The Okubo-Weiss Criteria in Two-Dimensional Hydrodynamic and   Magnetohydrodynamic Flows
  The Okubo [2]-Weiss [3] criterion is recast by using the 2D hydrodynamic Beltrami condition (Shivamoggi et al.[13]) that approximates the slow flow-variation ansatz imposed in its derivation. This turns out to provide an interesting interpretation of the Okubo-Weiss criterion very logically in terms of the topological properties of the underlying vorticity manifold. These developments are then extended to 2D quasi-geostrophic flows (via the potential divorticity framework), magnetohydrodynamic flows and electron magnetohydrodynamic flows (via the generalized magnetic flux framework) and the Okubo-Weiss criteria for these cases are considered. 
B. K. Shivamoggi, G. J. F. van Heijst and L.P.J. Kamp
physics.flu-dyn physics.plasm-ph