Abstract:
Double-diffusive processes are studied and parameterized, and their impacts on the
oceanic thermohaline circulation are investigated by single-hemisphere numerical models
and scaling analysis.
Scaling analysis on the thermohaline circulation has been done under three types of
surface boundary conditions. (a) Under "relaxation" conditions, there is a two-thirds
power law dependence of the meridional overturning rate (and the poleward heat
transport) on the diapycnal diffusivity. For any given external forcing, there is only
one equilibrium state for the thermohaline circulation. (b) Under "flux" boundary
conditions, there is a half power law dependence of the meridional overturning rate
on the diapycnal diffusivity. Only one mode is possible for given external forcing. (c)
Under "mixed" boundary conditions, multiple equilibria become possible. For given
thermal forcing, the existence of multiple equilibria depends on the relative contributions
of diapycnal diffusivity and the hydrologic forcing. Numerical experiments are
implemented to test the above scaling arguments. Consistent results have been obtained
under the above three types of boundary conditions. These provide a basis for
understanding how the thermohaline circulation depends on the diapycnal diffusivity,
which we know is influenced by the double-diffusive processes of "salt fingering" and
"diffusive layering" in some parts of the ocean.
In order to examine this issue, the double-diffusive processes are parameterized by diapycnal
eddy diffusivities for heat and salt that are different and depend on the local
density ratio, Rp= αTz/βSz. A background diffusivity is applied to represent turbulent
mixing in the stratified environment. The implementation of this double-diffusive
- parameterization in numerical models has significant impacts on the thermohaline
circulation. (a) Under "relaxation" boundary conditions, the meridional overturning
rate and the poleward heat transport are reduced, and water mass properties are
also changed. Similar results are obtained under "flux" boundary conditions. (b)
Under "mixed" boundary conditions, the critical freshwater flux for the existence of
the thermal mode becomes smaller with the double-diffusive parameterization. The
extent to which the thermohaline circulation is affected by double-diffusive processes
depends on the magnitude of the freshwater forcing.
