Nonlinear Processes in Oceanic and Atmospheric Flows


Stirring and mixing in the West Florida Shelf

Author: M. J. Olascoaga , RSMAS, University of Miami.

Names and affiliation of other authors:
F. J. Beron-Vera
RSMAS, University of Miami
Miami, USA

Oral or poster: Poster

Downloadable poster file:

Stirring and mixing in the West Florida Shelf

(nloa08-mjo-v2.pdf, 2479698 bytes)

Application of dynamical systems tools has recently revealed in simulated surface ocean currents a persistent Lagrangian coherent structure (LCS) on the West Florida Shelf (WFS). Consistent with satellite-tracked drifter trajectories, this LCS constitutes a cross-shelf transport barrier which constrains pollutant dispersal and also has important biological consequences. More precisely, red tides occur most frequently on the shoreside of this barrier, which suggests that it provides favorable conditions for their development by allowing for nutrient and dinoflagellate buildup. Here we carry out a detailed study of the nature of the surface ocean Lagrangian motion in the WFS. Examination of several diagnostics suggests that chaotic stirring dominates over turbulent mixing on timescales of up to two months or so. For instance: 1) the large-scale traits of simulated LCSs remain fairly unaltered under spatiotemporal truncations of the advection field; 2) the kinetic energy spectrum has a nearly -3 slope, implying marginally nonlocal dynamics; 3) the PDFs of finite-time Lyapunov exponents exhibit Gaussian cores and long tails with several extrema that fall more slowly than Gaussian toward large stretching rates, indicating spatially inhomogeneous fluid particle dispersion; and 4) Eulerian autocorrelation times of the velocity gradient are shorter than Lagrangian autocorrelation times, suggesting that the Lagrangian evolution is more irregular than the driving Eulerian flow.

*Satellite images from NASA and ESA

Nonlinear Processes in Oceanic and Atmospheric Flows. July 2-4, 2008. Castro Urdiales, Cantabria, Spain.