Turbulent mixing in the ocean is often parameterized in terms of the downscale energy transfer by internal waves. Expressed in terms of the vertical wavenumber spectrum of oceanic velocity shear ($V^2_z $) and isopycnal strain ($ \zeta^2_z $), the “finescale parameterization” relies on several parameters, including key assumptions relating to the spectral properties. Here we use an unsupervised learning model to identify spatial correlations between embedded parameters of the finescale parameterization based upon data from 1875 full-depth hydrographic profiles from 15 sections traversing the global ocean. The clustered patterns along the sections have marked horizontal and vertical spatial dependence associated with distinct modes of spectral variation. Two clustered regions are identified where the underlying spectra deviate significantly from the canonical Garrett-Munk (GM) spectrum, suggesting potential departures from implicit assumptions about the downscale energy cascade. Spectral composites in these two regions show intensification of variance in the low and high wavenumber regimes respectively, as well as distinction in overall spectral levels and geographic prevalence. Furthermore, these clusters are found to be associated with regions where parameterized estimates of the turbulent dissipation rate $\epsilon$ differ significantly (exceeding a factor of 5) from co-located in-situ observations measured using $\chi$-pod temperature microstructure. Extending the methodology to other hydrographic datasets has the potential to reveal reasons for this parameterization bias and to identify the dynamical underpinnings leading to more robust parameterizations of oceanic turbulent mixing.ere

The propagation characteristics of near-inertial waves (NIWs) and how mesoscale and submesoscale processes affect the waves’ vertical penetration (i.e., the chimney effect) are investigated using observations from a mooring array located in the northeast Atlantic. The year-long observations show that near-inertial motions are mainly generated by local wind forcing and that they radiate predominantly downward following several strong wind events. Once below the mixed layer, NIWs preferentially propagate equatorward primarily in the form of low modes. High-mode NIWs, however, are most likely dissipated locally near the base of the mixed layer. Enhanced near-inertial kinetic energy and vertical shear are found only in mesoscale anticyclones with Rossby number of O(0.1). By contrast, submesoscale motions with order one Rossby number have little effect on the trapping and vertical penetration of NIWs, due to their smaller horizontal scales and confined vertical extent compared to mesoscale eddies.

Diapycnal mixing shapes the distribution of climatically-important tracers, such as heat and carbon, as these are carried by dense water masses in the ocean interior. Here, we analyze a suite of observation-based estimates of diapycnal mixing to assess its role within the Atlantic Meridional Overturning Circulation. The rate of water mass transformation in the Atlantic Ocean’s interior shows that there is a robust buoyancy increase in the North Atlantic Deep Water (NADW), with a diapycnal circulation of up to 4 Sv between 24N and 32S in the Atlantic Ocean. Moreover, tracers within the southward-flowing NADW may undergo a substantial diapycnal transfer, equivalent to hundreds of metres in the vertical. This result is confirmed with a zonally-averaged numerical model of the AMOC and indicates that tracer mixing can lead to divergent global pathways and ventilation timescales following the upwelling of tracers in the Southern Ocean. These results point to the need for a realistic mixing representation in climate models in order to understand and credibly project the ongoing climate change.