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Two-Front Solutions of the SQG Equation and its Generalizations

The generalized surface quasi-geostrophic (GSQG) equations are transport equations for an active scalar that depend on a parameter $0<α\le 2$. Special cases are the two-dimensional incompressible Euler equations ($α= 2$) and the surface quasi-geostrophic (SQG) equations ($α= 1$). We derive contour-dynamics equations for a class of two-front solutions of the GSQG equations when the fronts are a graph. Scalar reductions of these equations include ones that describe a single front in the presence of a rigid, flat boundary. We use the contour dynamics equations to determine the linearized stability of the GSQG shear flows that correspond to two flat fronts. We also prove local-in-time existence and uniqueness for large, smooth solutions of the two-front equations in the parameter regime $1<α\le 2$, and small, smooth solutions in the parameter regime $0<α\le 1$.