Non-linear free surface

Near the ground, atmospheric surface pressure varies very significantly in the horizontal, due to topographic and dynamical undulations. Although isomorphic to sea surface height (SSH) variations in the ocean, the relative amplitude of surface pressure variations in the atmosphere (several %) are significantly larger than that of the free surface of the oceanic SSH. This motivates the implementation of a non-linear formulation of the free-surface equation (barotropic mode), which accurately takes into account surface pressure variations. The implementation is designed to accurately conserve any tracer (link to paper). This is especially useful in long coupled simulations.