Physics:Okubo-Weiss parameter

The Okubo-Weiss parameter, normally represented as 'W', is a measure of the relative importance of deformation and rotation, which is calculated as the sum of the squares of normal and shear strain minus the relative vorticity. This is widely applicable in fluid properties particularly in identifying and addressing the oceanic eddies. For a horizontally non-divergent flow in the ocean, the equation to calculate W is in the form W=s_n² +s_s² -ω². Eddies in which vorticity dominates strain are marked by negative W. Normally, in the analysis in a global perspective, closed contours of W =-2×10⁻¹² s⁻² were considered as eddies. Sea surface height (SSH) either wholly negative or wholly positive within such contours indicates cyclonic or anticyclonic polarity respectively. The centre of the eddy is defined to be the maximum or minimum SSH area within the W contour and the eddy diameter is defined as that of the circle with area equal to that enclosed by the W contour. The method may track noise-induced artifacts as eddies in certain cases, to avoid this, the W field is smoothed with half power filter cutoffs, for example of 1.5°×1.5° and only cases for which the W contour of at least 0.25° pixels or equivalent to an area about 50 km² can be considered.

References

• Okubo, A., 1970: Horizontal dispersion of floatable particles in the vicinity of velocity singularities such as convergences. Deep-Sea Res., 17, 445–454
• Weiss, J., 1991: The dynamics of enstrophy transfer in two-dimensional hydrodynamics. PhysicaD, 48, 273–294