The Lagrangian Potential Vorticity Balance during POLYMODE

Abstract

The terms in the Lagrangian potential vorticity equation are estimated by three different methods using clusters of SOFAR floats in the main (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 of SOFAR float and hydrographic data. The stretching term may be estimated below the main thermocline from float trajectories and knowing the topographic gradient. The 700 m mesoscale balance is either that of beta and vortex stretching balancing the material time derivative of relative vorticity (d?/dt), or that of beta and d?/dt balancing vortex stretching. The mean 700 m balance is the former type. The Lagrangian potential vorticity balances indicate internal convergences and divergences at 700 m. The mean and most of the synoptic 1300 m balances are achieved by beta and vortex stretching of equal magnitude balancing d?/dt. This is because topographic and planetary beta are of the same magnitude. Equivalently, the vertical velocity induced by eddy flow over topography in this area is dynamically important. This study emphasizes that the use of float cluster trajectories to obtain time series of the terms in the potential vorticity equation from a Lagrangian viewpoint is a powerful diagnostic tool for the study of ocean dynamics.