The dynamics of the ocean can be described by the incompressible Euler equations, completed by a transport equation on the density. For large scale dynamics, the ocean is assumed stratified, meaning that it is layered by sheets of constant density, namely isopycnals. The aim of this talk is twofold. First, we present the physical context an describe the main result, that is the study of perturbations around shear flows, which are natural equilibria of this system. Second, we present a particular point of the proof, namely the use of Alinhac’s good unknown.