A priori estimates for the 3D quasi-geostrophic system

The present article is devoted to the 3D dissipative quasi-geostrophic system (QG). This system can be obtained as limit model of the Primitive Equations in the asymptotics of strong rotation and stratification, and involves a non-radial, nonlocal, homogeneous pseudo-differential operator of order 2 denoted by Γ (and whose semigroup kernel reaches negative values). After a refined study of the non-local part of Γ, we prove apriori estimates (in the general L setting) for the 3D QG-model. The main difficulty of this article is to study the commutator of Γ with a Lagrangian change of variable. An important application of these a priori estimates, providing bound from below to the lifespan of the solutions of the Primitive Equations for ill-prepared blowing-up initial data, can be found in a companion paper.