Potential Vorticity Structure across the Gulf Stream: Observations and a PV-Gradient Model

Abstract

Potential vorticity (PV) structure across a baroclinic front is a property that determines the stability characteristics of that front, cross-frontal exchange, and the behavior of the vortical waves that this front enables. Hence, there has been much interest in estimating PV across the Gulf Stream (GS). However, PV estimations have typically encountered two problems—the horizontal resolution of sampling has been inadequate, and noise in measurements gets amplified by differentiation when estimating PV. This paper’s approach addresses both of these problems. The authors have used a unique set of highly resolved, simultaneous density and direct velocity measurements across the Gulf Stream, first to calculate the PV structure, and then to obtain its idealization within density layers using a PV-gradient (PVG) model. The PVG model inverts an input PV in layers to determine the isopycnal depths and velocities in isopycnal layers. By comparing the observed and PVG-modeled velocities and isopycnal positions, corrections to PV estimates can be made to improve agreement. In this way an idealized and smooth PV-gradient structure can be obtained that is dynamically consistent with the velocity and density fields and retains the correct magnitudes of the PV gradients while noise and insignificant details are filtered out. The GS PV structure can be successfully idealized as a “3-PVG-layer” representation, which has a strong positive PV gradient in the 18°C Water layer, a weak positive PVG in the upper main thermocline, and a weak negative PVG in the lower main thermocline. The associated velocity structure captures the core velocity throughout the upper main thermocline, the width, the vertical tilt, and the asymmetry of the flow with higher lateral shear on the cyclonic side.