Gunnar Voet

Yanai waves are observed in the central equatorial Pacific at 436–864 m depth, below the Equatorial Undercurrent. Applying maximum likelihood estimation to shipboard ADCP measurements enables identification of Yanai wave signals from irregularly sampled observations. Ray tracing links the observed wave to a tropical instability wave (TIW) passage ∼20° west of the survey site and ∼49 days earlier, where meridional velocity and sea surface temperature exhibit consistent variability with the Yanai wave. The influence of meridional wind on Yanai wave generation is unlikely due to the winds' high frequency and small amplitude. The estimated downward energy flux of Yanai wave (1.4 mW ) represents 14%–23% of the barotropic energy conversion sustaining TIWs at the equator. These results provide observational evidence linking a TIW passage to the observed subsurface Yanai wave and suggest that Yanai waves provide an important pathway for TIW energy dissipation and vertical energy redistribution.

Small-scale turbulent mixing in the ocean interior is vital in governing ocean circulation and tracer distributions, and hence global climate. However, the planetary extent of this role and its dependence on the microphysics of mixing remain inadequately understood. Here, we emphasize the variety of spatio-temporal scales on which such interior turbulent mixing can shape the climate system. In addition to its well-established role in facilitating the equilibration of deep branches of ocean circulation on centennial-to-millennial timescales, interior turbulent mixing is a leading determinant of oceanic tracer budgets on timescales as short as sub-annual. We highlight the importance of the co-dependence of vertical (diapycnal) mixing and lateral (isopycnal) stirring in establishing the large-scale impacts of oceanic turbulence. We conclude with a summary of theoretical, observational and computational bottlenecks in the way of a sufficiently accurate representation of mixing in Earth System Models, and discuss emerging opportunities for making progress in these areas.

We demonstrate the combined utility of distributed acoustic sensing (DAS) and repeat conductivity‐temperature‐depth (CTD) profiling for observing internal tide dynamics over a sloping seafloor. While DAS has been widely proposed as a method to infer ocean temperature variability from seafloor cables, quantitative in situ validation has been limited. Here we present the first published detailed comparison between DAS‐inferred bottom temperature fluctuations and near‐bottom temperature measured from high‐resolution yoyo‐CTD time series reaching 5 m above the seafloor, conducted over the EllaLink/GeoLab cable on the slope of Madeira. Linear regressions yield an empirical DAS temperature sensitivity of 21–37 µstrain , a factor 2.5–4 greater than commonly used theoretical estimates. The calibrated DAS record reveals semidiurnal internal tide bores propagating upslope to 1,300 m depth, where propagation halts and transitions to a complex interference pattern. These observations highlight the potential of combining seafloor fiber sensing with targeted in situ profiling to observe near‐bottom internal tide breaking processes.

The abyssal Ionian Sea is a deep region of interest for the entire ocean circulation of the Mediterranean Sea, since it plays an important role in the ventilation processes of the whole basin. Here we investigate spatial patterns of diapycnal mixing due to internal waves, over the bottom of the Ionian sub-basin. To identify regional features of the internal wave field in terms of vertical shear and strain, we analyze LADCP and CTD profiles, measured across the basin in 2007, covering various seafloor morphologies (shelf, shelf break, and abyssal plain). Our results show that increasing seafloor roughness reduces the variability of the shear-to-strain ratio, a pattern also influenced by correlations between slope and roughness. Roughness appears to constrain waves toward higher frequencies, with high shear-to-strain ratios associated with lower frequencies and flatter propagation angles, and low ratios linked to higher frequencies and steeper beams. Spectral analyses indicate that rougher regions enhance strain variance at small vertical scales while reducing shear variance at larger scales, leading to flatter shear spectra in the low-wavenumber band. Together, these findings suggest that roughness redistributes energy from large-scale toward small-scale, fundamentally altering the balance of internal wave energy across scales. These results expand our insights for 3D ocean circulation models, providing useful knowledge for ad hoc parameterization of mixing that should capture abyssal, internal wave–driven processes in the Mediterranean Sea.

Deep‐ocean upwelling, driven by small‐scale turbulence, plays a key role in climate by regulating the ocean's capacity to sequester heat and carbon. Recent theoretical studies have hypothesized that such upwelling may primarily occur within a bottom boundary layer (BBL) along the sloping seafloor. A dye experiment in a continental‐slope canyon during the BLT‐Recipes program revealed very rapid BBL‐focussed upwelling, endorsing this notion. Here, we elucidate the dynamical connection between the mixing and the upwelling. We show that along‐canyon upwelling stems from episodic turbulent mixing cells up to 250 m high, generated by tides sweeping up‐ and down‐canyon. The tidal currents support a vertical shear that periodically advects dense waters over slower‐flowing lighter waters, reducing BBL stratification. This triggers instabilities that mix the dense waters with neighboring lighter waters, resulting in net along‐boundary upwelling. Our findings substantiate the view that deep‐ocean upwelling can predominantly occur along the ocean's sloping boundaries.

Mixing in the ocean abyss sustains the deepest branches of the global overturning circulation, yet the processes that drive deep-ocean mixing remain poorly understood. Recent field measurements in a deep submarine canyon of the Rockall Trough have revealed that intense mixing occurs during strong, vertical shear-generated overturns exceeding 200 meters. These overturning events last only a few hours and occur at different tidal phases across sections of the canyon. We investigate the origin of the shear and its dependence on along-canyon location using a high-resolution numerical simulation and theory. The tidal shear is associated with internal Kelvin waves trapped by the canyon’s geometry, whose structure varies along the slope, resulting in distinct tidal phases of shear at different canyon locations. These waves are confined to the depth of the canyon, resulting in stronger shear than the low-mode internal waves that dominate in the ocean interior. Our results suggest that the strong shear associated with standing waves in canyon may play an important role in driving abyssal ocean mixing.

Deep-ocean upwelling, driven by small-scale turbulence, plays a key role in climate by regulating the ocean’s capacity to sequester heat and carbon. Recent theoretical studies have hypothesized that such upwelling may primarily occur within a bottom boundary layer (BBL) along the sloping seafloor. A recent dye experiment in a continental-slope canyon as part of the BLT-Recipes program revealed very rapid BBL-focussed upwelling, endorsing this notion. Here, we elucidate the mixing processes governing the upwelling. We show that along-canyon upwelling stems from episodic turbulent mixing cells up to 250 m high, generated by tides sweeping up- and down-canyon. The tidal currents support a vertical shear that advects dense waters over slower-flowing lighter waters, reducing BBL stratification. This triggers instabilities that mix the dense waters with neighboring lighter waters, resulting in a net along-boundary upwelling flow. Our findings substantiate the view that deep-ocean upwelling can predominantly occur along the ocean’s sloping boundaries.

Velocity and turbulence observations are used to estimate the forward cascade of kinetic energy from the internal tide to dissipation within a steep canyon. Two methods for computing cross‐frequency kinetic energy flux are compared to observed dissipation. One method, coarse graining, allows strongly nonlinear dynamics while the other assumes weak nonlinearity. Fluxes from both methods agree within a factor of 3 with dissipation estimates from a finescale parameterization which is often used in climate‐scale ocean models. Coarse graining predicts 68% of energy fluxing to dissipation from frequencies lower than 8cpd, while the weakly nonlinear method predicts 34%. The weighting of energy flux toward lower frequencies supports a shorter frequency‐space pathway to dissipation in the presence of topographic wave breaking than assumed by parameterizations. Enhanced near‐boundary mixing and upwelling has implications for the rate and spatial distribution of the upwelling branch of the global overturning circulation.