Analysis of Lagrangian Potential Vorticity Balance and Lateral Displacement of Water Parcels in Gulf Stream Meanders

Tao Song,Tom Rossby

doi:10.1175/1520-0485(1997)027<0325:aolpvb>2.0.co;2

Abstract

Abstract The balance of potential vorticity components following fluid parcel motion in Gulf Stream meanders was studied using RAFOS float data from the SYNOP Experiment. By introducing curvature dependent variations to the velocity and density fields, the authors relaxed the rigid field assumption used in earlier studies and examined closely 61 floats in the upper layers (13°–16°C) of the main thermocline. Float trajectories were segmented according to transition from crest to trough and trough to crest, and grouped by their positions relative to the current center. A total of 154 segments were collected to estimate the horizontal divergence and the mean lateral displacement of parcels under two distinct regimes: growing and decaying meanders. Both spatial and temporal changes in curvature affect the regions of divergence in a meandering stream. On the one hand, horizontal divergence increases with increasing curvature magnitude, while on the other hand, the divergence pattern itself changes going from g...

Highlights

The dynamics of meandering jets have been the focus of much meteorological research for over half a century, with special interest in the jet stream and its role in the development of midlatitude weather systems
From an analysis of the float tracks assuming conservation of potential vorticity (PV) and assuming the cross-stream structure of the down-stream velocity field was independent of curvature and downstream distance, Bower (1989) showed that there existed a systematic pattern of horizontal divergence: Divergence was found upstream of crests on the anticyclonic side, while convergence was present on the cyclonic side
With the relaxation of the rigid field assumption, the current center defined by the maximum downstream velocity was allowed to shift laterally as a function of the path curvature

Summary

Introduction

The dynamics of meandering jets have been the focus of much meteorological research for over half a century, with special interest in the jet stream and its role in the development of midlatitude weather systems. From an analysis of the float tracks assuming conservation of potential vorticity (PV) and assuming the cross-stream structure of the down-stream velocity field was independent of curvature and downstream distance, Bower (1989) showed that there existed a systematic pattern of horizontal divergence: Divergence (convergence) was found upstream of crests (troughs) on the anticyclonic side, while convergence (divergence) was present on the cyclonic side. Since divergence arises primarily from differential cross-stream motion of particles in the stream and particle motions are different between growing and decaying regimes, it is expected that a decaying meander could have significant impact on the divergence pattern established earlier To explore these issues and improve our understanding of the dynamics of fluid motion in the stream, the PV analysis methodology employed by Bower (1989) is applied to the 1988–90 RAFOS experiment.

Determination of growing and decaying meanders

Estimation of potential vorticity components

A case study

Divergence and convergence patterns in Gulf Stream meanders

Findings

Discussion