“…For a function w, we introduce the horizontal filter (related to the horizontal Helmholtz operator), given by [3,16,17], from the point of view of the numerical simulations, this filter is less memory consuming with respect to the standard one. Further, another interesting feature of this filter is that, even in the case of domains which are not periodic in the vertical direction, there is no need to introduce artificial boundary conditions for the Helmholtz operator (see, e.g., [3,8,9,10,11]).…”