Lagrangian spin parameter and coherent structures from trajectories released in a high-resolution ocean model

A study of the mesoscale eddy field in the presence of coherent vortices, by means of Lagrangian trajectories released in a high-resolution ocean model, is presented in this paper. The investigation confirms previous results drawn from real float data statistics (Veneziani et al., 2004) that the eddy field characteristics are due to the superposition of two distinct regimes associated with strong coherent vortices and with a typically more quiescent background eddy flow. The former gives rise to looping trajectories characterized by subdiffusivity properties due to the trapping effect of the vortices, while the latter produces nonlooping floats characterized by simple diffusivity features. Moreover, the present work completes the study by Veneziani et al. (2004) in regard to the nature of the spin parameter Ω, which was used in the Lagrangian stochastic model that best described the observed eddy statistics.The main result is that the spin obtained from the looping trajectories not only represents a good estimate of the relative vorticity of the vortex core in which the loopers are embedded, but it is also able to follow the vortex temporal evolution. The Lagrangian parameter Ω is then directly connected to the underlying Eulerian structure and could be used as a proxy for the relative vorticity field of coherent vortices

Pathways of Nordic Overflows from climate model scale and eddy resolving simulations

The overflows of cold, heavy waters from the Nordic Seas across the Greenland–Iceland–Scotland Ridges are simulated using the Hybrid Coordinate Model in a North Atlantic configuration. Results at three different horizontal model resolutions are compared to each other, to recent hydrographic sections and moored observations. Simulations in the finest grid employed, 1/12° resolution, show realistic overflow pathways, reasonable overflow and Deep Western Boundary Current mean velocities and transports, and overall reasonable North Atlantic three-dimensional temperature and salinity fields, namely the Atlantic Meridional Overturning Circulation (AMOC). In contrast, simulations at coarser grids of 1/3° and 1° resolution exhibit a range of significant problems owing to unresolved, dynamically vital features in the seafloor topography. This lack of resolution, for example of the Faroe Bank Channel, leads to unrealistic overflow pathways between Iceland and Scotland in the 1/3° and 1° cases. Accordingly, overflow mass transports are also unrealistic in this area. In the Denmark Strait Overflow the underlying topographical scales are larger, and pathways are reasonable even at coarse resolution. However, overflow speeds are too small in the 1/3° and 1° cases. Underestimated velocities in the 1° simulations are compensated by an overestimated sill cross-section, whereas it is too small in 1/3°. As such, the 1/3° and 1° simulations show both large under- and overestimations of volume transport at several locations. No significant improvement in modeled overflows takes place when the grid spacing is decreased from 1° to 1/3°. An experiment conducted with hand-tuned topography shows improved volume transports near the regions of modification, but somewhat increased errors in other parts of the deep circulation, indicating the complex response of the system to perturbations in bathymetry. These results demonstrate the importance of an accurate representation of the domain geometry, in particular the channels of the complex Iceland–Scotland ridge system, in order to reproduce the pathways of the deep AMOC

Lagrangian Data in a High Resolution Numerical Simulation of the North Atlantic. II: On the Pseudo-Eulerian Averaging of Lagrangian Data

In this paper, the statistical properties of the mean flow reconstruction using Lagrangian data are studied, considering the classical "binning" approach based on space-time averaging of finite difference velocity estimates. The work is performed numerically, using as the test flow a solution from a high resolution MICOM simulation of the North Atlantic. A set of trajectories are computed, simulating the motion of surface drifters initially launched on a regular 1 o \Theta 1 o array, transmitting positions every \Deltat = 12 hours, and analyzed over approximately 2 years of the simulation. The drifter distribution in time is influenced by the Ekman flow, resulting in maximum data concentration in the subtropical convergence regions and minimum concentration in the upwelling regions. Pseudo-Eulerian averages U pE , computed from Lagrangian data, are compared to "true" Eulerian averages UE , computed from grid point velocities inside 1 o \Theta 1 o bins for approximately 2 years..

North Brazil Current rings and transport of southern waters in a high resolution numerical simulation of the North Atlantic

Output from a very high resolution (1/12 deg.) North Atlantic simulation with the Miami Isopycnic Coordinate Ocean Model (MICOM) is analyzed in a region of the Tropical Atlantic characterized by the presence of the North Brazil Current (NBC) retroflection and North Brazil Current rings. The model mean and seasonal circulations present a good qualitative agreement with observations. Quantitatively, the modeled NBC in summer and fall does not completely retroflect into the North Equatorial Counter Current, and the model upper 100 m NBC is more intense than the observed values by 3-4 Sv. The modeled NBC generates a variety of rings, which we classify as ‘shallow’, ‘intermediate’, ‘deep’, and ‘subsurface’. An average of 8.3 rings of all types are generated per year, of which 6 are surface intensified, in good agreement with altimetry (5.7 rings per year, Goni and Johns, 2001). The transport of southern origin water by the the rings was estimated using two methods. First, the transport was computed kinematically from the rings' volume, resulting in an average transport of 6.6 Sv. Second, an estimation of southern water transport based on an explicit calculation of water mass content was done, resulting in an average transport of 7.5 Sv. The rings' contribution represents 0% of the total meridional transport from the surface to the intermediate water layers. Possible mechanisms operating in the model ring generation are briefly discussed

Parameterization of particle transport at submesoscales in the Gulf Stream region using Lagrangian subgridscale models

► Subgrid-scale error in particle transport is noted in mesoscale eddy resolving models. ► A hybrid approach of combining the effect of computed mesoscale eddies with Lagrangian subgrid-scale models is pursued. ► Improved particle transport over the submesocales is achieved. Ocean model fields are being routinely used for forecasting the spreading of pollutants, oil spills, and for biogeochemical transport. Recent observations and advances in our understanding of ocean processes indicate there is an explosion of flow instabilities in the submesoscale range. While submesoscale flows have a significant impact on transport at their own scales, they require much more extensive data sets and numerical computations. Therefore, transport carried out by submesoscale flows is quite challenging to approach deterministically. In this study, we put forward a hybrid approach by combining deterministic Lagrangian coherent structures (LCS) to compute transport over the mesoscale range with statistical Lagrangian subgridscale (LSGS) models for the underresolved submesoscale motions. We apply this approach to particle transport in the Gulf Stream region, which exhibits indications of submesoscale activity from both models and observations. We consider HYCOM solutions at two resolutions. In the 1/12° computation, mesoscale features are well resolved but submesoscales are not resolved, while the 1/48° computation captures some of the submesoscale flow instabilities as well. By using metrics of relative dispersion, we investigate three LSGS models and demonstrate that they can be useful in correcting the underestimation of submesoscale dispersion in the 1/12° solution, with respect to relative dispersion obtained from the 1/48° solution and an observational result

Resolution dependent relative dispersion statistics in a hierarchy of ocean models

In order to determine the effect of Eulerian spatial resolution on the two particle statistics of synthetic drifter trajectories, we examine a hierarchy of ocean models, starting from 2D turbulence simulations, progressing to idealized simulations of a buoyant coastal jet with ROMS, and finally to realistic HYCOM simulations of the Gulf Stream. In each case, particle dispersion at large time and space scales is found to be controlled by energetic meso-scale features of the flow that are relatively insensitive to the resolution of finer scale motions. In all cases, time-distance graphs given in terms of computed Finite Scale Lyapunov Exponents show an expected increase in the extent of exponential scaling with increasing spatial smoothing of the velocity field. The limiting value of the FSLE at small separation distances is found to scale remarkably well with the resolution of Eulerian velocity gradients as given by the average of positive Okubo–Weiss parameter values

Seasonality of the submesoscale dynamics in the Gulf Stream region

Frontogenesis and frontal instabilities in the mixed layer are known to be important processes in the formation of submesoscale features. We study the seasonality of such processes in the Gulf Stream (GS) region. To approach this problem, a realistic simulation with the Hybrid Coordinate Ocean Model is integrated for 18 months at two horizontal resolutions: a high-resolution (1/48°) simulation able to resolve part of the submesoscale regime and the full range of mesoscale dynamics, and a coarser resolution (1/12°) case, in which submesoscales are not resolved. Results provide an insight into submesoscale dynamics in the complex GS region. A clear seasonal cycle is observed, with submesoscale features mostly present during winter. The submesoscale field is quantitatively characterized in terms of deviation from geostrophy and 2D dynamics. The limiting and controlling factor in the occurrence of submesoscales appears to be the depth of the mixed layer, which controls the reservoir of available potential energy available at the mesoscale fronts that are present most of the year. Atmospheric forcings are the main energy source behind submesoscale formation, but mostly indirectly through mixed layer deepening. The mixed layer instability scaling suggested in the (Fox-Kemper et al., J Phys Oceanogr 38:1145–1165, 2008) parametrization appears to hold, indicating that the parametrization is appropriate even in this complex and mesoscale dominated area