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Isopycnal Mixing in the Eastern Tropical North Atlantic

Explore the dispersion of floats in the Atlantic using data from World Atlas 94, examining eddy diffusivity and dispersion patterns. Study reveals anisotropic dispersion and time-dependent diffusivities.

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Isopycnal Mixing in the Eastern Tropical North Atlantic

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  1. Isopycnal Mixing in the Eastern Tropical North Atlantic Dave Hebert, Tom Rossby and Long Zhou Graduate School of Oceanography, University of Rhode Island

  2. Plan views of temperature and O2 at the two nominal float deployment depths. Data is from the World Atlas 94 (Levitus and Boyer 1994). The nominal location of sound sources, spaced apart 720 km zonally and 500 km meridionally, are shown by diamonds. The location of the float deployments are shown by open circles. Dashed lines show transects used in the next slide.

  3. In this talk, we will examine the dispersion of the floats by the mesoscale eddies and examine the ‘effective eddy diffusivity’. But first, let’s look at the initial dispersion of the clusters of floats before the eddy stirring disperses the floats. Two short movies of initial dispersion to show that things can be quite complicated. This dispersion is a topic of future work.

  4. Example of gradual dispersion of the cluster of floats

  5. Example of sudden dispersion of the cluster of floats

  6. Isopycnal float trajectories for 600 days

  7. Single Particle (Absolute) Dispersion and Lagrangian Diffusivity For each cluster, the relative location of each float in the cluster was determined relative to the mean location of the cluster. This removed any mean velocity of the float in order us to determine particle dispersion.

  8. For time much shorter than the integral time scale, the dispersion increases as t2. For time much longer than the integral time scale, the dispersion increases as t . The Lagrangian eddy diffusivity is defined as

  9. ~0 2700 m2/s 1500 m2/s 840 m2/s 200 m2/s The Lagrangian eddy diffusivity as a function of time

  10. Conclusions • Dispersion is anisotropic: zonal dispersion is larger. • Meridional diffusivity appears to be constant with time (e.g. dispersion increases linearly) after the meridional integral time scale. • Zonal dispersion increases with time quadraticallyfor times longer than zonal integral time scale. This means that the zonal diffusivity increases with time. • Zonal diffusion appears to stop after 500 days. The reason is unkown at this time. • Integral time scales are much longer than previous Lagrangian studies at higher latitudes. Eddy diffusivities are smaller than those of previous studies.

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