Observations of Turbulence Generated by a Near-Inertial Wave Propagating Downward in an Anticyclonic Eddy

Two perpendicular microstructure turbulence and shipboard velocity sections were conducted at high horizontal resolution across an anticyclonic warm core ring. The observations showed elevated turbulence in the core of the eddy, coincident with regions of low Richardson number $\left(Ri\right)$. Shear leading to the low $Ri$ was associated with a downward-propagating near-inertial wave that appeared to be trapped in the negative vorticity associated with the eddy, as has been found previously. The magnitude of the turbulence production agreed well with the vertical divergence of the vertical energy flux of the wave. The mixing coefficient of the turbulence was near 0.2, which together with the correlation with low $Ri$ suggests that shear instability drives the turbulence. A high shear-to-strain ratio of 10.3 was found, as expected for a shear-dominated near-inertial wave. Fine-structure parameterizations using strain only and both shear and strain overestimate the turbulence by factors of 2.7 and 12 respectively.