Cartoon of the toy model for meridional circulation. We distinguish between an “outward transport zone” (rightward arrows) and an “inward transport zone” (leftward arrows), which are assigned fractions θ and 1 − θ (θ < 1/2) of the local column density, respectively. Integers n − 1, n, n + 1 represent radial locations whose separation corresponds to one vertical mixing timescale. Vertical mixing is symbolized by double vertical arrows.