A Divergence and Vorticity View of Nonlinear Oceanic Lee Wave Obtained by a Two-Vessel Survey

Abstract

The spatial distribution of the flow field inferred from shipboard measurements often suffers from a spatial-temporal aliasing effect. This study examined a detailed view of the horizontal divergence and relative vertical vorticity captured by a two-vessel survey to characterize velocity gradients and the resulting process in the presence of nonlinear lee waves. The three-dimensional structure of the horizontal velocity gradients and turbulent mixing within a nonlinear internal lee wave behind a shallow seamount was investigated. Synchronous two-vessel Acoustic Doppler Current Profilers provide in-situ measurements of velocity vectors that significantly minimize the spatial and temporal aliasing effect. The magnitude of horizontal divergence and relative vertical vorticity normalized by the planetary vorticity ($\delta /f$, $\zeta /f$ ∼ O (10)) is one order of magnitude greater than prior observations in the typical oceanic sub-mesoscale flow field using a two-vessel survey. Our analysis indicates that the spatial variations of horizontal divergence and relative vertical vorticity over the seamount are associated with flow-topography interactions. Owing to the bottom Ekman effect, the deflected Kuroshio enhances the relative vertical vorticity, ${\zeta }_z{=\partial }_{x}V-{\partial }_{y}U$, and horizontal components of relative vorticity, ${\zeta }_x=-{\partial }_{z}V$ and ${\zeta }_y={\partial }_{z}U$, resulting in vertical shear instability and symmetric instability. Instability hotspots are identified by the negative potential vorticity (PV) at the rear half of the nonlinear internal wave, where depressed isopycnals rebound. In situ observational surveys conducted on the lee of pinnacles further indicated that the enhanced turbulent eddy diffusivity and vertical nitrate gradient are collocated with the negative PV within a nonlinear internal lee wave.