The elliptical model of two-dimensional vortex dynamics. I: The basic state

Abstract
An approximate model of nonaxisymmetric vortices in a two‐dimensional fluid is developed in which each vortex is assumed to consist of a nested stack of elliptical regions of uniform vorticity. The model is governed by a set of coupled nonlinear ordinary differential equations for the aspect ratio, orientation, and centroid of the bounding contour of each region. It also admits a Hamiltonian formulation and possesses all the invariants of the original system. In Part II [Phys. Fluids A 3, XXX (1991)], the model is extended to include disturbances to the elliptical shape of each contour.