VERTICAL MIXING, ENERGY, AND THE GENERAL CIRCULATION OF THE OCEANS

Abstract

Key Words ocean circulation, ocean mixing, ocean circulation energy s Abstract The coexistence in the deep ocean of a finite, stable stratification, a strong meridional overturning circulation, and mesoscale eddies raises complex questions concerning the circulation energetics. In particular, small-scale mixing processes are necessary to resupply the potential energy removed in the interior by the overturning and eddy-generating process. A number of lines of evidence, none complete, suggest that the oceanic general circulation, far from being a heat engine, is almost wholly governed by the forcing of the wind field and secondarily by deep water tides. In detail however, the budget of mechanical energy input into the ocean is poorly constrained. The now inescapable conclusion that over most of the ocean significant "vertical" mixing is confined to topographically complex boundary areas implies a potentially radically different interior circulation than is possible with uniform mixing. Whether ocean circulation models, either simple box or full numerical ones, neither explicitly accounting for the energy input into the system nor providing for spatial variability in the mixing, have any physical relevance under changed climate conditions is at issue.

Role of vertical migration in biogenic ocean mixing

“…For example, plankton with characteristic body size of 10 −2 m, vertical migration speed of 10 −2 m s −1 and mean spacing of 10 −2 m have an estimated longitudinal (i.e., diapycnal) dispersion D v ≈ 10 −4 m 2 s −1 due to vortex shedding alone. This dispersion is three orders of magnitude greater than molecular diffusion of heat in the ocean, and five orders of magnitude greater than molecular diffusion of salt [Wunsch and Ferrari, 2004].…”

Propagating and evanescent internal waves in a deep ocean model

“…An important but challenging extension of this work should examine the effect of a turning depth on internal wave behavior at higher Reynolds number and lower Schmidt number, where wave breaking can occur. Such a study, necessarily three-dimensional, could provide insight into the poorly understood problem of mixing in the global oceans (Munk & Wunsch 1998;Wunsch & Ferrari 2004).…”

“…Approximately half of the internal wave energy in the ocean is produced by tidal flow over bottom topography (Munk & Wunsch 1998;Wunsch & Ferrari 2004). Oceanic buoyancy frequencies vary greatly, decreasing from the values in shallow water to become two orders of magnitude smaller in the well-mixed abyss (see example in §4).…”

Eddy energy sources and sinks in the South China Sea

“…As processes of smaller scale are not the focus of this paper, we treat them as one term. For detailed discussion on the eddy energy balance, readers are referred to Wunsch and Ferrari [2004] and Zhai and Marshall [2013].…”