Does the Potential Vorticity Distribution Constrain the Spreading of Floats in the North Atlantic?

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Abstract
Float trajectories are compared with the distribution of climatological potential vorticity, Q, on approximate isentropic surfaces for intermediate waters in the North Atlantic. The time-mean displacement and eddy dispersion are calculated for clusters of floats in terms of their movement along and across Q contours. For float clusters with significant mean velocities, the mean flow crosses Q contours at an angle of typically less than 20°–30° in magnitude in the ocean interior. The implied Peclet number in the ocean interior ranges from 1 to 19 with a weighted-mean value of 4.4. This mean Peclet number suggests that there is significant eddy mixing in the ocean interior: tracers should only be quasi-conserved along mean streamlines over a subbasin scale, rather than over an entire basin. The mean flow also strongly crosses Q contours near the western boundary in the Tropics, where the implied Peclet number is 0.7; this value may be a lower bound as Q contours are assumed to be zonal and relative vorticity is ignored. Float clusters with a lifetime greater than 200 days show anisotropic dispersion with greater dispersion along Q contours, than across them; float clusters with shorter lifetimes are ambiguous. This anisotropic dispersion along Q contours cannot generally be distinguished from enhanced dispersion along latitude circles since Q contours are generally zonal for these cases. However, for the null case of uniform Q for the Gulf Stream at 2000 m, there is strong isotropic dispersion, rather than enhanced zonal dispersion. In summary, diagnostics suggest that floats preferentially spread along Q contours over a subbasin scale and imply that passive tracers should likewise preferentially spread along Q contours in the ocean interior.

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