Author:
Achim Wirth 
In fluid dynamics we distinguish two types of motion : wave dynamics and vortical flow. The first is a periodical motion in time and/or space, while the second is a more persistent motion of eddies and currents. To explore the wave dynamics the divergence of the motion is considered whereas the (potential) vorticity is looked at to explore the vortical flow.
The image above shows the divergence (left) and the (potential) vorticity (right) for the same flow. At LEGI both types of motion and their interaction is studied.
For more information: Wirth, A., Inertia-gravity waves generated by near balanced flow in 2-layer shallow water turbulence on the beta-plane, Nonlinear Processes in Geophysics, European Geosciences Union (EGU), 2013, 20, pp.25-34.