  Many classical nova ejecta are enriched in CNO and Ne. Rosner et al. recently
suggested that the enrichment might originate in the resonant interaction
between large-scale shear flows in the accreted H/He envelope and gravity waves
at the interface between the envelope and the underlying C/O white dwarf. The
shear flow amplifies the waves, which eventually form cusps and break. This
wave breaking injects a spray of C/O into the superincumbent H/He. Using
two-dimensional simulations, we formulate a quantitative expression for the
amount of C/O per unit area that can be entrained into the H/He at saturation.
The fraction of the envelope that is enriched depends on the horizontal
distribution of shear velocity and the density contrast between the C/O white
dwarf and the H/He layer but is roughly independent of the vertical shape of
the shear profile. Using this parameterization for the mixed mass, we then
perform several one-dimensional Lagrangian calculations of an accreting white
dwarf envelope and consider two scenarios: that the wave breaking and mixing is
driven by the convective flows; and that the mixing occurs prior to the onset
of convection. In the absence of enrichment prior to ignition, the base of the
convective zone, as calculated from mixing-length theory with the Ledoux
instability criterion, does not reach the C/O interface. As a result, there is
no additional mixing, and the runaway is slow. In contrast, the formation of a
mixed layer during the accretion of H/He, prior to ignition, causes a more
violent runaway. The envelope can be enriched by <25% of C/O by mass
(consistent with that observed in some ejecta) for shear velocities, over the
surface, with Mach numbers <0.4.
