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

Abstract

A multiwavenumber theory is formulated to represent eddy diffusivities. It expands on earlier singlewavenumber theories and includes the wide range of wavenumbers encompassed in eddy motions. In the limiting case in which ocean eddies are only composed of a single wavenumber, the multiwavenumber theory is equivalent to the single-wavenumber theory and both show mixing suppression by the eddy propagation relative to the mean flow. The multiwavenumber theory was tested in a region of the Southern Ocean (708-458S, 1108-208W) that covers the Drake Passage and includes the tracer/float release locations during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). Cross-stream eddy diffusivities and mixing lengths were estimated in this region from the single-wavenumber theory, from the multiwavenumber theory, and from floats deployed in a global 1 /108 Parallel Ocean Program (POP) simulation. Compared to the single-wavenumber theory, the horizontal structures of cross-stream mixing lengths from the multiwavenumber theory agree better with the simulated float-based estimates at almost all depth levels. The multiwavenumber theory better represents the vertical structure of cross-stream mixing lengths both inside and outside the Antarctica Circumpolar Current (ACC). Both the single-wavenumber and multiwavenumber theories represent the horizontal structures of cross-stream diffusivities, which resemble the eddy kinetic energy patterns.

The Contribution of Striations to the Eddy Energy Budget and Mixing: Diagnostic Frameworks and Results in a Quasigeostrophic Barotropic System with Mean Flow

“…(1)] to the simplest system and thus allows us to focus on the basic physics about the mean flow effect on striations. Third, the constant mean flow assumption has been widely used in previous studies about striations, eddies, energetics, and mixing, leading to results/conclusions relevant to the ocean, at least to some extent (e.g., Arbic 2000;Killworth 1997;Flierl and Pedlosky 2007;Smith 2007;Ferrari and Nikurashin 2010;Hughes et al 2010;Venaille et al 2011;Chen et al 2015a).…”

“…As a key component of the oceanic circulation, they stir and mix tracers (e.g., heat and potential vorticity) and therefore greatly influence the oceanic circulation and climate variability. Given their importance, much effort has been made recently toward estimating and interpreting eddy energy budgets (e.g., Ferrari and Wunsch 2009;Xu et al 2011;Chen et al 2014a) and eddy mixing rates (e.g., Gille et al 2007;Griesel et al 2010;Rypina et al 2012;Klocker et al 2012a,b;Abernathey et al 2013;Chen et al 2014b;Chen et al 2015b) in the global ocean.…”

“…The temporal average (i.e., low-frequency component) of eddy motions has banded structures in most of the ocean that are often termed ''striations'' (e.g., Cox 1987;Galperin et al 2004;Maximenko et al 2005;Nakano and Hasumi 2005;Richards et al 2006;Maximenko et al 2008;Chen 2013). Spectral analysis of the output from an eddying global model suggests that these striations account for more than 10% of the zonal velocity variability in the upper 1000 m of the central North Pacific and east North Pacific regions (Chen et al 2015a).…”

“…Spectral analysis of the output from an eddying global model suggests that these striations account for more than 10% of the zonal velocity variability in the upper 1000 m of the central North Pacific and east North Pacific regions (Chen et al 2015a). This significant percentage suggests that striations are an important component of the oceanic eddy field and that their contribution to eddy energetics and tracer mixing is probably nonnegligible.…”

“…This significant percentage suggests that striations are an important component of the oceanic eddy field and that their contribution to eddy energetics and tracer mixing is probably nonnegligible. Yet, while their origin has received much attention, little effort has been made toward quantifying these contributions (e.g., Maximenko et al 2008;Schlax and Chelton 2008;Afanasyev et al 2012;Chen et al 2015a). …”