Joseph Henry LaCasce

Understanding oceanic transport and material fate has long relied on observations that primarily resolve mesoscale, largely geostrophic flows, leaving the role of submesoscale turbulence comparatively unobserved and poorly understood. Here, using new high-resolution drifter-cluster experiments with closest initial spacings below 1 km, we provide direct Lagrangian observations of how submesoscale turbulence modifies mesoscale eddy coherence and transport pathways. The clustered drifters resolve intense, transient ageostrophic motions that are fundamentally absent from geostrophic descriptions of eddy behavior. Analyzed in an eddy-centered reference frame, the observations reveal pronounced submesoscale-driven radial dispersion that facilitates substantial water-mass exchange between the eddy core and its periphery in both the mixed layer and the subsurface. Quantitative estimates indicate submesoscale horizontal diffusivity of O (10 to 100) m ² /s and a nonlocal dispersion regime across submesoscales. These results challenge the traditional view of mesoscale eddies as coherent structures containing a persistent water mass, with broad implications for interpreting and predicting the transport and fate of tracers in the ocean.

Mesoscale eddies play a key role in redistributing oceanic heat and salt, yet their global three‐dimensional (3D) transport structures remain poorly understood. Here, we estimate global meridional eddy heat and salt transports in the upper 1,800 m from satellite altimeter and Argo observations, revealing distinct vertical structures. Eddy heat transport reverses with depth, being poleward in the upper ocean but equatorward below, resulting in strong vertical cancellation and a weak net contribution to total meridional heat transport. In contrast, eddy salt transport exhibits pronounced latitude dependence, weakly equatorward at low latitudes but strongly poleward at mid‐latitudes, where it contributes nearly half of the total meridional salt transport. These contrasting behaviors primarily arise from differences in background temperature and salinity gradients, consistent with down‐gradient eddy fluxes. Our results provide novel observational insights into the global 3D structures of eddy heat and salt transports, offering a benchmark for evaluating ocean and climate models.

In the Nordic Seas, the main reservoir of heat is in the Lofoten Basin of the Norwegian Sea, where eddies detaching from the slope current bring warm water into the basin. The basin‐mean eddy diffusivity from surface drifters is about 2,000 . Subsurface horizontal diffusivities are calculated from Argo profiling float data with two independent methods to extend previous work. First, temperature anomalies are used to estimate eddy mixing lengths, which are then combined with root‐mean‐squared velocity fluctuations to obtain diffusivities in the upper 2,000 m. Second, diffusivities are calculated based on single particle across‐stream dispersion from the float trajectories at their parking depths near 1,000 m. The mean diffusivity across the Lofoten Basin is about 400 in the upper 1,000 m. With weak stratification, eddy‐driven stirring extends throughout the measured upper 2,000 m. From these diffusivities and the mean temperature gradient, the heat flux is calculated, which is directed into the central Lofoten Basin. Heat flux convergence leads to a mean warming of the Lofoten Basin by about 0.5C , which is comparable to the mean net surface heat flux. This heat is extracted from the poleward flowing Atlantic Water in the Norwegian Atlantic Current. Elevated diffusivities are also found on the slope of the Norwegian Basin, where cooling is noted. Our measurements provide a constraint on the heat budget of the Norwegian Sea.

The instability of baroclinic Rossby waves over two-dimensional topography is examined using a nonlinear model, a linear stability calculation and wave triads. In all cases, the Rossby waves are unstable, as seen previously over a flat bottom. But topography decreases the growth rates and changes the structure of the unstable waves. When the topographic height (or slope) exceeds a critical value, the instability is ‘locked’ to topography, in that the most unstable mode, particularly in the lower layer, resembles the bathymetry. In this limit, the growth rate becomes independent of topographic height. A triad calculation suggests that the growth rates in the locked state should depend on the lateral scale of the bathymetry but not its height, and that locking does not occur for topographic scales smaller than the surface deformation radius. The results suggest an alternate way that topographically locked flow can be generated, and indicate that baroclinic instability can be much different over steep bathymetry.

We investigate how a topographic slope impacts eddy diffusivities in a two‐layer quasi‐geostrophic model. There are asymmetric effects of retrograde slopes, where the layer interface and the topography tilt in the same direction, and prograde slopes, where the interface and topography tilt in opposite directions. Moreover, there is asymmetry between the upper and lower layer. Steep retrograde slopes suppress the eddy diffusivity in both layers compared to flat or weak slopes. With a strong prograde slope, coherent, long‐lived vortices form in the upper layer; as these are surface‐trapped, they are not influenced by topography or bottom friction, and the diffusivity in the upper layer is thus relatively unaffected by the slope. In the lower layer, however, the diffusivities decrease with slope magnitude for both prograde and retrograde slopes. We also compare the Lagrangian diffusivity, derived from particle tracking experiments, and the Eulerian diffusivity, based on the flux‐gradient relation for potential vorticity (PV). The two values agree in the upper layer, but not in the lower layer. We present a new expression relating Eulerian and Lagrangian diffusivities, and this correctly captures the differences seen in the lower layer. The difference occurs because bottom friction alters the PV along the particle tracks. The results underline the importance of considering both topographic slopes and bottom friction in parametrizations of mesoscale eddy stirring.

Calculating energy spectra in the ocean is impeded by the resolution limitations of satellite and shipboard acoustic Doppler current profiler (ADCP) data. An alternate approach is to use data from GPS-tracked surface drifters, which offer resolution to finer than 10 m. While direct spectral estimates from drifter velocities have been unsuccessful, indirect estimates, based on the dispersion of pairs of drifters, are possible. Using drifter data from the Gulf of Mexico, we infer wavenumber spectra by comparing pair dispersion to that expected with different spectral slopes. The measures include both time- and separation-based metrics and establish consistency between the two using the recently proposed cumulative inverse separation time (CIST), a separation-based measure similar to the finite-scale Lyapunov exponent (FSLE). The results indicate a K⁻² dependence from 100 m to several kilometers. The spectral slope is the same up to the deformation radius (∼50 km) in winter, while in summer, it is steeper, close to a K⁻³ dependence. Additional separation-based metrics suggest a shallower K^−5/3 spectrum in the 10–100-m range, but this cannot be confirmed with the time-based metrics. The results are consistent with previous drifter studies and help resolve differences between them. Differences between “original” and “chance” pairs are seen at the smallest scales (<1 km), where the dispersion is affected by the initial conditions. Furthermore, the FSLE fails to capture the smallest-scale dispersion, while a modified version, the finite amplitude growth rate (FAGR), is more successful. The findings will help guide future studies of submesoscale dispersion. Significance Statement: Wavenumber spectra of kinetic energy are central for characterizing turbulence and developing parameterizations for models. While satellite observations and shipboard velocity measurements can be used to produce spectra at the ocean surface at 10-km resolution, GPS-tracked surface drifters offer the possibility of spectral estimates at much finer scales. By comparing the dispersion of pairs of drifters with analytical predictions, we infer energy spectra over scales from roughly 10 m to several hundred kilometers. The spectra exhibit clear power-law dependencies, with distinct seasonal variations, and help reconcile previous observational estimates.

Cyclonic flow in enclosed basins arises from cyclonic vorticity input from either winds or eddies. Previous observations show winds account for about half of the circulation (integral of parallel velocity around a closed isobath) in the Lofoten Basin of the Norwegian Sea, implying a comparable contribution from eddies. In contrast, in the Norwegian Basin, winds explain much of the circulation. We use Argo data from 2012 to 2022, a linear wind‐forced and bottom friction‐regulated model, and a primitive equation model to investigate how winds and eddies drive the circulation. While Argo cannot resolve individual eddies, it does resolve mesoscale spatial variability. Absolute geostrophic velocities to 2,000 m are calculated by referencing geostrophic shear from objective maps of the profile data to the trajectories at 1,000 m. The time‐mean shear is anticyclonic. However, the flow is mostly cyclonic, bottom‐intensified, and isobath following. The linear model explains less than half of the observed circulation, suggesting a contribution from an unobserved eddy flux. This interpretation is supported by a numerical model with 800‐m horizontal resolution, which resolves the 10–15‐km deformation radius. In this model, the difference between the full and linear response comes from an eddy‐driven vorticity flux, which produces a largely cyclonic circulation. This result is consistent with the observed global prevalence of positive topostrophy (anticyclonic flow around elevations and cyclonic flow around basins). The observations also show a seasonal shift in the eddy flux, which maybe related to a decadal‐scale fresh inflow that alters the density gradient and influences eddy production.

The quasi-geostrophic two-layer model is a widely used tool to study baroclinic instability in the ocean. One instability criterion for the inviscid two-layer model is that the potential vorticity (PV) gradient must change sign between the layers. This has a well-known implication if the model includes a linear bottom slope: for sufficiently steep retrograde slopes, instability is suppressed for a flow parallel to the isobaths. This changes in the presence of bottom friction as well as when the PV gradients in the layers are not aligned. We derive the generalised instability condition for the two-layer model with non-zero friction and arbitrary mean flow orientation. This condition involves neither the friction coefficient nor the bottom slope; even infinitesimally weak bottom friction destabilises the system regardless of the bottom slope. We then examine the instability characteristics as a function of varying slope orientation and magnitude. The system is stable across all wavenumbers only if friction is absent and if the planetary, topographic and stretching PV gradients are aligned. Strong bottom friction decreases the growth rates but also alters the dependence on bottom slope. In conclusion, the often mentioned stabilisation by steep bottom slopes in the two-layer model holds only in very specific circumstances, thus probably plays only a limited role in the ocean.