  Equilibrium statistical mechanics of two-dimensional flows provides an
explanation and a prediction for the self-organization of large scale coherent
structures. This theory is applied in this paper to the description of oceanic
rings and jets, in the framework of a 1.5 layer quasi-geostrophic model. The
theory predicts the spontaneous formation of regions where the potential
vorticity is homogenized, with strong and localized jets at their interface.
Mesoscale rings are shown to be close to a statistical equilibrium: the theory
accounts for their shape, their drift, and their ubiquity in the ocean,
independently of the underlying generation mechanism. At basin scale, inertial
states presenting mid basin eastward jets (and then different from the
classical Fofonoff solution) are described as marginally unstable states. These
states are shown to be marginally unstable for the equilibrium statistical
theory. In that case, considering a purely inertial limit is a first step
toward more comprehensive out of equilibrium studies that would take into
account other essential aspects, such as wind forcing.
