The Lagrangian Potential Vorticity Balance during POLYMODE

Abstract

The terms in the Lagrangian potential vorticity equation are estimatedby three different methods using clusters of SOFAR floats in themain (700 m) and lower (1300 m) thermocline of the POLYMODE region.The Lagrangian stretching term, which is the most difficult to observe,has been estimated in the main thermocline using a combination ofSOFAR float and hydrographic data. The stretching term may be estimatedbelow the main thermocline from float trajectories and knowing thetopographic gradient. The 700 m mesoscale balance is either thatof beta and vortex stretching balancing the material time derivativeof relative vorticity (d?/dt), or that of beta and d?/dt balancingvortex stretching. The mean 700 m balance is the former type. TheLagrangian potential vorticity balances indicate internal convergencesand divergences at 700 m. The mean and most of the synoptic 1300m balances are achieved by beta and vortex stretching of equal magnitudebalancing d?/dt. This is because topographic and planetary beta areof the same magnitude. Equivalently, the vertical velocity inducedby eddy flow over topography in this area is dynamically important.This study emphasizes that the use of float cluster trajectoriesto obtain time series of the terms in the potential vorticity equationfrom a Lagrangian viewpoint is a powerful diagnostic tool for thestudy of ocean dynamics.