| Abstract: This paper discusses spurious diapycnal
mixing associated with the transport of density in a z-coordinate
ocean model. A general method, based on the work of Winters and
collaborators, is employed for empirically diagnosing an effective
diapycnal diffusivity corresponding to any numerical transport process.
This method is then used to quantify the spurious mixing engendered by
various numerical representations of advection. Both coarse and
fine resolution examples are provided that illustrate the importance of
adequately resolving the admitted scales of motion in order to maintain
a small amount of mixing consistent with that measured within the
ocean's pycnocline. Such resolution depends on details of the
advection scheme, momentum and tracer dissipation, and grid
resolution. Vertical transport processes, such as convective
adjustment, act as yet another means to increase the spurious mixing
introduced by dispersive errors from numerical advective fluxes. |