Mixing in two-dimensional vortex interactions

We examine the mixing of a passive tracer initially contained within several widely separated vortex patches in a two-dimensional, nearly inviscid, incompressible flow. The initial vortex positions and sizes are chosen so that they collapse toward a common center, resulting in a strong interaction. The area of tracer ejected from the vortices is found to be well correlated with the departure of each tracer contour from an ideal elliptical shape. This result appears to be more widely applicable, and, in particular, it may be useful for quantifying small-scale mixing in realistic atmospheric and oceanic flows. (C) 2000 American Institute of Physics. [S1070-6631(00)50412-7].