Researchers: (alphabetic order) - Garraffo, Zulema1 - Griffa, Annalisa1,2 - Mariano, Arthur1 - Reynolds, Andy3 - Veneziani, Milena1 - 1RSMAS, Miami - 2CNR, Italy - 3SILSOE, UK Sponsors: - ONR - NSF Publications: - Veneziani, M., A. Griffa, A.M. Reynolds, Z.D. Garraffo, and E.P. Chassignet, 2005: Parameterizations of Lagrangian spin statistics and particle dispersion in presence of coherent vortices. in press, J. Mar. Res., 63, issue 6, (PDF). - Veneziani, M., A. Griffa, Z.D. Garraffo, and E.P. Chassignet, 2005: Lagrangian spin parameter and coherent structures from trajectories released in a high-resolution ocean model. J. Mar. Res., 63, issue 4, 753-788, (PDF). - Maurizi A., A. Griffa, P.M. Poulain and F. Tampieri, 2004: Lagrangian turbulence in the Adriatic Sea as computed from drifter data: effects of inhomogeneity and nonstationarity. J. Geophys. Res., in press. - Veneziani M., A. Griffa, A.M. Reynolds and A.J. Mariano, 2004: Oceanic turbulence and stochastic models from subsurface Lagrangian data for the North-West Atlantic Ocean. J. Phys. Oceanogr., 34, (8), 1884-1906. - Reynolds, A.M. and M. Veneziani, 2004: Rotational dynamics of turbulence and Tsallis statistics. Phys. Lett. A, 327, 9-14. - Bauer, S., M.S. Swenson and A. Griffa, 2002: Eddy-mean flow decomposition and eddy diffusivity estimates in the tropical Pacific Ocean. 2: Results. J. Geophys. Res., 107, (C10), 3154-3171. - Garraffo, Z., A. Mariano, A. Griffa, C. Veneziani and E. Chassignet, 2001: Lagrangian data in a high resolution model simulation of the North Atlantic. 1: Comparison with in-situ drifters. J. Mar. Sys., 29, 157-176. - Garraffo, Z., A. Griffa, A. Mariano and E. Chassignet, 2001: Lagrangian data in a high resolution model simulation of the North Atlantic. 2: Mean flow reconstruction and sampling effects. J. Mar. Sys., 29, 177-200. - Falco P., A. Griffa, P.M. Poulain, E. Zambianchi, 2000: Transport properties in the Adriatic Sea as deduced from drifter data. J. Phys. Oceanogr., 30, (8), 2055-2071.

Lagrangian data provide information on ocean currents in terms of velocity and transport. Extensive data sets are available today, both at and below the ocean surface, thanks to a number of extensive field experiments. The data have been analyzed by a number of authors, providing significant contributions to our knowledge of the ocean circulation and transport. We have recently analyzed data sets of surface drifters in the Adriatic Sea and the Topical Pacific and a set of subsurface floats in the North Atlantic. Our focus is mostly on characterizing dispersion processes and testing suitable transport parameterizations, in particular in terms of Lagrangian Stochastic (LS) models. In the following, we provide some specific information on the North Atlantic study. The historical data set provided by 700 m acoustically-tracked floats has been analyzed in different regions of the north-western Atlantic Ocean. (Fig. 1, Fig. 2). In the Gulf Stream recirculation and extension regions, the autocovariances and crosscovariances of the Lagrangian velocity exhibit significant oscillatory patterns on time scales comparable with the Lagrangian decorrelation time scale. They are indicative of the presence of significant coherent structures and of sub- and super-diffusive behaviors in the mean spreading of water particles. Our main result is that the properties of the Lagrangian statistics can be considered as a superposition of two different regimes associated with looping and non-looping trajectories (Fig. 3), and that both regimes can be parameterized using a simple first-order Lagrangian stochastic model with spin parameter. The non-looping regime corresponds to an approximately homogeneous "background" flow, while the looping regime is characteristics of the coherent structures. The spin parameters couples the zonal and meridional velocity components, reproducing the effect of rotating vortices. It is considered as a random parameter whose probability distribution is approximately bi-modal, reflecting the distribution of loopers (finite spin) and non-loopers (zero spin). The simple model is found to be very effective in reproducing the statistical properties of the data (Fig. 4). Supplementary analysis has been performed using a synthetic data set of trajectories released in a high resolution Miami Isopycnic Model (MICOM) (Fig. 5). The goal is to investigate the relationship between Langrangian and Eulerian statistics, and in particular to verify whether the spin parameter can be interpreted as a relative vorticity estimate of the coherent structures (Fig. 6). Figures: Figure 1 Trajectory (spaghetti) plot of the acoustically tracked isobaric floats available in the North-Western Atlantic at 700 m depth from the Subsurface Float Data assembly Center (WFDAC) at Woods Hole. Figure 2 Map of the Eddy Kinetioc Energy obtained averaging the 700 m float data over 1 degree bins. Superimposed are boxes indicating regions of quasi-homogeneous EKE, where Lagrangian statistical analysis has been carried out. Figure 3 Sample of looping trajectories, "loopers", (left panels), and non looping trajectories, 'non-loopers" (right panel) in a region of approximately homogeneous EKE in the southern Gulf Stream recirculation (region RECW in Fig. 2). Arrows along trajectories are plotted every 5 days. Figure 4 Velocity autocovariance functions computed respectively from 700 m float data (top panels) and from simulated trajectories from a first order Lagrangian Stochastic Model (LSM) with spin (lower panels). First column show statistics from the overall data set, second column from non-loopers only and third column from loopers only. As it can be seen the agreement between data and LSM results is very satisfactory. Figure 5 Snapshot of relative vorticity map from a numerical simulation of a high resolution model in the North Atlantic, showing the presence of a rich population of coherent structures such as rings and vortices. The model is the Miami Isopycnic Model (MICOM) at 1/12 of degree resolution. Positive (negative) vorticity is indicated in red (blue). Figure 6 Video representing the evolution of the velocity field (sqrt EKE) from the MICOM simulation of the North Atlantic at 1/12 of degree resolution. The video spans a period of 1 year, during which high energy ring form and detach from the Gulf Stream, propagating south-eastward. Trajectories of simulated particles seeded in the rings are also shown with 60 day tails. (Click on the above image to play an 8MB animated gif.)