Diagnostics of diapycnal diffusion in z-level ocean models

In general ocean circulation models (OGCMs) diapycnal diffusion arises not<br/>only from the discretisation of the explicit diffusion, but also by numerically<br/>induced diffusion, caused, e.g., by common discretisations of advective<br/>transport.<br/>In the present study, three different diagnostics to analyse the mean<br/>diapycnal diffusivities of individual tracers (vertically and horizontally) are<br/>introduced: (i) The divergence method based on the work of Ledwell<br/>et al. (1998) infers the mean diapycnal diffusivity from the<br/>advection-diffusion equation. (ii) The tracer flux method based on the work<br/>of Griffies et al. (2000), that determines the diapycnal flux crossing an<br/>isopycnal layer, is modified for the analysis of mean diapycnal diffusivities<br/>of a passive tracer. (iii) The variance method based on the work of<br/>Morales Maqueda and Holloway (2006) is a more general approach<br/>as the diapycnal diffusion is analysed by the variance decay of the total<br/>tracer concentration.<br/>These methods can be used for the analysis of the diffusivity of passive<br/>tracer independent of the model set-up, e.g. the advection scheme used, but<br/>support only information about mean diapycnal diffusivity of that tracer<br/>field rather than for each individual layer. The applicability of these<br/>methods is tested in a set of 1- and 2-dimensional case studies. The effect<br/>of vertical advection and of diverging and converging isopycnals is shown<br/>separately. In all three methods used, the transformation of the tracer onto<br/>isopycnals leads to errors in the diagnosed diffusivities. It turns out that<br/>the tracer flux method is the most robust method and therefore the method<br/>of choice. In order to keep the errors as small as possible, longer time mean<br/>values should be analysed.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2008

Author dc:creator

• Getzlaff, Julia