A Lagrangian approach to the Loop Current eddy separation

F Andrade-Canto,L Zavala Sansón,J Sheinbaum

doi:10.5194/npg-20-85-2013

Abstract

Abstract. Determining when and how a Loop Current eddy (LCE) in the Gulf of Mexico will finally separate is a difficult task, since several detachment re-attachment processes can occur during one of these events. Separation is usually defined based on snapshots of Eulerian fields such as sea surface height (SSH) but here we suggest that a Lagrangian view of the LCE separation process is more appropriate and objective. The basic idea is very simple: separation should be defined whenever water particles from the cyclonic side of the Loop Current move swiftly from the Yucatan Peninsula to the Florida Straits instead of penetrating into the NE Gulf of Mexico. The properties of backward-time finite time Lyapunov exponents (FTLE) computed from a numerical model of the Gulf of Mexico and Caribbean Sea are used to estimate the "skeleton" of flow and the structures involved in LCE detachment events. An Eulerian metric is defined, based on the slope of the strain direction of the instantaneous hyperbolic point of the Loop Current anticyclone that provides useful information to forecast final LCE detachments. We highlight cases in which an LCE separation metric based on SSH contours (Leben, 2005) suggests there is a separated LCE that later reattaches, whereas the slope method and FTLE structure indicate the eddy remains dynamically connected to the Loop Current during the process.

Highlights

One of the most interesting features of the circulation in the Gulf of Mexico (GoM) is the Loop Current (LC)
Separation is usually defined based on snapshots of Eulerian fields such as sea surface height (SSH) but here we suggest that a Lagrangian view of the Loop Current eddy (LCE) separation process is more appropriate and objective
A strictly Lagrangian separation index is not defined, we found that Loop Current bulge (LCB) instantaneous hyperbolic saddle points and their strain direction provide information consistent with finite time Lyapunov exponents (FTLE) features such as their local orientation near the stagnation point

Summary

Introduction

One of the most interesting features of the circulation in the Gulf of Mexico (GoM) is the Loop Current (LC). Some authors have discussed several events where small cyclonic eddies on the periphery of the Loop Current influence the shedding of the LC rings (Fratantoni et al, 1998; Zavala-Hidalgo et al, 2003; Cherubin et al, 2006; Schmitz, 2005). Downstream from the LC, Sturges et al (2010) finds perturbations in the transport measurements from the cable between Miami and the Bahamas that precede several LCE detachments. All these findings suggest the detachment process is quite complicated, more so, since eddies may frequently detach and reattach from the LC during intrusion into the GoM

Objectives

Results

Conclusion