Effective Eddy Diffusivities Inferred from a Point Release Tracer in an Eddy-Resolving Ocean Model

• Published in: Journal of physical oceanography Vol. 39; no. 4; pp. 894 - 914
• Main Authors: LEE, Mei-Man, NURSER, A. J. George, COWARD, Andrew C, DE CUEVAS, Beverly A
• Format: Journal Article
• Language: English
• Published: Boston, MA American Meteorological Society 01.04.2009
• Subjects: Atmospheric models; Earth, ocean, space; Efficiency; Exact sciences and technology; Experiments; External geophysics; Marine; Physics of the oceans; Strain rate; Thermohaline structure and circulation. Turbulence and diffusion; Tracers; Turbulent flow; Eddy diffusion; Flow regime; Oceanic turbulence; Mixing process; tracers; digital simulation; Ocean dynamics; Computing method; Comparative study; Ocean model
• ISSN: 0022-3670; 1520-0485
• DOI: 10.1175/2008JPO3902.1

Abstract This study uses tracer experiments in a global eddy-resolving ocean model to examine two diagnostic methods for inferring effective eddy isopycnic diffusivity from point release tracers. The first method is based on the growth rate of the area occupied by the tracers (the equivalent variance). During the period when tracer dispersion is dominated by stirring, the equivalent variance is found to increase at a rate between the second power law (for a pure shearing flow regime) and the exponential law (for a pure stretching flow regime). The second method is based on the length of the tracer contours. In the framework of equivalent radius, the two methods of inferring eddy diffusivity can be understood as two different averagings over the tracer patch. Over a shorter period of tracer dispersion the two methods give different eddy diffusivities, and only over a longer time when tracer dispersion approaches the final stage of diffusion do they give a similar value of diffusivity. A new diagnostic quantity called stirring efficiency is introduced to indicate different flow regimes by measuring the efficiency of stirring against mixing. The new diagnostic quantity has the advantage that it can be calculated directly from the gradients of tracer distribution without needing to estimate strain rate or background diffusivity.