Relative Dispersion in the Antarctic Circumpolar Current

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  • 1 School of Oceanography, University of Washington, Seattle, Washington, USA
  • 2 Department of Geosciences, University of Oslo, Oslo, Norway
  • 3 Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, Florida, USA
  • 4 Department of Earth, Atmosphere and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
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Abstract

Stirring in the subsurface Southern Ocean is examined using RAFOS float trajectories, collected during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES), along with particle trajectories from a regional eddy permitting model. A central question is the extent to which the stirring is local, by eddies comparable in size to the pair separation, or non-local, by eddies at larger scales. To test this, we examine metrics based on averaging in time and in space. The model particles exhibit non-local dispersion, as expected for a limited resolution numerical model that does not resolve flows at scales smaller than ~ 10days or ~ 20–30km. The different metrics are less consistent for the RAFOS floats; relative dispersion, kurtosis and relative diffusivity suggest non-local dispersion as they are consistent with the model within error, while finite size Lyapunov exponents (FSLE) suggests local dispersion. This occurs for two reasons: (i) limited sampling of the inertial length scales and relatively small number of pairs hinder statistical robustness in time-based metrics, and (ii) some space-based metrics (FSLE, 2nd order structure functions), which do not average over wave motions and are reflective of the kinetic energy distribution, are probably unsuitable to infer dispersion characteristics if the flow field includes energetic wave-like flows that do not disperse particles. The relative diffusivity, which is also a space-based metric, allows averaging over waves to infer the dispersion characteristics. Hence, given the error characteristics of the metrics and data used here, the stirring in the DIMES region is likely to be non-local at scales of 5-100km.

Corresponding author address: Dhruv Balwada, School of Oceanography, University of Washington, Washington, WA, USA. E-mail: dbalwada@uw.edu

This article is included in the The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) Special Collection.

Abstract

Stirring in the subsurface Southern Ocean is examined using RAFOS float trajectories, collected during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES), along with particle trajectories from a regional eddy permitting model. A central question is the extent to which the stirring is local, by eddies comparable in size to the pair separation, or non-local, by eddies at larger scales. To test this, we examine metrics based on averaging in time and in space. The model particles exhibit non-local dispersion, as expected for a limited resolution numerical model that does not resolve flows at scales smaller than ~ 10days or ~ 20–30km. The different metrics are less consistent for the RAFOS floats; relative dispersion, kurtosis and relative diffusivity suggest non-local dispersion as they are consistent with the model within error, while finite size Lyapunov exponents (FSLE) suggests local dispersion. This occurs for two reasons: (i) limited sampling of the inertial length scales and relatively small number of pairs hinder statistical robustness in time-based metrics, and (ii) some space-based metrics (FSLE, 2nd order structure functions), which do not average over wave motions and are reflective of the kinetic energy distribution, are probably unsuitable to infer dispersion characteristics if the flow field includes energetic wave-like flows that do not disperse particles. The relative diffusivity, which is also a space-based metric, allows averaging over waves to infer the dispersion characteristics. Hence, given the error characteristics of the metrics and data used here, the stirring in the DIMES region is likely to be non-local at scales of 5-100km.

Corresponding author address: Dhruv Balwada, School of Oceanography, University of Washington, Washington, WA, USA. E-mail: dbalwada@uw.edu

This article is included in the The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) Special Collection.

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