# Transport by Lagrangian Vortices in the Eastern Pacific

## TL;DR

This study uses a frame-invariant method to identify rotationally coherent Lagrangian vortices in the Eastern Pacific, revealing their limited role in meridional transport compared to incoherent motions, challenging previous Eulerian-based estimates.

## Contribution

It introduces an objective Lagrangian vortex detection method and quantifies the limited contribution of RCLVs to meridional transport in the Eastern Pacific.

## Key findings

- RCLVs are smaller and shorter-lived than SSH eddies.
- RCLVs contribute less than 1% to meridional dispersion.
- Eddy transport is mainly due to incoherent motions outside RCLV cores.

## Abstract

Rotationally coherent Lagrangian vortices (RCLVs) are identified from satellite-derived surface geostrophic velocities in the Eastern Pacific (180$^\circ$-130$^\circ$ W) using the objective (frame-invariant) finite-time Lagrangian-coherent-structure detection method of Haller et al. (2016) based on the Lagrangian-averaged vorticity deviation. RCLVs are identified for 30, 90, and 270 day intervals over the entire satellite dataset, beginning in 1993. In contrast to structures identified using Eulerian eddy-tracking methods, the RCLVs maintain material coherence over the specified time intervals, making them suitable for material transport estimates. Statistics of RCLVs are compared to statistics of eddies identified from sea-surface height (SSH) by Chelton et al. 2011. RCLVs and SSH eddies are found to propagate westward at similar speeds at each latitude, consistent with the Rossby wave dispersion relation. However, RCLVs are uniformly smaller and shorter-lived than SSH eddies. A coherent eddy diffusivity is derived to quantify the contribution of RCLVs to meridional transport; it is found that RCLVs contribute less than 1% to net meridional dispersion and diffusion in this sector, implying that eddy transport of tracers is mostly due to incoherent motions, such as swirling and filamentation outside of the eddy cores, rather than coherent meridional translation of eddies themselves. These findings call into question prior estimates of coherent eddy transport based on Eulerian eddy identification methods.