EDDY RESOLUTION VERSUS EDDY DIFFUSION IN A DOUBLE GYRE GCM .2. MIXINGOF PASSIVE TRACERS

The parameterization of the effect of the unresolved scales of motion on a passive tracer field in large-scale numerical ocean models is ana lyzed through a combination of Lagrangian and Eulerian velocities. The primitive equation isopycnal model discussed by Bleck and Boudra is u sed in Part I to simulate the trajectory of particles in an eddy-resol ving double gyre circulation. From these trajectories, and from the as sociated Eulerian velocity field, a Lagrangian isopycnal diffusivity h eld and a deformation-dependent diffusivity distribution were estimate d (Part I). Here (Part II), the same velocity field is used to simulat e the evolution of idealized passive tracer fields in fine (20 km) and coarse (200 km) resolution models. The main goal of this work is to t est the limitations that coarse horizontal resolution imposes on the a dvective-diffusive equation by comparing the evolution of a passive tr acer held in a high-resolution version of the model and in an ''off li ne'' version. This off-line velocity field is meant to represent the v elocity resulting from a coarse spatial resolution simulation. The mod el ocean is represented by a typical double gyre circulation thoroughl y presented in the literature. The consistency in the comparison of th e evolution of various tracer fields from the high- and coarse-resolut ion simulations is anticipated by parameterizing the eddy diffusivity field in the coarse-resolution model with an eddy diffusivity distribu tion obtained from the eddy velocity field in the high-resolution vers ion of the model. Likewise, the advective velocity field used to repre sent the velocity from a coarse-resolution models is the Eulerian mean velocity held estimated from the eddy-resolving model. The evolution of the tracer field in the eddy-resolving experiment is analyzed in te rms of the tracer mixing across the model subpolar and subtropical gyr es. It is observed that the intergyre tracer exchange is the result of three main mechanisms: the subgrid-scale diffusivity, the transport o f anomalous tracers trapped during the formation (and shedding) of edd ies, and the occurrence of a phase shift between the meandering stream lines and the meander formed by the tracer front. The coarse-resolutio n tracer simulations presented here indicate that there is no eddy dif fusivity field consistently determined that when combined with a coars e advective velocity field can totally reproduce the tracer distributi on and the meridional tracer fluxes comparable to those obtained from the eddy-resolving version of the model. These simulations suggest, ho wever, that the use of a spatially dependent horizontal diffusivity fi eld greatly improves the accuracy of the passive tracer simulations (c ompared to the eddy-resolving simulations) over the use of a constant eddy diffusion coefficient.