Global well-posedness of slightly supercritical SQG equations and gradient estimate

Abstract: We prove the global regularity of smooth solutions for a dissipative surface quasi-geostrophic equation with both velocity and dissipation logarithmically supercritical compared to the critical equation. By this, we mean that a symbol defined as a power of logarithm is added to both velocity and dissipation terms to penalize the equation's criticality. Our primary tool is the nonlinear maximum principle which provides transparent proofs of global regularity for nonlinear dissipative equations. Combining the nonlinear maximum principle with a modulus of continuity, we prove a uniform-in-time gradient estimate for the critical and slightly supercritical surface quasi-geostrophic equation. It improves the previous double exponential bound by Kiselev-Nazarov-Volberg to the single exponential. In addition, we prove eventual exponential decay of the solutions.