Oceanic three-dimensional Lagrangian coherent structures: A study of a mesoscale eddy in the Benguela upwelling region

We study three dimensional (3D) oceanic Lagrangian coherent structures (LCSs) in the Benguela upwelling region obtained from an output of the regional ocean modeling system (ROMS). To do that, we first computed finite-size Lyapunov exponent (FSLE) fields in the region volume that characterize mesoscale stirring and mixing. Average FSLE values generally decreased with depth, but there was a local maximum at a depth of approximately 100 m. LCSs are extracted as the ridges of the calculated FSLE fields. They present a “curtain-like” geometry in which the strongest attracting and repelling structures appear as quasivertical surfaces. LCSs around a particular cyclonic eddy, pinched off from the upwelling front, are also calculated. The LCSs are confirmed to provide pathways and barriers to transport into and out of the eddy.

► 3D Lagrangian coherent structures (LCS) computed in the Benguela upwelling region.
► LCS have a vertical curtain-like geometry.
► LCS act as barriers/pathways at any depth to transport in/out of mesoscale eddies.
► A global decrease of mixing activity with depth is shown from averaged FSLE fields.
► But a local maximum is identified at approx. 100 m depth.