Does the potential vorticity distribution constrain the spreading of floats in the North Atlantic?

Float trajectories are compared with the distribution of climatological pot ential vorticity, Q, on approximate isentropic surfaces for intermediate wa ters in the North Atlantic. The time-mean displacement and eddy dispersion are calculated for clusters of Boats in terms of their movement along and a cross Q contours. For float clusters with significant mean velocities, the mean flow crosses Q contours at an angle of typically less than 20 degrees- 30 degrees 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 t he ocean interior: tracers should only be quasi-conserved along mean stream lines over a subbasin scale, rather than over an entire basin. The mean flo w 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 a s Q contours are assumed to be zonal and relative vorticity is ignored. Flo at clusters with a lifetime greater than 200 days show anisotropic dispersi on with greater dispersion along Q contours, than across them; Boat cluster s 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. Howe ver, 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 sum mary, diagnostics suggest that floats preferentially spread along Q contour s over a subbasin scale and imply that passive tracers should likewise pref erentially spread along Q contours in the ocean interior.