Diagnostics of diapycnal diffusion in z-level ocean models. Part II: 3-Dimensional OGCM

We present a robust method for diagnosing total diapycnal diffusivities, i.e. explicitly applied plus numerically induced diffusivities, from tracer release experiments in numerical z-level models. To this extent, numerical experiments differing only in the advection scheme used (CTRD using 2nd order centred differences, UPWIND using the upwind/upstream advection scheme, QUICK using the quicker advection scheme after Farrow and Stevens (1995) and FCT after Gerdes et al. (1991)) are analysed and compared. To obtain regionally resolved estimates of diapycnal diffusivities, individual inert dye tracers are released in dynamically different regions of a North Atlantic model, namely (i) in the interior of the subtropical gyre and (ii) in the western boundary current. Diagnosed diffusivities are robust with respect to changes in temporal and spatial sampling of the simulated dye tracer for both advection schemes and for both regions. The numerically induced diffusivity is generally positive, but can become negative for centred differences advection numerics after several months of simulated tracer dispersion.