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As two fluid particles separate in time, the entire spectrum of eddy motions is being sampled from the smallest to the largest scales. In large-scale geophysical systems for which the Earth rotation is important, it has been conjectured that the relative diffusivity should vary respectively as D 2 and D 4/3 for distances respectively smaller and larger than a well-defined forcing scale of the order of the internal Rossby radius (with D the r.m.s. separation distance). Particle paths data from a mid-latitude float experiment in the central part of the North Atlantic appear to support these statements partly: two particles initially separated by a few km within two distinct clusters west and east of the mid-Atlantic ridge, statistically dispersed following a Richardson regime (D 2 ∼ t 3 asymptotically) for r.m.s. separation distances between 40 and 300 km, in agreement with a D 4/3 law. At early times, and for smaller separation distances, an exponential growth, in agreement with a D 2 law, was briefly observed but only for the eastern cluster (with an e-folding time around 6 days). After a few months or separation distances greater than 300 km, the relative dispersion slowed down naturally to the Taylor absolute dispersion regime. IntroductionObservations of the separation of pairs of particles is one of the few experimental methods available to examine the spatial structure of geophysical turbulent flows. Richardson (1926) proposed that the relative diffusivity of an ensemble of pairs should scale as the 4/3 power of the (r.m.s.) separation distance. In his review of the subject, Corrsin (1962) emphasized the concept that turbulent eddies much smaller or much larger than the separation scale are relatively inefficient in further separation at the difference of eddies near the separation scale: the small eddies cause independent random walks of each member of the pair while the larger ones move the pair coherently as a single unit. As a result the relative velocity of the pair changes with the separation and is a non-stationary random variable. This accelerating property of relative diffusion has been used to infer properties of the energy wavenumber spectrum in the inertial range. The key quantity in the inertial range of three-dimensional turbulence is the energy dissipation rate. When the relative diffusivity is assumed to depend only on this energy dissipation rate and on separation, Richardson's law is recovered (Obukhov 1941;Batchelor 1952). The study of particle dispersion is important because of the interest in transport and mixing of chemicals in large-scale geophysical systems. While Taylor's (1921) single-particle dispersion theory relates to tracer dispersal from a fixed geographical origin and for very large times, two-particle dispersion relates to the spreading of a cloud of tracer from its centre of gravity (Batchelor 1952) and will

International audienceDuring the first decade of the twenty-first century, more than 6000 Argo floats have been launched over the World Ocean, gathering temperature and salinity data from the upper 2000 m, at a 10-day or so sampling period. Meanwhile their deep displacements can be used to map the ocean circulation at their drifting depth (mostly around 1000 m). A comprehensive processing of the whole Argo dataset collected prior to 1 January 2010 has been performed to produce a world-wide dataset of deep displacements. This numerical atlas, named ANDRO, after a traditional dance of Brittany meaning a swirl, comprises some 600 000 deep displacements. These displacements, based on Argo or GPS surface locations only, have been fully checked and corrected for possible errors found in the public Argo data files (due to incorrect decoding or instrumental failure). Park pressures measured by the floats while drifting at depth are preserved in ANDRO (less than 2% of the park pressures are unknown): 63% of the float displacements are in the layer (900, 1100) dbar with a good (more or less uniform) degree of coverage of all the oceans, except around Antarctica (south of 60°S). Two deeper layers--(1400, 1600) and (1900, 2100) dbar--are also sampled (11% and 8% of the float displacements, respectively) but with poorer geographical coverage. Grounded cycles (i.e., if the float hits the sea bottom) are excluded. ANDRO is available online as an ASCII file