The Cross-Stream Potential Vorticity Front and Its Role in Meander-Induced Exchange in the Gulf Stream

Abstract

The potential vorticity (PV) front observed in the upper thermocline at the northern edge of the Gulf Stream has been thought of as an inhibitor to lateral motion of parcels on isopycnals. The role of the PV front in relation to lateral motion and cross-stream exchange was investigated by monitoring the evolving PV field in the vicinity of Lagrangian parcels at the northern edge of the stream. The observational study involved shipboard acoustic Doppler current profiler (ADCP) and conductivity?temperature?depth surveys to sample the velocity and density fields around water parcels as they flowed through meanders. The observations reveal that the PV field in the vicinity of parcels moves laterally across the stream and evolves as parcels negotiate meander crests and troughs, allowing parcels to move across the stream without changing their Lagrangian PV. This lateral motion of the PV front suggests that it is more a response to than an inhibitor of the lateral motion of parcels in the meanders. The relative contributions of the components of PV to the front indicated a rich structure of changes in shear and stretching across the stream. Also, temperature changes of Lagrangian parcels were linked to small-scale mixing processes associated with observed intrusive features. Estimated diffusion coefficients suggest that horizontal mixing could be the main mechanism of mixing in the upper thermocline at the northern edge of the stream.