  Direct numerical simulation data show that the variance of the coupling term
in passive scalar advection by a random velocity field is smaller than it would
be if the velocity and scalar fields were statistically independent. This
effect is analogous to the "depression of nonlinearity" in hydrodynamic
turbulence. We show that the trends observed in the numerical data are
qualitatively consistent with the predictions of closure theories related to
Kraichnan's direct interaction approximation. The phenomenon is demonstrated
over a range of Prandtl numbers. In the inertial-convective range the depletion
is approximately constant with respect to wavenumber. The effect is weaker in
the Batchelor range.
