driven by sea-surface differential heating How is the thermal circulation set up? The major physical processes involved are schematically shown in Figure 5.116. Assume that the ocean was initially of uniform temperature T0 and motionless, so that no north-south differences in sea level or bottom pressure would
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