Internal wave-driven mixing (key_ zdftmx_new)

!----------------------------------------------------------------------- &namzdf_tmx_new ! new tidal mixing parameterization ("key_zdftmx_new") !----------------------------------------------------------------------- nn_zpyc = 1 ! pycnocline-intensified dissipation scales as N (=1) or N^2 (=2) ln_mevar = .true. ! variable (T) or constant (F) mixing efficiency ln_tsdiff = .true. ! account for differential T/S mixing (T) or not (F) /

The parameterization of mixing induced by breaking internal waves is a generalization of the approach originally proposed by St. Laurent et al. [2002]. A three-dimensional field of internal wave energy dissipation is first constructed, and the resulting diffusivity is obtained as

where is the mixing efficiency and is a specified three dimensional distribution of the energy available for mixing. If the ln_mevar namelist parameter is set to false, the mixing efficiency is taken as constant and equal to 1/6 [Osborn, 1980]. In the opposite (recommended) case, is instead a function of the turbulence intensity parameter , with the molecular viscosity of seawater, following the model of Bouffard and Boegman [2013] and the implementation of de Lavergne et al. [2016]. Note that is bounded by , a limit that is often reached when the mixing efficiency is constant.

In addition to the mixing efficiency, the ratio of salt to heat diffusivities can chosen to vary as a function of by setting the ln_tsdiff parameter to true, a recommended choice). This parameterization of differential mixing, due to Jackson and Rehmann [2014], is implemented as in de Lavergne et al. [2016].

The three-dimensional distribution of the energy available for mixing, , is constructed from three static maps of column-integrated internal wave energy dissipation, , , and , combined to three corresponding vertical structures (de Lavergne et al., in prep):

In the above formula, denotes the height above bottom, denotes the WKB-stretched height above bottom, defined by

The parameter (given by nn_zpyc in namzdf_tmx_new namelist) controls the stratification-dependence of the pycnocline-intensified dissipation. It can take values of 1 (recommended) or 2. Finally, the vertical structures and require the specification of the decay scales and , which are defined by two additional input maps. is related to the large-scale topography of the ocean (etopo2) and is a function of the energy flux , the characteristic horizontal scale of the abyssal hill topography [Goff, 2010] and the latitude.

Gurvan Madec and the NEMO Team

NEMO European Consortium2017-02-17