Separating Small-Scale Vortical Motions and Internal Waves in Sargasso Sea

Three-dimensional measurements of temperature, salinity, and velocity were taken simultaneously in the upper Sargasso Sea using a swarm of 20 EM-APEX floats, half of which were capable of measuring turbulence. Horizontal divergence, relative vertical vorticity, vortex stretching, and linear potential vorticity are computed at horizontal scales of 1–10 km. These measurements allow us to separate internal waves from the small-scale vortical mode. Specifically, horizontal divergence, relative vertical vorticity, vortex stretching, and potential vorticity are decomposed into internal wave and vortical mode components. Their spectral properties in two-dimensional space, vertical wavenumber and frequency, are revealed. Furthermore, the observed energy spectra, including both potential and kinetic energy, are decomposed into the internal wave and vortical mode components in vertical wavenumber and frequency spaces, based on their distinct polarization relations. The observed vortical mode has a low Rossby number, implying a geostrophic or quasi-geostrophic balance. The Burger number of the vortical mode is computed as a function of vertical wavenumber and frequency and is used to identify the dynamic balance of the vortical mode. Consistent with results from the previous internal wave experiment (IWEX) and other studies, the vortical mode energy is much weaker than that of internal waves, but is not negligible. Effects of the small-scale vortical mode and internal waves on lateral mixing will be discussed.