A Synoptic Map of Isopycnic Potential Vorticity in the Seasonal Thermocline

Abstract

The distribution of isopycnic potential vorticity (IPV) has been mapped in the seasonal thermocline from a single ship using relative velocity measurements plus absolute ship motion, and density profiles from a CTD mounted on an undulating towed vehicle. The advantage of this technique is that the measurements are quasi-synoptic and no geostrophic assumptions have to be made since the current measurements are independent of the hydrographic data. The data were collected during an 11-day survey at the North Atlantic Polar Front near the Charlie Gibbs Fracture Zone (50°?52°N, 33°?37°W) in summer 1981. The isopycnic distributions of temperature and potential vorticity are significantly correlated. The synoptic-scale IPV was calculated from the density field first with planetary vorticity alone and then with absolute vorticity (i.e. using the measured relative vorticity). Both versions show a meander structure with a wavelength of about 200 km and 100 km amplitude. A mesoscale front lies between the meander trough and ridge. Although it is possible to detect a mean IPV difference between trough and ridge from hydrographic data alone, the crossfront gradient of IPV was underestimated by excluding the relative vorticity. The contribution of the enstrophy to the variance of potential vorticity is however small, indicating that the Rossby radius of the flow is about 12 km and thus generally smaller than the scales resolved of about 30 km.