Reconciling float-based and tracer-based estimates of lateral diffusivities

Abstract

Lateral diffusivities are computed from synthetic particles and tracers advected by a velocity field derived from sea-surface height measurements from the South Pacific, in a region west of Drake Passage. Three different estimates are compared: (1) the tracer-based "effective diffusivity" of Nakamura (1996), (2) the growth of the second moment of a cloud of tracer and (3) the single-and two-particle Lagrangian diffusivities. The effective diffusivity measures the cross-stream component of eddy mixing, so this article focuses on the meridional diffusivities for the others, as the mean flow (the ACC) is zonally oriented in the region. After an initial transient of a few weeks, the effective diffusivity agrees well with the meridional diffusivity estimated both from the tracer cloud and from the particles. This proves that particleand tracer-based estimates of eddy diffusivities are equivalent, despite recent claims to the contrary. Convergence among the three estimates requires that the Lagrangian diffusivities be estimated using their asymptotic values, not their maximum values. The former are generally much lower than the latter in the presence of a mean flow. Sampling the long-time asymptotic behavior of Lagrangian diffusivities requires very large numbers of floats in field campaigns. For example, it is shown that hundreds of floats would be necessary to estimate the vertical and horizontal variations in eddy diffusivity in a sector of the Pacific Southern Ocean.

Energy pathways and structures of oceanic eddies from the ECCO2 state estimate and simplified models

“…Klocher et al, 2012b). To our best knowledge, though the effect of mean flow on tracer mixing induced by the total eddies has been studied (e.g.…”

A Multiwavenumber Theory for Eddy Diffusivities and Its Application to the Southeast Pacific (DIMES) Region

“…where G denotes the O(1) mixing efficiency, u rms is the eddy velocity magnitude, and k denotes the eddy diffusivity (e.g., Taylor 1915;Klocker and Abernathey 2014). Bates et al (2014) pointed out that ocean studies often assume that L mix is of the same order of magnitude as the eddy size (e.g., Holloway 1986;Haine and Marshall 1998).…”

Physical controls on copepod aggregations in the Gulf of Maine

“…Uncertainty in n depends crucially upon the number of drifters, analogous to the results of Klocher et al (2012). This work is the first to my knowledge to attempt to quantify uncertainty in Kpir.…”

“…As noted in the introduction, Klocher et al (2012) showed that the diffusivity computed from drifters in the Southern Ocean depends directly on the number of drifters used in the computation. We wish to know whether the number of drifters in the GOM is sufficient to discern spatial variability in n as well as the asymmetry in the K tensor.…”

“…For example, the accuracy of single-particle diffusivity estimates has been shown to depend to a high degree on the number of drifters used in the calculation (Klocher et al (2012)). Using a numerical model and model drifters, we can quantify the uncertainty in diffusivity estimates given a certain number of drifters, and determine whether current observations are sufficient to make the desired inferences, or whether additional observational effort should be undertaken.…”