John R. Taylor

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.

The coastal ocean in the northern Gulf of Mexico is highly productive and socioeconomically important. Here, stratification can inhibit vertical exchange, with consequences for ecosystem health and hypoxia. Previous work emphasized the role of wind-driven turbulent mixing as a mechanism to overcome the stratification barrier. We identify symmetric instability (SI) as an additional and more energy-efficient pathway linking surface and bottom waters. In high-resolution observations, diagonal bands of overturning motions, a telltale sign of SI, connect the sea surface to the bottom and produce intrusions of temperature and oxygen anomalies. Overturning persists for 2 days after instability-favorable wind ceases. During this time, vertical advective fluxes exceed turbulent fluxes by an order of magnitude, ventilating low-oxygen bottom waters and transporting surface heat downward. Our results show that SI facilitates vertical exchange that can outlive direct wind forcing, highlighting an instability-driven mechanism that may be important in coastal oceans more generally.

Submesoscale eddies are important in setting the stratification in the ocean surface mixed layer and transporting energy between large and small scale motions. However, the study of submesoscale on a global scale has been hindered by a shortage of global, long-term datasets. To meet this need, we apply an unsupervised machine learning method adapted from the profile classification model (PCM) to density profiles collected by Argo floats over the global ocean from 2000-2021, producing the first global observational climatology of submesoscale restratification. The method classifies individual vertical profiles based on the shape of the density profile in the ocean surface mixed layer. The fraction of profiles that exhibit a shape characteristic of submesoscale is referred to as the submesoscale restratification (SR) index. The SR index peaks in spring in both hemispheres and lags the maxima of mixed layer depth by one month, suggesting that submesoscale eddies play an important role in restratifying the mixed layer. Hotspots of SR index can be found in the Norwegian Sea and the Drake Passage in spring. This method enables the study of the spatial and temporal distributions of submesoscale restratification on a global scale.

Layer formation can occur within stratified fluids, often associated with the effect of ‘double diffusion’ where the fluid buoyancy depends on two components with differing molecular diffusivities (e.g. heat and salt in seawater). However, since layering also occurs in one-component stratified fluids, the generation mechanism for layers is often unclear. In this paper, we present a framework that unifies multiple-layer generation mechanisms across both one- and two-component stratified fluids. We demonstrate how these mechanisms can be assessed using simulations of double-diffusive intrusions. Our simulations illustrate the importance of the negative turbulent diffusivity for buoyancy in contributing to layer formation.

Buoyancy-driven exchange flows in geophysical contexts often exhibit significant interfacial turbulence leading to a partially mixed intermediate layer between two counterflowing layers. In this paper we perform a three-layer hydraulic analysis of such flows, highlighting the dynamical importance of the middle mixed layer. Our analysis is based on the viscous, shallow water, Boussinesq equations and includes the effects of mixing as a non-hydrostatic pressure forcing. We demonstrate the superior predictive accuracy of three-layer hydraulics over the more classical two-layer approach by applying it to direct numerical simulation data in stratified inclined duct exchange flows where turbulence is controlled by a modest slope of the duct. The three-layer model predicts a region bounded by two control points in the middle of the duct, linked to the onset of instability and turbulence, whereas a two-layer model only predicts one control point. We show that the nonlinear characteristics of the three-layer model correspond to linear long waves perturbing a three-layer mean flow. We also provide the first evidence of long-wave resonance, as well as resonance between long and short waves, and their connection to turbulence. These results challenge current parameterisations for turbulent transport, which typically overlook long waves and internal hydraulics induced by streamwise variations of the flow.

This study examines the interplay between water column structure, tidal currents, and basal melting at a site beneath Ronne Ice Shelf, using a 3‐year data set of oceanographic measurements, and a collocated year‐long time series of radar‐derived melt rate estimates. Currents at the site are characterized by mixed semidiurnal tides with strong spring‐neap variability, superimposed on a nontidal flow. The product of current speed and thermal driving, both measured approximately 19 m from the ice base, explains 88% of the melt rate variability. Although current speed is the dominant driver of this variability, thermal driving also contributes non‐negligibly on spring‐neap and longer timescales. The semidiurnal tidal ellipses feature marked vertical variations, transitioning from nearly rectilinear in the mid‐water column to more circular and clockwise (CW)‐rotating near the ice. This depth‐dependence of the semidiurnal tide is attributed to the differential influence of boundary friction on the CW and anticlockwise (ACW) rotary components near the critical latitude (where the tidal frequency equals the Coriolis frequency). A theoretical model, which assumes depth‐independent eddy viscosity, successfully reproduces the observed 3‐year mean vertical structure of the tidal ellipses. Considering the total tidal current rather than individual constituents, ice base friction damps both the time‐mean flow speed and the tidal fluctuations, with attenuation varying over the spring‐neap cycle, peaking during spring tides. The observed latitude‐ and time‐dependent effects of ice base friction on the barotropic tide are not captured in parameterizations that estimate tide‐induced friction velocity by scaling the time‐averaged barotropic tidal speed with a constant drag coefficient.