The previous discussion made no approximation other than to employ a hydrostatic and Bousinnesq fluid, as well as assumptions in Section 7.1 leading to the expression for the surface pressure. The purpose of this section is to discuss a version of the barotropic system which has been is implemented by Killworth et al (1991) and Dukowicz and Smith (1994). This method is also available in MOM, but is not recommended due to its inability to conserve (see Griff ies, Pacanowski, Schmidt, and Balaji (2000) for details).