Option rigid_lid_surface_pressure enables the method developed by Smith, Dukowicz, and Malone (1992) which is hereafter referred to as SDM. Instead of taking the curl of the vertically integrated momentum equations to drop the surface pressure, the divergence is taken which yields an elliptic equation for the surface pressure instead of a stream function. Its main advantage is that Neumann boundary conditions apply instead of Dirichlet boundary conditions, and there is no need to solve island integrals which perform poorly on SIMD29.10 parallel computers. However, elliptic equation solvers converge very slowly using this method.