Velocity and turbulence observations are used to estimate the forward cascade of kinetic energy from the internal tide to dissipation within a steep canyon. The semidiurnal tide is the only significant kinetic energy source within the canyon, simplifying the energetic pathway analysis. 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 are greater than dissipation estimates from the finescale parameterization which is often used in global climate models. Coarse graining reveals energy flux to dissipation from lower internal wave frequencies. These results support the idea that there may be greater kinetic energy cascade due to wave breaking near topography than accounted for by current parameterizations. Enhanced near boundary mixing and upwelling has implications for the rate and spatial distribution of the upwelling branch of the global overturning circulation.

It is well established that small scale cross-density (diapycnal) turbulent mixing induced by breaking of overturns in the interior of the ocean plays a significant role in sustaining the deep ocean circulation and in regulation of tracer budgets such as those of heat, carbon and nutrients. There has been significant progress in the fluid mechanical understanding of the physics of breaking internal waves. Connection of the microphysics of such turbulence to the larger scale dynamics, however, is significantly underdeveloped. We offer a hybrid theoretical-statistical approach, informed by observations, to make such a link. By doing so, we define a bulk flux coefficient, $\Gamma_B$, which represents the partitioning of energy available to an ‘ocean box’ (such as a grid cell of a coarse resolution climate model), from winds, tides, and other sources, into mixing and dissipation. $\Gamma_B$ depends on both the statistical distribution of turbulent patches and the flux coefficient associated with individual patches, $\Gamma_i$. We rely on recent parameterizations of ~$\Gamma_i$~ and the seeming universal characteristics of statistics of turbulent patches, to infer $\Gamma_B$, which is the essential quantity for representation of turbulent diffusivity in climate models. By applying our approach to climatology and global tidal estimates, we show that on a basin scale, energetic mixing zones exhibit moderately efficient mixing that induces significant vertical density fluxes, while quiet zones (with small background turbulence levels), although highly efficient in mixing, exhibit minimal vertical fluxes. The transition between the less energetic to more energetic zones marks regions of intense upwelling and downwelling of deep waters. We suggest that such upwelling and downwelling may be stronger than previously estimated, which in turn has direct implications for the closure of the deep branch of the ocean meridional overturning circulation as well as for the associated tracer budgets.

Tropical cyclones (TCs) are powered by heat fluxes across the air-sea interface, which are in turn influenced by subsurface physical processes that can modulate storm intensity. Here, we use data from 6 profiling floats to recreate 3D fields of temperature (T), salinity (S), and velocity around Super Typhoon Mangkhut (western North Pacific, September 2018). Observational estimates of vorticity (ζ) and divergence (Γ) agree with output from a 3D coupled model, while their relation to vertical velocities is explained by a linear theoretical statement of inertial pumping. Under this framework, inertial pumping is described as a linear coupling between ζ and Γ, whose cycles cause periodic displacements in the ocean thermocline and generate near-inertial waves (NIWs). Vertical profiles of T and S show gradual mixing of the upper ocean with diffusivities as high as κ~10^-1 m^2 s^-1, which caused an asymmetric cold wake of sea surface temperature (SST). We estimate that rain layer destruction used ~ 10% of energy used for mixing near the TC track, therefore inhibiting SST cooling. Lastly, watermass transformation analyses suggest that κ > 3x10^-3 m^2 s^-1 above ~110 m depth and up to 600 km behind the TC. These analyses provide an observational summary of the ocean response to TCs, demonstrate some advantages of ζ and Γ for the study of internal wave fields, and provide conceptual clarity on the mechanisms that lead to NIW generation behind TCs.

We present improvements in the modeling of the vertical wavenumber spectrum of the internal gravity wave continuum in high-resolution regional ocean simulations. We focus on model sensitivities to mixing parameters and comparisons to McLane moored profiler observations in a Pacific region near the Hawaiian Ridge, which features strong semidiurnal tidal beams. In these simulations, the modeled continuum exhibits high sensitivity to the background mixing components of the K-Profile Parameterization (KPP) vertical mixing scheme. Without the KPP background mixing, stronger vertical gradients in velocity are sustained in the simulations and the modeled kinetic energy and shear spectral slopes are significantly closer to the observations. The improved representation of internal wave dynamics in these simulations makes them suitable for improving ocean mixing estimates and for the interpretation of satellite missions such as the Surface Water and Ocean Topography (SWOT) mission.

A near-inertial wave stalling and breaking in a critical layer was observed for a week by a pair of autonomous velocity and density profilers on concentric paths in an ocean eddy. Profiler observations also provide estimates of the eddy’s vertical vorticity and shear, quantities needed to test theories of downward near-inertial wave (NIW) energy flux and loss from inertial wave–eddy interactions. The unique observations of the wave’s intrinsic frequency ᵱ4;ᵅ6; and vertical wavenumber ᵅA; provide a novel estimate of the vertical group velocity Cgz from changes in ᵱ4;ᵅ6; with respect to ᵅA;. Shear, strain, energy flux convergence, and parameterized turbulence are all elevated near 140 m depth, near the bottom of the strongest eddy velocities. Our observations are consistent with a downgoing NIW’s group velocity decreasing owing to wave–eddy interactions, providing important clues on global energetics of NIW mixing.

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.