Lagrangian Statistics from Oceanic and Atmospheric Observations

Journal of Atmospheric and Oceanic Technology, 2003

The statistical properties of Lagrangian velocities in a high-resolution numerical simulation of the North Atlantic Ocean are analyzed and discussed in the framework of particle dispersion parameterizations. Consistent with previous analyses of float trajectories, the modeled velocity distribution is shown to be non-Gaussian, both at the surface and at 1500 m. These results can have significant implications on oceanographic research, as they suggest that current parameterizations of particle dispersion by linear stochastic processes or eddy-diffusivity approaches may be incorrect, since they assume Gaussian velocity distributions. The results also indicate the need for empirical parameterizations of particle dispersion based on nonlinear stochastic processes. It is shown that, even for a truly non-Gaussian dataset, a Gaussian probability distribution function can be spuriously recovered when the sampling density is too low. The best compromise between data sampling and space averaging when a limited amount of data are available, as is the case in most field observations, is then identified.

2016

Lagrangian data can provide relevant information on the advection and diffusion properties of geophysical flows at different scales of motion. In this study, the dispersion properties of an ensemble of trajectories transported by a surface ocean flow are analyzed. The data come from a set of Lagrangian drifters released in the South Brazilian Bight, during several oceanographic campaigns. Adopting a dynamical systems approach, the attention is primarily focused on scale-dependent indicators, like the finite-scale Lyapunov exponent. The relevance of mechanisms like two-dimensional turbulence for the dispersion process is addressed. Some deviations from the classical turbulent dispersion scenario in two-dimensions are found, likely to be ascribed to the nonstationary and nonhomogeneous characteristics of the flow. Relatively small-scale features (of order 1-10 km) are also considered to play a role in determining the properties of relative dispersion as well as the shape of the kineti...

2000

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 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 in regard to the nature of the spin parameter ⍀, which was used in the Lagrangian stochastic model that best described the observed eddy statistics.

Journal of Physical Oceanography, 2004

The historical dataset provided by 700-m acoustically tracked floats is analyzed in different regions of the northwestern Atlantic Ocean. The goal is to characterize the main properties of the mesoscale turbulence and to explore Lagrangian stochastic models capable of describing them. The data analysis is carried out mostly in terms of Lagrangian velocity autocovariance and cross-covariance functions. In the Gulf Stream recirculation and extension regions, the autocovariances and cross covariances exhibit significant oscillatory patterns on time scales comparable to the Lagrangian decorrelation time scale. They are indicative of sub-and superdiffusive behaviors in the mean spreading of water particles. The main result of the paper is that the properties of Lagrangian data can be considered as a superposition of two different regimes associated with looping and nonlooping trajectories and that both regimes can be parameterized using a simple first-order Lagrangian stochastic model with spin parameter ⍀. The spin couples the zonal and meridional velocity components, reproducing the effects of rotating coherent structures such as vortices and mesoscale eddies. It is considered as a random parameter whose probability distribution is approximately bimodal, reflecting the distribution of loopers (finite ⍀) and nonloopers (zero ⍀). This simple model is found to be very effective in reproducing the statistical properties of the data.

… Analysis and Prediction …

As more high-resolution observations become available, our view of ocean mesoscale turbulence more closely becomes that of a "sea of eddies." The presence of the coherent vortices significantly affects the dynamics and the statistical properties of mesoscale flows, with important consequences on tracer dispersion and ocean stirring and mixing processes. Here we review some of the properties of particle transport in vortex-dominated flows, concentrating on the statistical properties induced by the presence of an ensemble of vortices. We discuss a possible parameterization of particle dispersion in vortex-dominated flows, adopting the view that ocean mesoscale turbulence is a two-component fluid which includes intense, localized vortical structures with non-local effects immersed in a Kolmogorovian, low-energy turbulent background which has mostly local effects. Finally, we report on some recent results regarding the role of coherent mesoscale eddies in marine ecosystem functioning, which is related to the effects that vortices have on nutrient supply. frequency mesoscale variability (i.e. the medium-size fluctuations in the general circulation), for the mesoscale and sub-mesoscale coherent vortices (vortical motions at scales smaller than the internal Rossby radius of deformation, McWilliams 1985), or for a generic complicated motion in the presence of turbulence.

As two fluid particles separate in time, the entire spectrum of eddy motions is being sampled from the smallest to the largest scales. In large-scale geophysical systems for which the Earth rotation is important, it has been conjectured that the relative diffusivity should vary respectively as D^2 and D^4/3 for distances respectively smaller and larger than a well-defined forcing scale of the order of the internal Rossby radius (with D the r.m.s. separation distance). Particle paths data from a mid-latitude float experiment in the central part of the North Atlantic appear to support these statements partly: two particles initially separated by a few km within two distinct clusters west and east of the mid-Atlantic ridge, statistically dispersed following a Richardson regime (D^2 ∼ t^3 asymptotically) for r.m.s. separation distances between 40 and 300 km, in agreement with a D^4/3 law. At early times, and for smaller separation distances, an exponential growth, in agreement with a D^2 law, was briefly observed but only for the eastern cluster (with an e-folding time around 6 days). After a few months or separation distances greater than 300 km, the relative dispersion slowed down naturally to the Taylor absolute dispersion regime.

Journal of Marine Systems, 2001

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 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 in regard to the nature of the spin parameter ⍀, which was used in the Lagrangian stochastic model that best described the observed eddy statistics.

Limnology and Oceanography, 2017

In situ measurements were undertaken to characterize particle fields in undisturbed oceanic environments. Simultaneous, co-located depth profiles of particle fields and flow characteristics were recorded using a submersible holographic imaging system and an acoustic Doppler velocimeter, under different flow conditions and varying particle concentration loads, typical of those found in coastal oceans and lakes. Nearly one million particles with major axis lengths ranging from 14 lm to 11.6 mm, representing diverse shapes, sizes, and aspect ratios were characterized as part of this study. The particle field consisted of marine snow, detrital matter, and phytoplankton, including colonial diatoms, which sometimes formed "thin layers" of high particle abundance. Clear evidence of preferential alignment of particles was seen at all sampling stations, where the orientation probability density function (PDF) peaked at near horizontal angles and coincided with regions of low velocity shear and weak turbulent dissipation rates. Furthermore, PDF values increased with increasing particle aspect ratios, in excellent agreement with models of spheroidal particle motion in simple shear flows. To the best of our knowledge, although preferential particle orientation in the ocean has been reported in two prior cases, our findings represent the first comprehensive field study examining this phenomenon. Evidence of nonrandom particle alignment in aquatic systems has significant consequences to aquatic optics theory and remote sensing, where perfectly random particle orientation and thus isotropic symmetry in optical parameters is assumed. Ecologically, chain-forming phytoplankton may have evolved to form large aspect ratio chains as a strategy to optimize light harvesting.

Ocean Modelling, 2010

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.

Journal of Physical Oceanography, 2013

A large-scale tool for systematic analyses of the dispersal and turbulent properties of ocean currents and the subsequent separation of dynamical regimes according to the prevailing trajectories taxonomy in a certain area was proposed by Rupolo. In the present study, this methodology has been extended to the analysis of model trajectories obtained by analytical computations of the particle advection equation using the Lagrangian open-source software package Tracing the Water Masses of the North Atlantic and the Mediterranean (TRACMASS), and intercomparisons have been made between the surface velocity fields from three different configurations of the global Nucleus for European Modelling of the Ocean (NEMO) ocean/sea ice general circulation model. Lagrangian time scales of the observed and synthetic trajectory datasets have been calculated by means of inverse Lagrangian stochastic modeling, and the influence of the model field spatial and temporal resolution on the analyses has been ...