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

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. Plain Language Summary: Near‐inertial waves—or internal gravity waves with a frequency near the inertial frequency—are typically excited at the sea surface by variable wind stress forcing. These waves can be trapped and amplified at the base of mesoscale (horizontal scales of about 10–100 km) anticyclonic eddies, especially after storms or strong wind events, situations in which high near‐inertial energy is generated. In this article we present observational evidence of near‐inertial wave trapping during low to moderate wind conditions. We also show that these trapped waves lead to deep mixing near the base of the eddy. The observation of near‐inertial wave packets on two different dates and the low to moderate winds persistent during the survey suggest that our findings may be common in other similar open‐ocean eddies. Mesoscale anticyclonic eddies of the World Ocean may therefore act as deep mixing structures as they propagate westward. Key Points: Observational evidence demonstrates the signature of near‐inertial wave trapping near the base of a mesoscale anticyclonic eddy during low to moderate wind conditionsThe trapped near‐inertial waves produce low fine‐scale Richardson numbers and enhanced strain variance, suggesting turbulence productionEnhanced vertical mixing may presumably be found inside similar open‐ocean eddies under normal atmospheric conditions [ABSTRACT FROM AUTHOR]