Near-Inertial Wave Trapping Near the Base of an Anticyclonic Mesoscale Eddy Under Normal Atmospheric Conditions

Abstract

Observational evidence demonstrates the signature of near-inertial wave (NIW) trapping by a long-lived westward propagating mesoscale anticyclonic eddy under normal atmospheric conditions. Cross-eddy sections of density and shipboard acoustic Doppler current profiler data show the downward (upward phase) propagation of NIW packets with vertical wavelengths of 200 m within the eddy, and wave energy amplification in a critical vertical layer located near the base of the anticyclone. Elevated strain variance, enhanced turbulent kinetic energy dissipation rates obtained from a fine-scale parameterization, and the occurrence of low (≤1) gradient Richardson numbers are consistent with turbulence production and mixing from 400 m to at least the maximum sampling depth of 725 m associated with the trapped NIWs. The observation of NIW packets near the eddy base at two different stages of the survey separated by 10 days and the persistence of low to moderate winds before and during the eddy sampling strongly suggest that our findings may be common in similar open-ocean anticyclonic eddies under normal wind conditions.