The classical approach to the study of tracer advection in large-scale 
geophysical flows is based on separating the effects of the mean flow (that is 
slowly variable in space and time) from that turbulent dynamics acting on 
smaller scales. When it is not resolved, the turbulent velocity is often 
considered as a random variable, and its effects are usually described in term 
of an eddy diffusivity. It has been shown in the past years that the presence of 
coherent structures may modifie the dispersion properties. We here study the 
dispersion of tracers in a statistically stationary two-dimensional turbulent 
field, through the analysis of statistical characteristics as the dispersion 
coefficient, the distribution of displacements, the velocity autocorrelation 
function and the first exite time. We then develope a non linear stochastic 
model which takes into account of the nonlocal effects coherent structures have 
on the velocity field. We show, on the other hand, that the modelization of the 
advecting field as a sum of coherent vortices fails in reproducing the turbulent 
dynamics, indicating that both the coherent structures and a lower energy 
vorticity component (the "background") are necessary for reproducing the 
advecting field. The parameterization, which provides a large improvement in the 
description of the turbulent statistics, uses few parameters, whose values can 
be set from observative lagrangian properties, such as autocorrelation function 
and velocity distribution.