# Enduring Lagrangian coherence of a Loop Current ring assessed using independent observations

## TL;DR

This study combines satellite ocean color and drifter data to demonstrate the long-lasting Lagrangian coherence of a Loop Current ring, revealing limitations in current models and emphasizing the importance of nonlinear dynamical systems metrics.

## Contribution

It provides observational evidence of enduring Lagrangian coherence in a Loop Current ring and evaluates the performance of data-assimilative models in capturing this behavior.

## Key findings

- Observed a Loop Current ring with long-lasting Lagrangian coherence.
- Data-assimilative models failed to reproduce the observed coherence.
- Highlights the importance of nonlinear dynamical systems metrics for model assessment.

## Abstract

Ocean flows are routinely inferred from low-resolution satellite altimetry measurements of sea surface height assuming a geostrophic balance. Recent nonlinear dynamical systems techniques have revealed that surface currents derived from altimetry can support mesoscale eddies with material boundaries that do not filament for many months, thereby representing effective transport mechanisms. However, the long-range Lagrangian coherence assessed for mesoscale eddy boundaries detected from altimetry is constrained by the impossibility of current altimeters to resolve ageostrophic submesoscale motions. These may act to prevent Lagrangian coherence from manifesting in the rigorous form described by the nonlinear dynamical systems theories. Here we use a combination of satellite ocean color and surface drifter trajectory data, rarely available simultaneously over an extended period of time, to provide observational evidence for the enduring Lagrangian coherence of a Loop Current ring detected from altimetry. We also seek indications of this behavior in the flow produced by a data-assimilative system which demonstrated ability to reproduce observed relative dispersion statistics down into the marginally submesoscale range. However, the simulated flow, total surface and subsurface or subsampled emulating altimetry, is not found to support the long-lasting Lagrangian coherence that characterizes the observed ring. This highlights the importance of the Lagrangian metrics produced by the nonlinear dynamical systems tools employed here in assessing model performance.