On the global regularity for the supercritical SQG equation

Abstract

We consider the initial value problem for the fractionally dissipative quasi-geostrophic equation \[ \partial_t \theta + \mathcal{R}^\perp \theta \cdot \nabla \theta + \Lambda^\gamma \theta = 0, \qquad \theta(\cdot,0) =\theta_0 \] on $\mathbb{T}^2 = [0,1]^2$, with $\gamma \in (0,1)$. The coefficient in front of the dissipative term $\Lambda^\gamma = (-\Delta)^{\gamma/2}$ is normalized to $1$. We show that given a smooth initial datum with $\|\theta_0\|_{L^2}^{\gamma/2} \|\theta_0\|_{\dot{H}^2}^{1-\gamma/2}\leq R$, where {\em $R$ is arbitrarily large}, there exists $\gamma_1 = \gamma_1(R) \in (0,1)$ such that for $\gamma \geq \gamma_1$, the solution of the supercritical SQG equation with dissipation $\Lambda^\gamma$ does not blow up in finite time. The main ingredient in the proof is a new concise proof of eventual regularity for the supercritical SQG equation, that relies solely on nonlinear lower bounds for the fractional Laplacian and the maximum principle.