In this paper, the importance of model horizontal resolution in identifying the nature of mixing and dispersion is investigated by comparing two data-assimilative, high-resolution simulations (4km and 1km), one of which is submesoscale-resolving. By employing both Eulerian and Lagrangian metrics, upper-ocean differences between the mesoscale- and submesoscale-resolving simulations are examined in the northeastern Gulf of Mexico, a region of high mesoscale and submesoscale activity. The nature of mixing in both simulations is identified by conducting Lagrangian experiments to track the generation of Lagrangian Coherent Structures (LCSs) and their associated transport barriers. Finite-time Lyapunov exponents (FTLE) fields show higher separation rates of fluid particles in the submesoscale-resolving case which indicate more vigorous mixing, with differences being more pronounced in the shelf regions (depths<=500m). The extent of the mixing homogeneity is examined by using probability density functions (PDFs) with results suggesting that mixing is heterogeneous in both simulations, but some homogeneity is exhibited in the submesoscale-resolving case. The FTLE fields also indicate that chaotic stirring dominates turbulent mixing in both simulations regardless of the horizontal resolution. In the submesoscale-resolving experiment, however, smaller scale LCSs emerge as noise-like filaments that suggest a larger turbulent mixing component than in the mesoscale-resolving experiment. The impact of resolution is then explored by investigating the spread of oil particles at the location of the Deepwater Horizon oil spill.

This study utilizes Deep Neural Networks (DNN) to improve the K-Profile Parameterization (KPP) for the vertical mixing effects in the ocean’s surface boundary layer turbulence. The DNNs were trained using 11-year turbulence-resolving solutions, obtained by running a large eddy simulation model for Ocean Station Papa, to predict the turbulence velocity scale coefficient and unresolved shear coefficient in the KPP. The DNN-augmented KPP schemes (KPP_DNN) have been implemented in the General Ocean Turbulence Model (GOTM). This implementation is stable for long-term integration and as efficient as existing variants of KPP schemes. Three different KPP_DNN schemes, varying in input and output variables, have been developed and trained. The performance of models using the KPP_DNN schemes is compared with that of those using popular deterministic first-order and second-moment closure turbulent mixing parameterizations. Solution comparisons show that the simulated mixed layer is cooler and deeper, aligning closely with observations when wave effects are included in parameterizations. In the KPP framework, changes to the velocity scale of unresolved shear, which is used to calculate mixed layer depth, have a larger impact on the simulated mixed layer than do changes to the magnitude of diffusivity. In the KPP_DNN, changes to unresolved shear depend on not only on wave forcing, but also on the mixed layer depth and buoyancy forcing.

Uncertainty exists in the time-mean total transport of the Antarctic Circumpolar Current (ACC), the world’s strongest ocean current. The two most recent observational programs in Drake Passage, DRAKE and cDrake, yielded transports of 141 and 173.3 Sv, respectively. In this paper, we use a realistic 1/12 global ocean simulation to interpret these observational estimates and reconcile their differences. We first show that the modeled ACC transport in the upper 1000 m is in excellent agreement with repeat shipboard acoustic Doppler current profiler (SADCP) transects and that the exponentially decaying transport profile in the model is consistent with the profile derived from repeat hydrographic data. By further comparing the model results to the cDrake and DRAKE observations, we argue that the modeled 157.3 Sv transport, i.e. approximately the average of the cDrake and DRAKE estimates, is actually representative of the time-mean ACC transport through the Drake Passage. The cDrake experiment overestimated the barotropic contribution in part because the array undersampled the deep recirculation southwest of the Shackleton Fracture Zone, whereas the surface geostrophic currents used in the DRAKE estimate yielded a weaker near-surface transport than implied by the SADCP data. We also find that the modeled baroclinic and barotropic transports are not correlated, thus monitoring either baroclinic or barotropic transport alone may be insufficient to assess the temporal variability of the total ACC transport.

Fine-scale motions ($<$100 km) contribute significantly to the exchanges and dissipation of kinetic energy in the upper ocean. However, knowledge of ocean kinetic energy at fine-scales (in terms of density and transfers) is currently limited due to the lack of sufficient observational datasets at these scales. The sea-surface height measurements of the upcoming SWOT altimeter mission should provide information on kinetic energy exchanges in the upper ocean down to 10-15 km. Numerical ocean models, able to describe ocean dynamics down to $\sim$10 km, have been developed in anticipation of the SWOT mission. In this study, we use two state-of-the-art, realistic, North Atlantic simulations, with horizontal resolutions $ \sim $ 1.5 km, to investigate the distribution and exchanges of kinetic energy at fine-scales in the open ocean. Our results show that the distribution of kinetic energy at fine-scales approximately follows the predictions of quasi-geostrophic dynamics in summertime but is somewhat consistent with submesoscale fronts-dominated regimes in wintertime. The kinetic energy spectral fluxes are found to exhibit both inverse and forward cascade over the top 1000 m, with a maximum inverse cascade close to the average energy-containing scale. The forward cascade is confined to the ocean surface and shows a strong seasonality, both in magnitude and range of scales affected. Our analysis further indicates that high-frequency motions ($<$1day) play a key role in the forward cascade and that the estimates of the spectral fluxes based on geostrophic velocities fail to capture some quantitative aspects of kinetic energy exchanges across scales.

The South Atlantic Ocean plays an important role in the Atlantic meridional overturning circulation (AMOC), connecting it to the Indian and Pacific Oceans as part of the global overturning circulation system; yet the detailed time mean circulation structure in this region and the large-scale spatial pattern of the AMOC variability remain unclear. Using model outputs from a 60-year, eddying global ocean-sea ice simulation validated against observations at a zonal section at 34°S, a meridional section at 65°W in the Drake Passage, and a meridional section southwest of Africa, we show that the upper limb of the AMOC originates primarily from the Agulhas leakage and that, while the cold Pacific water from the Drake Passage does not contribute significantly to the AMOC, it does play a role in setting the temperature and salinity properties of the water masses in the subtropical South Atlantic. We also find that the North Atlantic deep water (NADW) in the lower limb of the AMOC flows southward as a deep western boundary current all the way to 45°S and then turns eastward to flow across the Mid-Atlantic Ridge near 42°S, and that the recirculation around the Vitoria-Trindade seamount chain brings some NADW into the Brazil Basin interior. Finally, we find that the modeled AMOC variability is coherent on interannual to decadal timescales from 35°S to about 35°N, where diapycnal water mass transformations between the upper and lower limbs of the AMOC are expected to be small.

The wavenumber spectral slope of sea surface height (SSH) computed within the mesoscale range from satellite altimetry exhibits a large spatial variability which, until now, has not been reproduced in numerical ocean models. This study documents the impacts of including internal tides, high-resolution bathymetry, and high-frequency atmospheric variability on the SSH wavenumber spectra in the Atlantic Ocean, using a series of 1/50° Equatorial and North Atlantic simulations with a realistic representation of barotropic/baroclinic tides and mesoscale-to-submesoscale variability. The results show that the inclusion of internal tides does increase high frequency SSH variability (with clear peaks near 120 km and 70 km) and flattens the spectra slope in the mesoscale range in a good agreement with observations. The surface signature of internal tides, mostly in the equatorial Atlantic but also in subtropical regions in the eastern North Atlantic, is the primary reason behind the observed large spatial variability of the spectral slope in the Atlantic. Internal tides are stronger in the tropical regions when compared to higher latitudes because of the stronger barotropic tides and stronger stratification in the upper layer of the water column. High-resolution bathymetry does play an important role in the internal tide generation on a local scale, but its impact on large-scale SSH variability and SSH wavenumber spectra is quite small. High-frequency wind variability plays only a minor role on the generation of high-frequency SSH variability.

Ocean circulation is dominated by turbulent geostrophic eddy fields with typical scales ranging from 10 km to 300 km. At mesoscales (> 50 km), the size of eddy structures varies regionally following the Rossby radius of deformation. The variability of the scale of smaller eddies is not well known due to the limitations in existing numerical simulations and satellite capability. But it is well established that oceanic flows (< 50km) generally exhibit strong seasonality. In this study, we present a basin-scale analysis of coherent structures down to 10\,km in the North Atlantic Ocean using two submesoscale-permitting ocean models, a NEMO-based North Atlantic simulation with a horizontal resolution of 1/60 (NATL60) and an HYCOM-based Atlantic simulation with a horizontal resolution of 1/50 (HYCOM50). We investigate the spatial and temporal variability of the scale of eddy structures with a particular focus on eddies with scales of 10 to 100\,km, and examine the impact of the seasonality of submesoscale energy on the seasonality and distribution of coherent structures in the North Atlantic. Our results show an overall good agreement between the two models in terms of surface wavenumber spectra and seasonal variability. The key findings of the paper are that (i) the mean size of ocean eddies show strong seasonality; (ii) this seasonality is associated with an increased population of submesoscale eddies (10\,–\,50\,km) in winter; and (iii) the net release of available potential energy associated with mixed layer instability is responsible for the emergence of the increased population of submesoscale eddies in wintertime.