For a two-dimensional problem and at microwave radar frequencies, in a previous paper, the field scattered by a highly-conducting large rough sea surface in the presence of a duct having a linear-square refractive index profile has been computed. The FB (Forward-Backward) method has been applied to solve the linear system obtained by discretizing the BIE (Boundary Integral Equation) from the MoM (Method of Moments). To reduce again the complexity in order to solve huge problems, the SDIM (Subdomain Decomposition Iterative Method) is applied and combined with the Forward-Backward Spectral-Acceleration (FB-SA) for the calculation of the surface currents on each sub-surface of the sea. The ACA algorithm is also applied to accelerate the coupling steps between the subsurface. Finally, this method is tested on the medium having a refractive index of parabolic profile, which is more realistic than a linear profile