A numerical model integrated the steady advection-diffusion equation by using a kinematic circulation with characteristics of a subtropical gyre, i.e., closed streamlines and western intensification. Homogenization of the interior concentration increased, and the gradients were forced to the boundaries as mixing, parameterized as eddy diffusion, decreased. As the western boundary current intensified, the value of the homogenized interior became closer to the southern boundary value. This indicated that cross-stream mixing was enhanced within the western boundary current. Tongues of high and low tracer concentration spiraled across streamlines toward the center of the gyre. These simulations will be helpful in the interpretation of lateral distributions of geochemical tracers, which are being generated by programs such as TTO.