A Lagrangian Description of Pulsation, Tilting, Axisymmetrization, and Meandering Trajectory in an Anticyclonic Eddy

Abstract

Mesoscale eddies pulsate, tilt with depth, and deform, but these processes are rarely resolved together from multi-depth Lagrangian observations. We analyze a long-lived intrathermocline anticyclonic eddy in the Canary Eddy Corridor using four drifters spanning 15–150 m and a novel, position-only ellipse-fitting method based on axis-aligned bounding boxes (AABB). Applied to the drifter positions, it provides hourly, depth-resolved time series of eddy-center motion and geometry (area, ellipticity, orientation, and inter-depth offset). At ∼100–150 m, the coherent core shows a ∼10-day pulsation in which area, deformation, and tilt co-vary. Contraction coincides with a more symmetric, more vertically aligned core and is followed by faster westward translation, whereas expansion coincides with stronger deformation, larger tilt, and slower translation. The center follows a westward meandering path: speeds peak along straighter segments and decrease at turning loops, and this speed–curvature modulation is phase-linked to the pulsation. Superimposed on these week-scale cycles, the 100 and 150 m centers undergo slow cyclonic precession (∼3.5 turns over ∼95 days) while both layers remain trapped. Together, these observations describe a coupled kinematic pattern linking pulsation, deformation/tilt, and non-steady translation. If similar week-scale variability is common, under-resolving it may bias inferred pathways and metrics related to retention, transport efficiency, and eddy–mean-flow interactions. Although we do not estimate 3-D velocities or energy budgets, the AABB framework provides a compact way to track coupled internal variability from sparse multi-depth drifter sampling.