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LatMix combines shipboard, autonomous, and airborne field observations with modeling to improve understanding of ocean stirring across multiple scales.
Dispersion of natural and anthropogenic tracers in the ocean is traditionally conceptualized as a two-stage process: The first step, stirring, is an adiabatic rearrangement of water parcels that does not change their potential temperature, salinity, or other tracer concentrations; it tends to stretch tracer patches into convoluted streaks and therefore enhances overall variance of tracer gradients. Molecular diffusion then acts to reduce small-scale gradients and effects the ultimate mixing (Eckart 1948; Garrett 2006). In practice, all small-scale processes not resolved in a particular numerical or analytic framework (e.g., Reynolds-averaged Navier- Stokes equations) are often lumped into mixing with the understanding that it may include unresolved stirring as well. Within the strongly stratified ocean interior, a clear distinction can be made between isopycnal processes, which act along surfaces of constant potential density (or, more strictly, neutral surfaces; Montgomery 1940; McDougall 1984), and diapycnal processes, which act across these surfaces (Gregg 1987; MacKinnon et al. 2013).
Interpretation of lateral dispersion of tracers in the ocean in terms of mixing is fraught with ambiguity. The unresolved flux JT of a tracer T ascribed to lateral mixing is commonly parameterized with Fickian diffusion law,
JT = -Kh∇T,
where Kh is the effective diffusivity and ∇T is the resolved tracer gradient. However, it has been long recognized that Kh depends strongly on the spatial and temporal scales being considered (Stommel 1949; Ozmidov 1958). Therefore, any estimate of Kh must be accompanied with the specification of scales, which are themselves somewhat arbitrarily defined (Okubo 1976). These ambiguities can be overcome by understanding and modeling the processes responsible for lateral stirring of tracers.
Stirring cascades variance from large scales, where it is produced, to O(1) cm scales, where it is removed by molecular diffusion (Stern 1975). On the mesoscale O(10-100) km, isopycnal stirring by geostrophic eddies is well understood (e.g., Smith and Ferrari 2009). Likewise, stirring by microscale (0.01-1 m) isotropic turbulence to the molecular scale has been studied for many decades and its physics is well established. In contrast, the dynamics that control stirring on the submesoscale (0.1-10 km), and the relative importance of various processes is not as well known....