LAGRANGIAN VELOCITY SPECTRA AT 700 M IN THE WESTERN NORTH-ATLANTIC

Pending an appropriate scaling of each trajectory by its Lagrangian in tegral timescale T-L, there exists a generic shape of the Lagrangian f requency spectrum for the trajectories of the 700-m dataset in western North Atlantic, which are stationary on the timescale of 200 days. Th e generic spectral shape contains a plateau at the lowest frequencies extending up to nu(0) similar to (30T(L))(-1), a power-law behavior wi th an intermediate spectral slope alpha = 0.25 between nu(0) and nu(1) similar to (3 similar to 4T(L))(-1), and a sleeper slope n greater th an or equal to 3 at larger frequencies. Such a steep slope at large fr equencies implies that most of Lagrangian dispersion can be ascribed t o low and intermediate frequency motions. The variance of the Lagrangi an acceleration computed from such a spectrum is finite, indicating co ntinuous particle accelerations and supporting a truly Lagrangian beha vior of the 700-m floats. The existence of an intermediate power-law b ehavior in the spectrum can be linked with the trapping of some trajec tories by persistent energetic structures and is associated with a ten dency for anomalous diffusion lasting up to 10T(L). The authors also i ntroduce an alternative method for computing T-L from a yardstick meas ure of Lagrangian decorrelation length from each individual trajectory .