| Large scale overturning in the ocean is sustained by gravitational potential energy (GPE)
generation. The direct impact of GPE generation on the circulation is most readily recognised
through the equivalence between the GPE and depth-integrated pressure of a water column.
The efficiency of GPE generation due to turbulent kinetic energy (TKE) dissipation is
commonly quoted as Î, the fraction of TKE used to mix the water column (the
remaining energy is dissipated as heat). However, this is only correct if there is no
exchange between GPE and available internal energy (AIE), due to contraction or
expansion following mixing. Such exchange does occur due to the non-linearity of the
equation of state. If the ratio of GPE-to-AIE conversion to mixing-energy from
TKE dissipation is (1 - ξ), then the ratio of GPE generation to TKE dissipation is
ξÎ.
In this study, ξ is determined globally from World Ocean Atlas data. It is found that
the regime ξ < 1 dominates in the pycnocline, where contraction associated with
cabbelling results in the loss of GPE. In the abyssal ocean, where there are weak positive
conservative temperature gradients, the regime ξ > 1 dominates due to expansion
associated with thermobaricity. The mean of ξ is 0.6 at 400 m depth, and 1.6 at 5000 m
depth, indicating that TKE dissipation is typically 2-3 times as effective at supplying
GPE to the local water column if it occurs in the abyssal ocean. The effect of this
result on published estimates of GPE generation from turbulence observations is
presented.
Finally, a numerical model is used to explore how the circulation and surface buoyancy
fluxes respond to exchange between GPE and AIE associated with mixing. It is found that
GPE converted to/from AIE during mixing is not recovered locally, and is only
recovered globally on centennial timescales. Therefore, ξÎ, as opposed to Î, is the
quantity of most use in determining the role of TKE dissipation in large scale ocean
circulation. |