Isopycnal Lagrangian statistics from the North Atlantic Current RAFOS float observations

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One hundred isopycnal floats were tracked on the 27.2 and 27.5 σσ surfaces in the Newfoundland Basin (NFB) from July 1993 to July 1995 to study the current structure and exchanges of waters between the subtropical and subpolar gyres. The float-mapped mean flow consists of weak flows in the NFB and a strong boundary current (the North Atlantic Current (NAC)), which separates from the boundary at the Northwest Corner, becoming a diffusive zonal drift. The NAC meanders are linked to topography and have similar patterns on the two isopycnals despite the fact that the upper layer velocities are twice as fast as the lower layer ones. Perturbation velocity from the mean is used to compute isopycnal turbulent dispersion and diffusivity. This large data set allows us to resolve a narrow mean NAC and results in a Gaussian turbulence. The turbulence approximately follows the classic Taylor dispersion theory. Integral timescales and length scales and turbulent isopycnal diffusivity are of 1.5-2.5 days, 20-30 km, and (1-7) × 103 m2 s-1, respectively. The timescale increases with depth and decreases with latitude, the length scale decreases with depth and longitude, and the diffusivity decreases with depth and from NAC to NFB. Compared to previous results from surface drifters and isobaric floats, our isopycnal statistics are more isotropic and agree better with the Taylor dispersion theory because (1) the mean velocity has a better resolution and (2) the isopycnal floats are better Lagrangian followers. The diffusivity scales better with the rms velocity and length scale than with the velocity variance and timescale.

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Journal of Geophysical Research: Oceans