Astronomical data reveals that approximately 3.5 terawatts (TW) of
tidal energy is dissipated in the ocean. Tidal models and satellite
altimetry suggest that 1 TW of this energy is converted from the
barotropic to internal tides in the deep ocean, predominantly around
regions of rough topography such as mid-ocean ridges. A global tidal
model is used to compute turbulent energy levels associated with the
dissipation of internal tides, and the diapycnal mixing supported by
this energy flux is computed using a simple parameterization.
The mixing parameterization has been incorporated into a coarse
resolution numerical model of the global ocean. This parameterization
offers an energetically consistent and practical means of improving
the representation of ocean mixing processes in climate models. Novel
features of this implementation are that the model explicitly accounts
for the tidal energy source for mixing, and that the mixing evolves
both spatially and temporally with the model state. At equilibrium,
the globally averaged diffusivity profile ranges from 0.3 cm2 s^-1
at thermocline depths to 7.7 cm2s-1 in the abyss
with a depth average of 0.9 cm^2 s^-1, in close
agreement with inferences from global balances. Water properties are
strongly influenced by the combination of weak mixing in the main
thermocline and enhanced mixing in the deep ocean. Climatological
comparisons show that the parameterized mixing scheme results in a
substantial reduction of temperature/salinity bias relative to model
solutions with either a uniform vertical diffusivity of 0.9 cm^2
s^-1 or a horizontally uniform bottom-intensified
arctangent mixing profile. This suggests that spatially varying bottom
intensified mixing is an essential component of the balances required
for the maintenance of the ocean′s abyssal stratification.
|
|
|