Presenter: Andreas Thurnherr (Lamont-Doherty Earth Observatory)

Description:
Oceanic internal waves are generated primarily at the boundaries and propagate into the interior where they interact with each other to form the internal-wave field, which is closely linked with the turbulent mixing that drives the lower limb of the global overturning circulation. Not least because turbulent mixing is difficult to measure, it has become common practice to infer turbulence and mixing levels from estimates of the energy in the internal wave field. Kinetic and potential energy levels are usually derived from vertical wavenumber spectra of vertical shear and of the vertical straining of the background density field, respectively, from profiles of horizontal velocity from Lowered ADCP and vertical stratification from CTD measurements, respectively. Here, we present measurements of vertical divergence (dw/dz), an alternate quantitative measure of the internal-wave field. Using LADCP-derived vertical velocity from repeat-hydrography sections, spectra of vertical divergence were analyzed and compared to spectra of shear and strain. The divergence spectra conform as well to the shape of the Garrett-Monk model as the corresponding shear and strain spectra. More importantly, the data indicate that vertical divergence carries essentially the same information content as vertical shear. In particular, the divergence-to-strain variance ratio is closely correlated with the shear-to-strain variance ratio, the two ratios share the same scaling with N, and are furthermore characterized by consistent spatial and temporal patterns. Due to differences in processing, LADCP-derived vertical velocities are less affected by short measurement range due to a lack of particles in the water column than vertical shear, making vertical divergence particularly suitable for the deep ocean. There is a large data base of publicly available LADCP vertical velocity data that can be analyzed for vertical divergence, including all US GO-SHIP repeat hydrography cruises since 2015.