Float trajectories in simple kinematic flows

Abstract

A simple kinematic model has been used to compute Lagrangian trajectories. Although it is certainly too simple to model geophysical flows, it has provided insights into the behavior of Lagrangian tracers. In particular, the existence of trapping regions has been shown to greatly increase the dispersion rate of tracers and to lead to net tracer displacements when the Eulerian mean flow is zero. In general, the spectrum of spatial scales present in the trajectories is wider than the Eulerian spectrum and is biased towards shorter wavelengths; the disparity between the 100-km Eulerian scale and the much shorter length scales experienced by the tracers is demonstrated. The estimation of the Eulerian parameters of the field from Lagranian observations must be done with a great deal of care, particularly if the eddy flow velocities are comparable to or exceed the mean flow. With a limited number of tracers it is extremely difficult to estimate, with any degree of confidence, the properties of either the mean Eulerian flow or the eddy field. Clearly, more effort must be spent to better understand the behavior of tracers in more realistic flows, to devise data analysis techniques, and to relate the Eulerian and Lagrangian spectra.