Fast Times at Lofoten High – Interactions Between Near-Inertial Oscillations and a Strong Mesoscale Eddy

Abstract

This study investigates interactions between near-inertial oscillations (NIOs) and a mesoscale eddy in the Lofoten Basin, Norwegian Sea, focusing on pressure gradients in the eddy. Using surface drifters deployed during the Northern Ocean Rapid Surface Evolution (NORSE) project, we observe large spatial variations in near-inertial kinetic energy among drifters co-located within the Lofoten Basin Eddy (LBE), a semipermanent anticyclonic vortex. NIOs in the upper ocean can subsequently propagate vertically as near-inertial internal waves, with their eventual breakdown contributing significantly to upper ocean mixing. We show that a Lagrangian slab model incorporating sea surface height derived pressure gradient terms accurately captures the observed wind work and energy input into the system, while a wind-forced only slab model fails to do so. The drifter data reveal multiple distinct near-inertial kinetic energy events, but not all appear to be initiated by wind forcing. We propose that systematic correlations between near-inertial velocities and the pressure gradients sampled along Lagrangian trajectories, modulated by intrinsic near-inertial oscillations of the eddy center itself, can drive a net energy gain for NIOs near the eddy center leading to increased near-inertial kinetic energy close to the core. These findings highlight the importance of eddy pressure gradients and sub-eddy scale dynamics in shaping near-inertial energy distributions, with implications for ocean mixing parameterizations and the interpretation of emerging high-resolution altimetry data from missions such as SWOT.