Statistical Analysis of the Surface Circulation in the California Current System Using Satellite-Tracked Drifters

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  • 1 Scripps Institution of Oceanography, La Jolla, California
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Abstract

A kinematic description of the surface circulation in the southern California current System is presented using the statistics of the 7–11 month long trajectories of 29 satellite-tracked mixed layer drifters. The drifters were released north of 30°N and traveled southward at an average speed of 3–4 cm s−1 along Baja California through an inhomogeneous field of mesoscale eddies of 15 cm s−1 rms variability. Lagrangian and Eulerian statistics of the variations about this mean southward drift are computed. The drifter ensemble mean Lagrangian decorrelation time scale is 4–5 days and the Lagrangian decorrelation space scale is 40–50 km. The computation of dispersion of single particles about the mean drift shows that the theory of diffusion by homogeneous random motion (Taylor's theory) describes these dispersive motions well. Ensemble mean diffusivities of about 4 × 107 cm2 s−1 are found. On a 200 × 200 km2 spatial average, single-partial diffusivities are found to be proportional to the kinetic energy of the locally inhomogeneous fluctuations. Particle-pair statistics are used to study the relative dispersion of particles. The relative diffusivities depend on the initial separation and on the duration of drift. The results are compared to Richardson's 4/3 power law. The Eulerian spatial and temporal correlation of the velocity field indicates that the eddy field is isotropic for scales less than 200 km. The zero time lag correlation indicates an Eulerian length scale of 80 km. The 25-day lagged correlation function indicates that a 2 cm S−1 northwestward propagation of features exists roughly perpendicular to the mean flow.

Abstract

A kinematic description of the surface circulation in the southern California current System is presented using the statistics of the 7–11 month long trajectories of 29 satellite-tracked mixed layer drifters. The drifters were released north of 30°N and traveled southward at an average speed of 3–4 cm s−1 along Baja California through an inhomogeneous field of mesoscale eddies of 15 cm s−1 rms variability. Lagrangian and Eulerian statistics of the variations about this mean southward drift are computed. The drifter ensemble mean Lagrangian decorrelation time scale is 4–5 days and the Lagrangian decorrelation space scale is 40–50 km. The computation of dispersion of single particles about the mean drift shows that the theory of diffusion by homogeneous random motion (Taylor's theory) describes these dispersive motions well. Ensemble mean diffusivities of about 4 × 107 cm2 s−1 are found. On a 200 × 200 km2 spatial average, single-partial diffusivities are found to be proportional to the kinetic energy of the locally inhomogeneous fluctuations. Particle-pair statistics are used to study the relative dispersion of particles. The relative diffusivities depend on the initial separation and on the duration of drift. The results are compared to Richardson's 4/3 power law. The Eulerian spatial and temporal correlation of the velocity field indicates that the eddy field is isotropic for scales less than 200 km. The zero time lag correlation indicates an Eulerian length scale of 80 km. The 25-day lagged correlation function indicates that a 2 cm S−1 northwestward propagation of features exists roughly perpendicular to the mean flow.

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