Why Potential Vorticity Is Not Conserved along Mean Streamlines in a Numerical Southern Ocean

Potential vorticity (PV) is used as an indicator of the forcing processes and dissipation at work in the Southern Ocean. Output from the Semtner–Chervin model run with quarter-degree resolution is considered on isopycnal surfaces along Montgomery streamfunctions. Numerical results are compared with hydrographic measurements. Although simple hypotheses might suggest that subsurface PV should be unaffected by wind forcing and constant along streamlines, these results indicate that even at about 1000-m depth, PV varies along mean streamlines in both the numerical model output and in the in situ observations. The changes in PV are largely represented by stratification changes rather than shifts in the Coriolis parameter or in relative vorticity. In the numerical model output, a combination of mechanisms is responsible for these changes in PV, including transient tracer fluxes, transient momentum fluxes, diffusive processes, and long-term tracer drift.