Float-Derived Isopycnal Diffusivities in the DIMES Experiment

Abstract

As part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES), 210 subsurface floats were deployed west of the Drake Passage on two targeted density surfaces. Absolute (single particle) diffusivities are calculated for the floats. The focus is on the meridional component, which is less affected by the mean shear. The diffusivities are estimated in several ways, including a novel method based on the probability density function of the meridional displacements. This allows the determination of the range of possible lateral diffusivities, as well as the period over which the spreading can be said to be diffusive. The method is applied to the float data and to synthetic trajectories generated with the Massachusetts Institute of Technology General Circulation Model (MITgcm). Because of ballasting problems, many of the floats did not remain on their targeted density surface. However, the float temperature records suggest that most occupied a small range of densities, so the floats were grouped together for the analysis. The latter focuses on a subset of 109 of the floats, launched near 105°W. The different methods yield a consistent estimate for the diffusivity of 800 ± 200 m2 s−1. The same calculations were made with model particles deployed on 20 different density surfaces and the result for the particles deployed on the neutral density surface γ = 27.7 surface was the same within the errors. The model was then used to map the variation of the diffusivity in the vertical, near the core of the Antarctic Circumpolar Current (ACC). The results suggest mixing is intensified at middepths, between 1500 and 2000 m, consistent with several previous studies.

Direct Estimate of Lateral Eddy Diffusivity Upstream of Drake Passage

“…This value agrees with an independent estimate based on the dispersion of 72 acoustically tracked isopycnal floats, deployed on the same isopycnal surface as the tracer (see LaCasce et al 2014). The main objective of this paper is to explain how we obtained this estimate.…”

Role of Mesoscale Eddies in Cross-Frontal Transport of Heat and Biogeochemical Tracers in the Southern Ocean

“…(2) and from Lee et al's (2007) method. This value of k lies at the low end of observational and theoretical values of 500 to 2000 m 2 s 21 for the Southern Ocean (e.g., Ferrari and Nikurashin 2010;Abernathey et al 2010;LaCasce et al 2014). We note that k is expected to vary in space and time because of mean flow suppression (e.g., Ferrari and Nikurashin 2010).…”

Isopycnal Eddy Diffusivities and Critical Layers in the Kuroshio Extension from an Eddying Ocean Model

“…While the ''level-off'' can be judged by eye (e.g., Griesel et al 2010;Klocker et al 2012a,b;Griesel et al 2014;LaCasce et al 2014), we provide a simple quantitative criterion to judge whether k(t) asymptotes to a constant in a statistical sense, given the limited number and duration of float data available for assessing k.…”