Lagrangian Velocity Spectra at 700 m in the Western North Atlantic.
Résumé
Pending an appropriate scaling of each trajectory by its Lagrangian integral timescale TL, there exists a generic shape of the Lagrangian frequency spectrum for the trajectories of the 700-m dataset in western North Atlantic, which are stationary on the timescale of 200 days. The generic spectral shape contains a plateau at the lowest frequencies extending up to ν0∼(30TL)−1, a power-law behavior with an intermediate spectral slope α = 0.25 between ν0 and ν1∼(3∼4TL)−1, and a steeper slope n ≥ 3 at larger frequencies. Such a steep slope at large frequencies implies that most of Lagrangian dispersion can be ascribed to low and intermediate frequency motions. The variance of the Lagrangian acceleration computed from such a spectrum is finite, indicating continuous particle accelerations and supporting a truly Lagrangian behavior of the 700-m floats. The existence of an intermediate power-law behavior in the spectrum can be linked with the trapping of some trajectories by persistent energetic structures and is associated with a tendency for anomalous diffusion lasting up to 10TL. The authors also introduce an alternative method for computing TL from a yardstick measure of Lagrangian decor-relation length from each individual trajectory.